• fractionation;
  • therapy;
  • cancer;
  • radioimmunotherapy;
  • radiation;
  • antibody;
  • radionuclide;
  • radioisotope


  1. Top of page
  2. Abstract
  3. Heterogeneity of Macromolecules in Tumors
  4. Preclinical Radioimmunotherapy Evidence
  5. Clinical Radioimmunotherapy Evidence
  6. Fractionated or Hyperfractionated Radiotherapy and Radioimmunotherapy
  7. Comparison of the Radiobiologic Aspects of Fractionation for External Beam Radiotherapy and Radioimmunotherapy
  8. Discussion
  9. Acknowledgements

Although fractionation can be used in a discrete radiobiologic sense, herein it is generally used in the broader context of administration of multiple, rather than single, doses of radionuclide for radioimmunotherapy (RIT) or other targeted radionuclide therapies. Fractionation is a strategy for overcoming heterogeneity of monoclonal antibody (MAb) distribution in the tumor and the consequent nonuniformity of tumor radiation doses. Additional advantages of fractionated RIT are the ability to 1) provide patient-specific radionuclide and radiation dosing, 2) control toxicity by titration of the individual patient, 3) reduce toxicity, 4) increase the maximum tolerated dose (MTD) for many patients, 5) increase tumor radiation dose and efficacy, and 6) prolong tumor response by permitting treatment over time. However, fractionated RIT has logistic and economic implications. Preclinical and clinical data substantiate the advantages of fractionated RIT, although the radiobiology for conventional external beam radiotherapy does not provide a straightforward rationale for RIT unless fractionation leads to more uniform distribution of radiation dose throughout the tumor. Preclinical data have shown that toxicity and mortality can be reduced while efficacy is increased, thereby providing inferential evidence of greater uniformity of radiation dose. Direct evidence of superior dosimetry and tumor activity distribution has also been found. Clinical data have shown that toxicity can be better controlled and reduced and the MTD extended for many patients. It is clear that fractionated RIT can only fulfill its potential if the effects of critical issues, such as the number and amount of radionuclide doses, the radionuclide physical and effective half-life, and the dose interval, are better characterized. Cancer 2002;94:1332–48. © 2002 American Cancer Society.

DOI 10.1002/cncr.10304

The benefit from fractionation of the total dose of radiation for external beam radiotherapy (EBRT) is well established; multiple doses, usually given daily, extend the total radiation dose that can be given to the malignancy by decreasing normal tissue toxicity.1, 2 Indeed, hyperfractionation—that is, multiple doses of radiation each day—has been shown to be superior to daily doses of EBRT,3–5 although it has logistic and economic disadvantages. Sealed radionuclide source implant radiotherapy involves continuous, high-dose-rate radiation rather than the multiple, short bursts of radiation characteristic of EBRT. The critical importance of fractionation for EBRT depends on the steepness of the dose-response relationship for therapeutic advantage.6

Radionuclide treatment, whether radioimmunotherapy (RIT) or other, involves continuous and continuously decreasing low-dose-rate radiation that appears to destroy cells primarily through apoptosis,7 rather than through the necrosis characteristic of the cellular effects of EBRT and chemotherapy. There is evidence suggesting that RIT may have greater therapeutic efficacy for equivalent total radiation dose and dose rates when compared with EBRT.8–10 There are two broad approaches to radionuclide dose (activity; mCi) schedules in RIT. One of these is the administration of a single, large dose of radionuclide (radiolabeled monoclonal antibody [MAb]), which is often associated with bone marrow reconstitution (transplantation); this approach has been used by Press et al.11 and, more recently, in pivotal Phase III trials of Bexxar™ (Corixa Corp., Seattle, WA; SmithKline Beecham Corp., Philadelphia, PA) and Zevalin™ (IDEC Pharmaceuticals, San Diego, CA). Potential advantages of a single large dose of radionuclide include less sublethal damage that can be repaired and avoidance of treatment interruption because of antiglobulin, e.g., human antimouse MAb (HAMA), development. A second approach to dose schedule is to divide the total dose of radionuclide into multiple-doses, referred to as “fractionated”, RIT.12–14 Preclinical and clinical evidence indicate that a larger total radionuclide dose can be administered when fractionated, regardless of whether the individual doses are low,13 near the nonmyeloablative maximum tolerated dose (MTD),14 or near the myeloablative MTD.15

The fractionation of RIT was first formally described in 1985,16 although multiple doses were given in earlier RIT trials. Although fractionation for conventional radiotherapy has come to have specific radiobiologic implications relative to tissues with acute (early) and delayed (late) response, the term is generally used herein in the broader dictionary sense of “dividing into fractions or parts”—that is, the use of multiple, rather than single, radionuclide doses. There is a strong rationale for fractionated RIT because of known tumor biology that leads to significant nonuniformity of the distribution of macromolecules in malignant tissue.17

An advantage of RIT over immunotherapy is that a radionuclide can be chosen with emissions that have a multicellular range, thereby distributing the radiation dose more uniformly in the malignant tissue. However, nonuniform distribution of radiation throughout the tumor persists, with some regions receiving higher and others lower radiation doses. When a single, large radionuclide dose of radiolabeled MAb is given, some regions of the malignancy may be underirradiated while other regions are overirradiated. The purposes of this publication are to provide rationales and data favoring fractionation for RIT (Table 1), to identify the disadvantages (Table 2), to assess its efficacy, to characterize preferred methods for implementation, and to further stimulate preclinical and clinical examination of this strategy.

Table 1. Advantages of Fractionated Radioimmunotherapy
1.More uniform distribution of MAb and radiation dose
2.Patient-specific radionuclide and radiation dose
3.Control toxicity by titration of an individual patient
4.Reduced toxicity
5.Increased MTD for many patients
6.Increased tumor radiation and efficacy
7.Prolongation of tumor response
Table 2. Disadvantages of Fractionated Radioimmunotherapy
1.Lower radiation dose rate
2.Complex strategy to implement
3.Treatment interruption
4.Increased cost
5.Potential delay in tumor regression

Heterogeneity of Macromolecules in Tumors

  1. Top of page
  2. Abstract
  3. Heterogeneity of Macromolecules in Tumors
  4. Preclinical Radioimmunotherapy Evidence
  5. Clinical Radioimmunotherapy Evidence
  6. Fractionated or Hyperfractionated Radiotherapy and Radioimmunotherapy
  7. Comparison of the Radiobiologic Aspects of Fractionation for External Beam Radiotherapy and Radioimmunotherapy
  8. Discussion
  9. Acknowledgements

RIT produces a less uniform radiation dose distribution within tumor than EBRT. The inability of MAbs and other macromolecules to penetrate uniformly throughout a tumor and bind to all cells results in a heterogeneous radiation dose deposition in the tumor.17–22 Tumor control is more difficult with a nonuniform dose distribution because some regions of the tumor may be underdosed.23–25

Terms such as “guided missile” and “magic bullet” are occasionally used to describe MAb targeting. A more realistic concept depends upon an understanding of the physiologic factors involved in tumor “targeting.” To achieve a greater MAb concentration in tumor than in normal tissues, administered MAb must first be distributed in a volume that includes the tumor cells. Then the MAb must react with and remain bound to antigen on tumor cells; unbound MAb must clear its distribution volume prior to significant dissociation of tumor-bound MAb.

The factors that affect blood flow and macromolecule delivery to tumors have been described in detail.17, 26–28 Dvorak et al.28 have described tumor architecture as consisting of tumor vessels, stroma, and parenchyma that serve as barriers to the delivery of macromolecules (Fig. 1). Vascular density and blood flow varies widely within different regions of a tumor29, 30 and affects the uniformity of macromolecule delivery.31–33 In addition, elevated interstitial fluid pressure inhibits the inward diffusion of MAbs from the periphery toward the center of the tumor.34–36 Measurements in experimental tumors have shown that the interstitial pressure is substantially lower in the periphery and the hydrostatic pressures in the vascular and interstitial spaces are nearly equal, which limits the convective delivery of macromolecules to solid tumors.36–38 Heterogenous tumor uptake may also occur due to the existence of a “binding site barrier” in which a high ratio between tumor antigen concentration and the concentration of MAb in the tumor milieu can lead to a MAb “sink,” which requires progressive saturation of antigen sites in parenchymal perivascular regions before MAb diffuses to deeper tumor regions.19, 20, 39–41 Other factors contributing to heterogeneous uptake of macromolecules include antigenic heterogeneity and modulation.

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Figure 1. Schematic representation of solid tumor structure. Solid tumors consist of parenchymal (tumor cell) units (A, B, and C) enveloped in stroma. Blood vessels and focal sites of vascular leakage from hyperpermeable blood vessels are concentrated at the tumor-host interface but also traverse tumor stroma. Parenchymal units may consist of (A) loosely packed tumor cells, typical of lymphomas, melanomas, and poorly differentiated carcinomas, or (B) tightly packed cells linked together by occlusive intercellular junctions and an enveloping basement membrane, typical of well-differentiated carcinomas. In (C), a well-differentiated carcinoma with occlusive junctions exhibits focal invasion at a site of basement membrane dissolution. From: Abrams PG, Fritzberg A, Editors. Radioimmunotherapy of Cancer. New York: Marcel Dekker, Inc., 2000:107–35. Reprinted with permission from Marcel Dekker, Inc.

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The result of these phenomena is that MAbs have highly nonuniform distributions. Despite radionuclides with radiations that traverse many cell diameters, nonuniform radiation dose to different regions of the tumor continues to be a problem for RIT. Griffith et al.42 performed quantitative autoradiography of iodine-131(131I)–Lym-1 MAb in Raji B-cell lymphoma xenografts to obtain a correlation of film density with dose determined by sectioned microthermoluminescent dosimeters implanted in tumors (Fig. 2). The measured absorbed dose heterogeneity varied up to 400%. Roberson and Buchsbaum23 investigated the tumor uptake of 131I-labeled 17-1A MAb in subcutaneous LS174T colon cancer xenografts as a function of time after injection. Three-dimensional (3-D) activity distributions were determined from serial section autoradiographs and used to construct a mathematic description of spatial and temporal changes in dose rate distributions. The characteristic pattern was high tumor surface deposition at early times after injection and a slow diffusion of the activity toward the center of the tumor at later times. The average 3-D dose rate distributions for 1 and 4 days after injection clearly illustrated the heterogeneity in dose rate throughout the tumor as a function of time after radiolabeled MAb injection (Fig. 3).

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Figure 2. An autoradiographic image of a section from a Raji human B-cell lymphoma tumor from a mouse given iodine-131(131I)–Lym-1 monoclonal antibody. The straight cursored line of interest passed directly over two micro-TLD providing an activity distribution, showing that radiation dose varied greatly in different tumor regions. Griffith MH, Yorke, ED, Wessels BW, DeNardo GL, Neacy WP. Direct dose confirmation of quantitative autoradiography with micro-TLD measurements for radioimmunotherapy. J Nucl Med 1988;29:1795–1809. Reprinted with permission from the Society of Nuclear Medicine, Inc.

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Figure 3. Radial dependence of dose rate for 300 μCi injection of iodine-131 (131I)–labeled 17-1A monoclonal antibody in LS174T xenografts. Plotted are histograms for each of 30 radial increments (tumor center = 0; tumor surface = 30; for a 10-mm diameter tumor, each radial increment represented 0.167 mm). Each histogram represents the number of cubic voxels experiencing dose rates within a radial interval (dose rate-volume histograms). Voxel dimensions were 0.2 or 0.25 mm on a side. Top and Bottom, 1 and 4 days after injection, respectively. Roberson PL, Buchsbaum DJ. Reconciliation of tumor dose response to external beam radiotherapy versus radioimmunotherapy with 131iodine-labeled antibody for a colon cancer model. Cancer Res 1995;55(suppl):5811–6. Reprinted with permission from the American Association for Cancer Research, Inc.

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Fractionation of the total amount of radionuclide into a series of smaller doses represents a strategy for achieving more uniform distribution of the radiation dose throughout the tumor. Subsequent doses of radiolabeled MAb can access regions different from those accessed earlier, if blood flow has been improved and reductions in tumor size have led to reductions in interstitial pressure and redistribution of blood flow.

Under certain conditions, fractionated administration of radiolabeled MAbs and peptides has been shown to be efficacious;43–47 multiple administrations have caused less toxicity than single administrations.45, 46, 48, 49 Other methods of reducing the effects of MAb heterogeneity include the use of radionuclides with longer ranges, e.g., yttrium-90 (90Y) or rhenium-188 (188Re);24 combined modality RIT with EBRT added; or other strategies.50, 51

Preclinical Radioimmunotherapy Evidence

  1. Top of page
  2. Abstract
  3. Heterogeneity of Macromolecules in Tumors
  4. Preclinical Radioimmunotherapy Evidence
  5. Clinical Radioimmunotherapy Evidence
  6. Fractionated or Hyperfractionated Radiotherapy and Radioimmunotherapy
  7. Comparison of the Radiobiologic Aspects of Fractionation for External Beam Radiotherapy and Radioimmunotherapy
  8. Discussion
  9. Acknowledgements

A number of preclinical studies demonstrate the advantages of dose fractionation for RIT. A fractionated RIT study of beagle dogs was conducted to assess toxicity.52 90Y-B72.3 was administered as a single dose (range, 1–2.5 mCi/kg) or 2 doses of 0.9 mCi/kg on Days 0 and 4 or 0 and 8 (total, 1.8 mCi/kg), or as 2 mCi/kg for the first dose and 0.9 mCi/kg for the second dose on Day 8 (total, 2.9 mCi/kg). Bone marrow was the dose-limiting tissue, while liver was the second dose-limiting tissue. A 20% decrease in bone marrow toxicity was observed with fractionation; an even greater decrease in liver toxicity accompanied fractionated RIT.

To determine whether fractionation of dose provided an advantage for RIT, MAb B72.3 immunoglobulin (Ig)G labeled with 131I was given to athymic mice bearing LS174T human colon adenocarcinoma xenografts.45 The LS174T xenograft, in which 30–60% of cells express the TAG-72 antigen, was used to reflect the heterogeneity of the TAG-72 antigen seen in tumors from patients. Otherwise lethal doses were fractionated to assess toxicity and the effect on tumor growth. In contrast to a single 600-μCi dose of 131I-B72.3 IgG, after which 60% of the mice died of toxic effects, two 300-μCi doses of 131I-B72.3 IgG (total, 600 μCi) reduced or eliminated tumor growth in 90% of mice, and only 10% of the mice died of toxic effects (Fig. 4). Furthermore, dose fractionation permitted escalation to three weekly doses of 300 μCi of 131I-B72.3 IgG (total, 900 μCi), resulting in even more extensive tumor reduction or elimination and minimal toxic effects. In a further investigation of acute toxic effects, mice receiving multiple doses were sacrificed at 2 weeks, the time of peak marrow toxicity,53 and at 7 weeks after MAb administration. In all cases, normal tissues showed no evidence of toxicity upon histologic examination. The bone marrow was normocellular with all three cell lines present in normal proportion.

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Figure 4. Percentage of mice demonstrating antitumor effect, defined as an increase of less than 50% in tumor volume when compared with growth of the untreated control mice, during the observation period (35–45 days) after one to three doses of iodine-131 (131I)–B72.3 at weekly intervals. Mortality is the percentage of mice that died during the observation period. Prepared from data in: Schlom J, Molinolo A, Simpson JF, Siler K, Roselli M, Hinkle G, et al. Advantage of dose fractionation in monoclonal antibody–targeted radioimmunotherapy. J Natl Cancer Inst 1990;82:763–71.

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A study by Buchsbaum et al.46 to investigate shorter fractionation intervals also demonstrated that fractionated RIT provided a therapeutic advantage with increased tumor cures and regression and decreased toxicity. For example, one dose of 600 μCi 131I-CC49 to LS174T tumor-bearing mice was lethal to 25% of mice, and no tumors disappeared (Fig. 5). When three doses of 300 Ci 131I-CC49 (total, 900 μCI) were given within 1 week, tumors disappeared in 40% of the mice, accompanied by 30% mortality. Moreover, three doses of 300 μCi 131I-CC49 reduced the tumor recurrence rate dramatically. The data also showed that a higher concentration of 125I-CC49 was maintained in the tumor for a longer period of time after fractionated treatments than after a single treatment with 131I-CC49. Fractionated dose and continuous infusion of 131I-CC49 (total radioactivity dose equal in both groups) were also compared in this model; mice that received three doses within 1 week had longer survival and tumor doubling times than the mice that received continuous infusion.54 Tumor radiation dose was higher and bone marrow dose lower in the groups that received multiple doses. A higher concentration of 125I-CC49 was maintained in the tumor periphery for a longer period of time following two treatments with 131I-CC49 at a 3-day interval than after a single dose of 131I-CC49, so that the radiation time and the total radiation dose were increased in the tumor. Using serial section autoradiography to reconstruct tumor activity distributions, Buchsbaum et al.54 and Roberson et al.55 compared the 3-D dosimetry of LS174T human colon carcinoma xenographs in athymic mice for a single dose, three dose fractions, and continuous infusion of 131I-CC49 over 7 days. Radiation dose rate nonuniformities were reduced by fractionated and continuous infusions; fractionated doses produced superior dosimetric results when compared with single dose or continuous infusion.

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Figure 5. Percentage of mice demonstrating an antitumor effect (complete response, defined as no visible or palpable tumor) after one to three doses of iodine-131 (131I)–CC49 given within a 1-week interval. Mortality is the percent of mice that died during the observation period (180 days). Prepared from data in: Buchsbaum D, Khazaeli MB, Liu T, Bright S, Richardson K, Jones M, et al. Fractionated radioimmunotherapy of human colon carcinoma xenografts with 131I-labeled monoclonal antibody CC49. Buchsbaum D, Khazaeli MB, Liu T, Bright S, Richardson K, Jones M, et al. Fractionated radioimmunotherapy of human colon carcinoma xenografts with 131I-labeled monoclonal antibody CC49. Cancer Res 1995;55:5881s–5887s.

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Beaumier et al.56 evaluated the use of 186Re-labeled NR-LU-10 MAb in mice bearing SHT-1 small cell lung cancer xenografts. When compared with a single dose of 430 μCi 186Re-NR-LU-10, fractionation into two to four doses (total, 492–603 μCi) within 7–10 days was associated with significantly delayed tumor growth and reduced toxicity, and actually allowed more radiolabeled MAb to be administered.

Studies by Blumenthal et al.,57, 58 who found that tumor vascularity was profoundly altered following radiolabeled MAb treatment, even when fractionated, are of some concern. Between 7 and 21 days after 150 μCi 131I-Mu-9, the number of blood vessels, vascular volume, blood flow, and vascular permeability in GW-39 human colon cancer xenografts in athymic mice were profoundly reduced, as was the tumor uptake of a second dose of radiolabeled MAb. The macroscopic radiation dose to the tumor was about 4200 centigrays (cGy), although that to the vessels was many times greater because of preferential perivascular localization of radiolabeled MAb. The threshold for decreased vascular permeability was a macroscopic tumor dose of 800 cGy, at which other alterations of tumor vascularity were not observed. Subsequently, Blumenthal et al.59 reported that vascular permeability, the most sensitive of the vascular parameters studied, was actually increased in 40% and unchanged in an additional 20% of 10 other tumor xenograft models at 14 days after a fixed 1500 cGy RIT dose to tumor. Doubling the tumor dose to 3000 cGy by doubling the radionuclide amount further produced mixed results in the vascular permeability measurements. Despite the care with which these investigations were conducted, their relevance to the clinical circumstance is uncertain because of the variability of the results in different tumor models, the high radiation doses to tumor vessels due to preferential perivascular localization, and the difficulties of delivering tumor doses of these magnitudes to patients.

Studies of RIT fractionation have generally shown beneficial effects in preclinical models. A higher total radionuclide dose could be delivered in several fractions than could be tolerated as a single dose, and tumor control was improved with fractionation. Tumor concentrations of radiolabeled MAb were preserved over multiple treatment doses, and there was direct and indirect evidence of more uniform radiation dose distribution in the tumor after multiple doses.

Clinical Radioimmunotherapy Evidence

  1. Top of page
  2. Abstract
  3. Heterogeneity of Macromolecules in Tumors
  4. Preclinical Radioimmunotherapy Evidence
  5. Clinical Radioimmunotherapy Evidence
  6. Fractionated or Hyperfractionated Radiotherapy and Radioimmunotherapy
  7. Comparison of the Radiobiologic Aspects of Fractionation for External Beam Radiotherapy and Radioimmunotherapy
  8. Discussion
  9. Acknowledgements

Although most RIT trials have involved the use of multiple radionuclide doses, few, thus far, have been intended to evaluate fractionation and only one, by Meredith et al.,60 has been designed to provide direct comparisons of single and multiple dosing. Several conclusions can be drawn from the multiple-dose trials that have used a fractionation strategy (Tables 1 and 2). First, fractionation is an effective method for titrating the radionuclide dose and the associated toxicity for an individual patient. In a sense, fractionation is a form of patient-specific radionuclide dosing, wherein the total radionuclide dose is determined by the accumulated amount of radionuclide that ultimately leads to Grade 3 or 4 toxicity in the patient. Second, fractionation provides an added level of safety when radionuclide doses are given at 2- to 8-week intervals, because radionuclide dose reduction and dose delay techniques can be used to ameliorate future toxicity in response to the observed acute toxicities from the earlier radionuclide dose(s). In RIT trials that have used 131I-Lym-1 for non-Hodgkin lymphoma (NHL) and chronic lymphocytic leukemia, these strategies have allowed patients with extensive marrow malignancy to be safely and successfully treated (Fig. 6). 13 Third, fractionation is difficult to execute in malignancies associated with immunocompetence unless humanized antibodies or immunosuppressant drugs are used.61 On the other hand, in a RIT trial designed to determine the MTD of a minimum of two and a maximum of four doses of 131I-Lym-1 mouse MAb for NHL, HAMA interrupted treatment for only 10% of the patients.14 Fourth, fractionated RIT can also be used in association with bone marrow reconstitution;15 greater lung radiation doses were tolerated by the patients given fractionated, high-dose RIT than were reported to be dose-limiting by Press et al. for single-dose RIT.11, 62 Finally, tumor targeting is clearly preserved over multiple therapeutic doses of radiolabeled MAb.

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Figure 6. A male age 45 years with non-Hodgkin lymphoma (NHL) unresponsive to five chemotherapy regimens had normal blood counts prior to a series of doses of iodine-131(131I)–Lym-1. Peripheral blood cells (left scale) are expressed as the percentage of pretreatment baseline (platelets, 307 k/mm3 (squares); granulocytes, 4.5 k/mm3 (circles); white blood cells (WBC), 6.7 k/mm3 (triangles); Hematocrit (HCT), 40.5% (diamonds)); bars (right scale) indicate accumulated 131I (total radioactivity dose 184 mCi/m2; 328 mCi). G1–G4 indicate hematologic toxicity grade based on conventional criteria for the nadir value after each treatment dose. Peripheral blood cells decreased dramatically after the first treatment dose, likely due to diffuse marrow NHL observed by marrow imaging and biopsy. The second and fourth treatment doses were dose-delayed and dose-reduced due to hematologic toxicity. By fractionating the 131I-Lym-1 dose, the patient's treatment could be titrated achieving substantial total doses of 131I and a durable partial remission. Marrow eligibility criteria (less than 25% marrow NHL) often used for NHL radioimmunotherapy would have precluded treatment or, if single-dose data from maximum tolerated dose trials were used, the patient would have incurred life-threatening hematologic toxicities.

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Thirty patients with Stage III or IV B-cell malignancies (25 NHL and 5 chronic lymphocytic leukemia [CLL]) who had progressed despite standard treatment entered a trial to assess 131I-Lym-1 toxicity and, secondarily, efficacy.13 Lym-1, a mouse MAb, binds an antigen that is expressed on malignant human B cells.63 At trial entry, bone marrow from 12 of 14 NHL patients showed substantial infiltration of the marrow by malignant cells on microscopic examination. An additional 7 of 11 NHL patients had historical evidence for extensive marrow malignancy, and all 5 patients with CLL had diffuse involvement of the bone marrow. Of the 30 patients, 15 had gradable hematologic abnormalities at trial entry, including 5 patients who had Grade 3 or 4 abnormalities. Patients were treated with doses of 30 or 60 mCi of 131I-Lym-1 at 2- to 6-week intervals; 11 of the 30 patients completed the intended 300 mCi. Treatment was interrupted by hematologic toxicity in 3 patients and the development of HAMA in 3 patients. There were no deaths due to toxicity, and no bleeding episodes or neutropenic sepsis. Tumor regression was great enough to qualify 57% as responders (13 NHL and 4 CLL patients). The responses from this low-dose, fractionated RIT were remarkable because the patients' hematologic status made them poor candidates. Otherwise untreatable patients were treated with fractionated RIT, achieving responses while morbidity was controlled.

Based on this strategy of fractionating the total 131I and radiation dose, a dose escalation trial was designed to define the MTD and efficacy of the first two of a maximum of four doses of 131I-Lym-1 given 4 weeks apart.14 131I was escalated from 40 to 100 mCi/m2 of body surface area. The nonmyeloablative MTD for each of two doses of 131I-Lym-1 given 4 weeks apart was 100 mCi/m2 in patients with not more than 25% marrow NHL.14 All three entries in this patient cohort had complete responses. Two of three patients in the 100 mCi/m2 cohort tolerated the study maximum of four treatment doses of 131I-Lym-1. Total 131I received by these three patients was 355, 626, and 810 mCi, respectively, contributing 121, 207, and 275 cGy and 103, 194 and 275 cGy to the body and marrow (from the blood and body), respectively. Despite total radionuclide doses at the MTD level approximating those reported to be dose-limiting in NHL for single-dose RIT with bone marrow reconstitution, fractionated RIT without bone marrow reconstitution was well tolerated; there were no instances of significant bleeding or neutropenic sepsis.

A noteworthy trial of fractionated RIT was implemented by Divgi et al.,64 in which 131I-chimeric G250 MAb was given to patients with metastatic renal cell carcinoma. As a strategy to avoid hospitalizing patients, an initial dose of 30 mCi was given; then, at intervals of several days, the patient was given additional doses, dependent upon measurements of the body content of 131I, to bring the body content back to 30 mCi. One might describe the approach as “topping off” the body content of 131I. Although the trial had not been completed at the time this article was written, tumor targeting was excellent for all doses up to that time.

A study to determine the potential of fractionated RIT for patients with recurrent Hodgkin disease was conducted by Vriesendorp et al.65, 66 Ninety patients received 90Y-labeled polyclonal rabbit antihuman ferritin IgG, 57 patients received a single dose (0.3–0.5 mCi/kg), and 33 patients received 0.25 mCi/kg doses on Days 0 and 7. HAMA occurred in about 5% of the patients. In this study, fractionation did not provide the expected decrease in hematologic toxicity or increase in tumor response.

Meredith et al.60 reported 12 patients with metastatic colon cancer who were treated with 131I–chimeric B72.3 at total doses of 28 or 36 mCi/m2. The MTD for a single dose was 36 mCi/m2 with marrow suppression as the dose-limiting toxicity. The degree of bone marrow suppression in response to a total dose of 36 mCi/m2 was significantly less, when fractionated in two or three weekly fractions, than that seen with the same amount given as a single dose (Fig. 7). To our knowledge, this study represents the only controlled trial of radiolabeled MAb fractionation in humans.

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Figure 7. Fractionation versus nadir white blood cells (WBC) and platelet grade. Comparison of mean toxicity score (sum of WBC and platelet grade) for groups of patients treated with total doses of 28 mCi/m2 or 36 mCi/m2 iodine-131 (131I)–ch B72.3 given as one, two, or three weekly fractions. From: Meredith RF, Khazaeli MB, Liu T, Plott G, Wheeler RH, Russell C, et al. Dose fractionation of radiolabeled antibodies in patients with metastatic colon cancer. J Nucl Med 1992;33:1648–53. Reprinted with permission from the Society of Nuclear Medicine, Inc.

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Fractionated or Hyperfractionated Radiotherapy and Radioimmunotherapy

  1. Top of page
  2. Abstract
  3. Heterogeneity of Macromolecules in Tumors
  4. Preclinical Radioimmunotherapy Evidence
  5. Clinical Radioimmunotherapy Evidence
  6. Fractionated or Hyperfractionated Radiotherapy and Radioimmunotherapy
  7. Comparison of the Radiobiologic Aspects of Fractionation for External Beam Radiotherapy and Radioimmunotherapy
  8. Discussion
  9. Acknowledgements

Most modern radiation treatment is given in a fractionated manner in order to increase antitumor efficacy and spare normal tissues. Historically, the advantages of fractionation, due to differing effects between tissues, were first noted when it was not possible to sterilize a ram with a single dose of radiation to the testes without causing skin breakdown; sterilization could be accomplished with skin tolerance by fractionation of the radiation.1 Since then, it has been determined that dose per fraction, time between fractions, dose rate, total dose, and overall treatment time influence normal tissue and tumor effects.2 To preserve normal tissues, the larger the dose per fraction, the fewer fractions can be given and the lower the total dose for equivalent risk of complications.

Although there is a spectrum of rates at which normal tissues respond to radiation, responses are roughly divided into acute and late responses.67 This “approximate division” is based on the proliferation kinetics of the cells, with tissues with acute response undergoing rapid renewal versus late-responding tissues that have infrequent cell division and turnover. Tolerance-limiting early-responding cells include basal cells of the skin, crypt cells of the intestines, and hematopoeitic stem cells. Examples of tolerance-limiting late-responding cells include renal tubular cells, fibroblasts, and smooth muscle cells in arterial walls. Rapidly growing tumors generally respond in a manner similar to early-responding normal tissues.

There are radiobiologic mechanisms that explain differences between early- and late-responding normal tissues and between tumors and normal tissues.67 These factors include repair of sublethal or potentially lethal damage, redistribution in the cell cycle, and regeneration from surviving stem cells. For tumors, reoxygenation of hypoxic areas between fractions is important.

Several mechanisms favor fractionation for differential tumor and normal tissue effects, including the decreased ability of tumor cells to repair sublethal and potentially lethal damage between fractions that are closely timed. There is a wealth of information, accumulated over several decades, that has allowed extensive analysis of tumor control and normal tissue effects resulting from EBRT dose fractionation schemes. Analysis of clinical, animal model, and in vitro studies of radiation effects allows application of tables and equations for prediction of equivalence between various schemes for early and late effects.68

Standard fractionation for EBRT is usually considered as 1.8 or 2 Gy per fraction, one fraction per day, 5 days per week. For simplicity of further discussion and examples, 2-Gy fractions will be considered the standard, unless specifically stated otherwise.

Hyperfractionation, such as giving EBRT at least twice per day, decreases the radiation dose per fraction and the time interval between fractions. Because hyperfractionation decreases late effects at the expense of increasing acute effects, higher total doses can be delivered with the same risk of late complications. A general rule of thumb applicable to most tissues is that 1.2 Gy twice daily can be given to a total dose of about 115% of that for single daily 2-Gy fractions with the same risk of late complications. In taking advantage of the differences between fast-growing tumors and decreased late effects by hyperfractionation, the higher total tumor doses usually result in improved tumor control rates. Accelerated fractionation may use conventional doses per fraction, but it spaces fractions more closely together, such that overall treatment time is reduced to deliver the same total dose. This scheme can also increase tumor control, but normal tissue late effects are increased. Hypofractionation uses larger doses per fraction, often spaced further apart and to a smaller total dose than standard fractionation.

Head and neck cancer is a disease for which fractionation has been extensively studied and principles applied to clinical trials. For these fast-growing, relatively radiosensitive tumors, fractionation increases tumor control rates. Results from the national cooperative Radiation Therapy Oncology Group 9003 trial for advanced head and neck cancer confirmed the outcomes for various fractionation regimens, which were predicted from prior analyses.3 In this trial, hyperfractionated and accelerated fractionation schemes offered a control advantage over standard fraction or split course schemes, while late effects were similar. As predicted, the results showed the influence of treatment duration on tumor control. Due to tumor regrowth, higher doses were needed for equivalent control if delivered over longer intervals.3–5 After adjustment for the dose per fraction (using an alpha/beta ratio of 15 Gy), 7 days' shortening of the treatment duration for hyperfractionation or accelerated fractionation increased tumor control rates for advanced head and neck cancers treated with various altered fractionation schemes.4

Some RIT treatments have been given at a near-tolerance dose with repetition of that dose after a period of normal dose-limiting tissue recovery, rather than a true fractionation of the MTD. This scheme is similar to that frequently used for chemotherapy that can be cycled every 3–4 weeks due to bone marrow suppression and recovery kinetics. For fast-growing tumors, this regimen may be suboptimal because it allows surviving tumor cell proliferation to occur between treatments, whereas with true fractionation, a short time interval prohibits tumor cells from proliferating while allowing recovery of normal tissues. For example, regrowth of malignancy has been reported in a lymphoma patient treated with RIT; initial regression was followed by regrowth of some lymph nodes within 3 weeks after treatment, before the marrow had recovered sufficiently to allow the next treatment.44 This experience illustrates that the time between fractions is limited by normal tissue recovery, in the absence of marrow support, even when the time between treatments is suboptimal for tumor control. For this patient's lymphoma, which was sensitive to treatment but had rapid growth kinetics, smaller doses given more frequently may have provided better control. This suggestion is supported by results reported by DeNardo, et al.,13 after using dose fractionation schedules for 131I-Lym-1, and Shen et al., after modeling of marrow recovery for timing between fractions.69

RIT is inherently hyperfractionated because it delivers continuously decreasing, low-dose-rate radiation as the radionuclide decays (often over days). Furthermore, fractionation regimens that have a greater chance of tumor control than single dosing have been achieved. Some theoretic advantages of RIT fractionation, in addition to those already discussed, include decreasing the effects of heterogeneity, as discussed in detail by O'Donoghue et al.70 Parameters for applying fractionation principles to RIT have been analyzed.71, 72 In addition to factors discussed above that apply to EBRT, the relatively low-dose-rate effects of RIT have also been considered. To compensate for anticipated decreased effectiveness of the low dose rate of RIT, Fowler72 suggested that a total dose increase of 20% would be required to achieve antitumor efficacy comparable to that of EBRT. However, this adjusted dose increase cannot be applied as an absolute rule, since the results of comparing the efficacy of RIT with that of EBRT have been variable; some have reported that RIT was more efficacious than high-dose-rate radiation.8–10 The variety of responses may be indicative of different underlying mechanisms that affect radiation sensitivity, as summarized by Knox et al.8 In their analysis, they found that the size of the survival curve shoulder [alpha/beta ratio] and the tumor doubling time were important determinants of the magnitude of dose rate effects in a given tumor type. Study of underlying mechanisms for some tumors revealed a correlation between tumor sensitivity to low-dose-rate radiation and G2/M block.73 Moulder et al.74 found a therapeutic gain for fractionated low-dose-rate when compared with conventional-dose-rate fractionated radiotherapy for gastrointestinal and renal damage. Guidelines for the optimal fractionation of RIT have not been determined, since many of the dose rate experiments have not fractionated the low-dose-rate radiation or taken into account other factors that may be important for normal tissue toxicity as well as tumor control.

Comparison of the Radiobiologic Aspects of Fractionation for External Beam Radiotherapy and Radioimmunotherapy

  1. Top of page
  2. Abstract
  3. Heterogeneity of Macromolecules in Tumors
  4. Preclinical Radioimmunotherapy Evidence
  5. Clinical Radioimmunotherapy Evidence
  6. Fractionated or Hyperfractionated Radiotherapy and Radioimmunotherapy
  7. Comparison of the Radiobiologic Aspects of Fractionation for External Beam Radiotherapy and Radioimmunotherapy
  8. Discussion
  9. Acknowledgements

From the earliest days of radiotherapy, it has been recognized that radiation effects on biologic systems are highly dependent on the temporal pattern of exposure. In any particular tissue, a series of small high-dose-rate fractions enables a larger total dose to be tolerated than would be possible using a single, high-dose-rate exposure. Conversely, a radiation dose delivered as a single acute exposure will be more damaging to normal tissues than the same total dose divided into smaller fractions. This indicates that the relationship between dose and response is nonlinear for high dose rates. By itself, however, it does not constitute a rationale for fractionation. The other key factor is the dissociation of response in different tissues when the pattern of radiation exposure changes. The relative sensitivity of tissues to radiation is not invariant but may be altered by manipulating the treatment structure. This is the major rationale for fractionation of EBRT. For EBRT, the dose limitation is generally imposed by delayed reactions in late-responding normal tissues, often associated with the development of fibrosis.

Mechanistically, the major radiobiologic advantage associated with fractionation of EBRT is the differential increase in repair of radiation damage in late-responding normal tissues compared with tumors. This appears to be a consequence of differences in the shapes of the underlying dose-response relationships and can be understood in terms of the linear-quadratic (LQ) model. First advanced as a model for normal tissue and tumor responses to radiation in the early 1980s,75, 76 the LQ model has been of great value in the analysis of clinical data and in the design of EBRT fractionation schemes. It is based on the assumption that cell survival and, more generally, dose-response relations for high-dose-rate exposures can be decomposed into linear and quadratic components.

The clonogenic survival of mammalian cells in culture following single doses of radiation can generally be described by the equation

  • equation image(1)

where Fs is the surviving fraction, d is the dose delivered, and α and β are parameters that describe the shape of the survival curve. One possible biophysical interpretation of these parameters is that α equals the rate of cell kill by a single-hit mechanism and β equals the rate of cell kill caused by the interaction of two sublethal hits. However, this is probably an oversimplification.77

Assuming that each fraction has an identical effect, the surviving fraction after n fractions of size d is

  • equation image(2)

where D = nd is the total dose.

If the quantity E = -ln(Fs) is used as a metric for biologic effect, we can write

  • equation image(3)

In this notation, the quantity (1 + d/(α/β)) represents the relative effectiveness (RE) of the total dose, D, when delivered at a high dose rate in fractions of size d.68, 75

  • equation image(4)

The quantity D(1+d/(α/β)) has units of dose and may be thought of as the dose required to produce the biologic effect, E, if given as a very large number of very small fractions. This quantity has been called the “biologically effective dose,” or BED.72

  • equation image(5)

For computational purposes, there is a linear relationship between BED and biologic response.

  • equation image(6)

This formalism is not the only one possible for the LQ model, but it has the advantage that for any treatment schedule the BED is given by

  • equation image(7)

By making some assumptions about the nature and kinetics of repair of radiation damage, it is possible to calculate the RE for radiation delivered over protracted times. In particular, this type of analysis may be extended to treatment with biologically targeted radionuclides.

Although numeric values of α and β may be derived from the analysis of survival data for tumor cells grown in culture, it is usually not possible to assign meaningful values to these parameters in the context of a normal tissue dose-response relationship. However, it is possible to estimate the ratio α/β. The α/β ratio has emerged as an important concept in clinical radiobiology because it characterizes the degree of nonlinearity associated with the dose-response relationship. Normal tissues fall into two broad categories: acutely responding and late-responding. Acute responses develop over days to weeks; examples include radiation damage in various epithelia and the hematopoietic system. These tissue responses are generally associated with high values of (≥ 10 Gy), indicating that the dose-response relationship is relatively insensitive to changes in fraction size. Late effects, developing typically over months to years, include radiation damage to the liver, central nervous system, and fibrotic changes in endothelial tissue. Generally, the severity of late effects is not predictable on the basis of acute effects. Late responses are associated with low values of the α/β ratio (∽3 Gy), indicating a pronounced dependency on fraction size. Data from experimental tumors and from the analysis of clinical dose-response relationships indicate that tumors usually behave like acutely responding normal tissues.

It is well known that prolonging, or inserting time gaps into, conventional fractionated radiotherapy reduces tumor control rates unless total doses are increased.78–83 This is usually attributed to allowing more time for tumor cell proliferation.84–86 In terms of the LQ model, the simplest assumption is that cellular proliferation is exponential with a growth rate, λ. The biologic effect can then be written as

  • equation image(8)

where E(t) is now explicitly dependent on time, t.

This represents the effect of the radiation exposure modified by concurrent cellular proliferation. Although an oversimplification, this model may be applicable to systems, such as tumors and acutely responding normal tissues, where rapid cellular proliferation is an important factor, but not late-responding tissues, where proliferation over the course of treatment is unlikely. The above equation indicates that the biologic effect produced in a proliferating cell population is determined primarily by the total dose, modified by the radiobiologic effectiveness of that dose and the time over which it is delivered.

Some of the radiobiologic factors that are applicable to fractionated EBRT may also be applicable to RIT. However, there are fundamental differences in the patterns of radiation and toxicity between the two modalities. Radionuclide therapy is customarily a systemic rather than a local treatment modality. The dose-limiting toxicity is usually due to the hematopoietic system, unless bone marrow reconstitution is also used. The hematopoietic system is an acutely responding system with a limited tolerance for radiation. In radionuclide therapy, organs are unlikely to be irradiated in relative isolation as occurs in EBRT, and it is also unlikely that one segment of an organ will experience a very high dose while another segment has a dose of zero. Radiation dose rates have lower orders of magnitude in radionuclide therapy than in EBRT. Another significant difference is the much greater level of microscopic nonuniformity of dose for radionuclide therapy. These contrasts mean that we have to be selective in how the lessons derived from fractionated EBRT are applied to RIT.

The major radiobiologic advantage of fractionation in EBRT is the differential increase in repair between late-responding normal tissues and tumors caused by delivering radiation in smaller individual doses. Does this rationale apply to radionuclide therapy?

For radiation delivered at a constant dose rate, r, over a period of time, T, Dale68 calculated that the equation for the relative effectiveness (RE) is

  • equation image(9)

where μ is a time constant that characterizes the rate of repair of radiation damage that is presumed to be a monoexponential process.

For radiation by an exponentially decaying dose rate with an effective decay constant, λ, where the radiation time is long enough that the dose rate decays all the way to zero, the analogous equation for RE is

  • equation image(10)

where r0 is the initial dose rate.68

Other formulae for RE have been provided when the pattern of dose rate has been more complex. However, a number of observations that remain valid for the more complex versions may be made using the simpler formula (Equation 10).

There is usually a significant difference between the rates of repair and decay. Values derived for the repair half-time for mammalian cells in culture and normal tissues in patients fall within a range of minutes to hours.10, 87 In contrast, most clinical applications of radionuclide therapy deliver radiation dose with an effective halftime of several days. In terms of Equation 10, this means μ λ. For a repair halftime of 1.5 hours, a reasonable approximation, μ is about 0.5 hr−1. Comparing Equation 10 with the corresponding Equation 4 for the RE of fractionated EBRT, it can be seen that r0/μ plays an analogous role to that of the fraction size, d. The sparing effect of treatment with a decaying dose rate will be equivalent to that of conventional fractionated EBRT if r0/μ = 2 Gy or r0 = 2 μGy/hr. For μ = 0.5 hr−1, this corresponds to an initial dose rate of 1 Gy/hr. If the initial dose rate produced by radionuclide therapy is less than this, the treatment is already more sparing than conventional fractionated EBRT, suggesting that there is little to be gained in terms of differential repair by delivering radionuclide therapy in a fractionated manner. Contrast this with the sort of dose rates achievable in patients with targeted radionuclide therapy. To provide an approximate value, we note that tumor concentrations of radionuclide are typically 0.01–0.02% injected dose per gram. For an administered activity of 100 mCi of 131I, the activity per gram of tumor assuming instantaneous uptake is 10–20 μCi/g. The absorbed dose rate produced by this concentration is 4–8 cGy/hr, assuming electronic equilibrium and ignoring photon radiation. This is significantly less than the previous calculation of the 2-Gy equivalent dose rate. The high-dose-rate fraction size that is theoretically equivalent to exponentially decaying dose rates of initial value r0 is given by

  • equation image(11)

For the initial dose rates quoted above, equivalent fraction sizes are generally less than 0.5 Gy for reasonable values of μ. Moreover, the dose rates in normal tissues must be significantly less than the values for tumors if targeted radionuclide therapy is a rational approach. These considerations suggest that fractionation of RIT will provide little further sparing due to repair of radiation damage.

Another element of the sparing effect of fractionated treatments is due to proliferative regeneration in rapidly dividing cell populations. We now consider how this principle relates to the concept of RIT fractionation. It is apparent that moving from a single large dose of radiolabeled MAb to a series of smaller doses, each separated by a matter of some days, will result in a lower average dose rate delivered over a longer time period. For similar total doses, uniform dose distributions, and cell populations with time-independent radiosensitivity, this pattern of radiation is expected to result in reduced biologic effectiveness.88 However, as fractionation also spares the dose-limiting hematopoietic system, it enables an increase in the total administered activity.45, 46, 89 The relative proliferation rates of the targeted tumor and the hematopoietic progenitor cells then become key factors. Fractionation would be advantageous if there was faster proliferative regeneration in the hematopoietic system than in the tumor. This may be applicable to a slowly growing tumor cell population, but, given the importance of treatment duration in EBRT, it is unlikely to provide a significant advantage for many tumors.

After radiation, the progression of dividing cells through the cell cycle is delayed. The delay depends on both dose and dose rate, occurs only at specific points in the cell cycle, and is similar for both surviving and nonsurviving cells. The net result is that many cells accumulate in the G2/M and G1/S boundaries and alter the mitotic index. The length of the delay and the decrease in mitotic index are both functions of dose.90 It is well known that the radiosensitivity of cells is a function of their position within the cell cycle, cells in late S phase being the most resistant and those in G2/M phase being the most radiosensitive. Cells that are in the most sensitive phase, when radiation occurs, will be preferentially killed. For acute radiation exposures, this produces a partial cell synchrony and a change in the overall sensitivity of the population. In the case of fractionated EBRT, it is believed that this synchrony is rapidly lost because of natural variation in the rates at which cells pass through the cycle, in a process called redistribution. For continuous radiation exposures, it is possible that certain dose rates enable limited cell cycle progression but produce a check at the radiosensitive G2/M boundary. This may be the mechanism underlying the inverse dose rate effect observed in vitro for low-dose-rate radiation, although this is not absolutely established.91 Radiosensitization due to induced cell-cycle blocks may be relevant for combined EBRT and low-dose-rate radiation92 and, in principle, may render RIT more effective than otherwise expected. However, it is not clear what additional advantage this mechanism confers on fractionated RIT.

The oxygenation status of cells is a major determinant of their radiosensitivity. The oxygen effect is greatest for sparsely ionizing radiation (e.g., beta particles) and is absent for densely ionizing radiation (e.g., alpha particles). Fractionated EBRT is thought to allow previously hypoxic tumor regions to reoxygenate. Reoxygenation may also be of importance in the context of RIT fractionation. The effectiveness of single-dose RIT may be limited by hypoxic subpopulations of tumor cells, whereas multiple doses of RIT may enable tumor sensitization by reoxygenation processes. It would be very interesting to perform an experiment comparing fractionated and single-dose alpha-emitter (e.g., bismuth-213) RIT. As the oxygen effect is absent from densely ionizing radiation, any therapeutic advantage seen with fractionated treatments must be due to other mechanisms.

One other mechanism that is highly significant for RIT is the nonuniformity of radiation dose within tumors. Nonuniformity of tumor uptake of MAb is an almost invariable finding when tumors are biopsied following RIT,93, 94 even in cases where the target antigen is homogeneously expressed. This is a major radiobiologic difference between RIT and EBRT. Mathematical modeling studies suggest that nonuniform dose distributions become proportionately less effective as the mean dose increases.24 “Dose escalation” may not lead to a significant increase in tumor responses. In addition, the negative impact of dosimetric nonuniformity is expected to be most severe for radiosensitive tumors. In this context, fractionated RIT may confer a therapeutic benefit because the loss of effectiveness due to nonuniformity is minimized. The degree to which the pattern of radionuclide uptake changes from fraction to fraction is a critical factor in the comparative effectiveness of fractionated versus single-dose RIT. Fractionation should have a therapeutic advantage if different doses target different subpopulations of tumor cells.This could come about through time-dependent changes in tumor capillary blood flow or modifications to tumor architecture caused by the effects of preceding doses. Experimental data with trace-labeled MAb indicate no change in the distribution for two successive doses.95 However, experimentation with therapeutic levels of radionuclide has clearly shown that fractionated RIT produces superior dosimetric results and tumor activity distributions.54, 55 The results of this experiment are of critical importance to the question of the therapeutic advantage of fractionated RIT at a radiobiologic level.


  1. Top of page
  2. Abstract
  3. Heterogeneity of Macromolecules in Tumors
  4. Preclinical Radioimmunotherapy Evidence
  5. Clinical Radioimmunotherapy Evidence
  6. Fractionated or Hyperfractionated Radiotherapy and Radioimmunotherapy
  7. Comparison of the Radiobiologic Aspects of Fractionation for External Beam Radiotherapy and Radioimmunotherapy
  8. Discussion
  9. Acknowledgements

Because MAbs are macromolecules, they have difficulty penetrating the tumor.96, 97 The mechanisms underlying poor penetration and heterogenous distribution of MAbs in tumors have been elegantly examined by Jain.98, 99 Nonuniform and inadequate blood flow, elevated interstitial pressure, necrotic regions, and absent antigenic targets on some cells contribute to heterogenous distribution of MAbs. Radionuclides with radiation that traverses many cell diameters distribute the radiation dose more uniformly. Despite the use of these radionuclides, nonuniform dose to different regions of the tumor remains an obstacle for RIT.9, 100

Press et al.62 have demonstrated that a single, large dose of 131I-labeled anti-CD20 MAb was remarkably effective treatment for NHL when autologous marrow reconstitution was used. However, this may not be the best approach, because normal tissues and some regions of the tumor are overradiated to assure that all regions of the tumor are adequately radiated. In early trials, the strategy of fractionating RIT into a series of smaller radionuclide doses was used. Fractionation of RIT provided an opportunity to explore toxicity at a time when the toxicity was largely unknown.

Despite its logistic disadvantages, there are other reasons for, and advantages to, fractionating RIT. A major purpose for fractionated RIT is its use as a strategy for overcoming underradiation of tumor regions. In the presence of nonuniform radiation dose, an increase in dose from an increase in radionuclide amount given as a single dose becomes progressively less effective. This is also true for multiple dosing unless the MAb, and therefore the radiation dose, distribution is different from dose to dose. Subsequently administered doses of radiolabeled MAb can access regions different from those accessed earlier, because blood flow has improved and reductions in tumor size have led to reductions in interstitial pressure. There is considerable evidence—namely, better efficacy—to support this thesis, but no one has yet developed a study design to fully document this premise. Preclinical studies provide powerful evidence of better radiation dose distribution from fractionation, reflected in increased therapeutic response to equivalent radionuclide doses and to higher but equitoxic radionuclide doses when the doses are fractionated. Furthermore, larger amounts of radionuclide can be given in multiple doses than as a single dose, with equal or reduced toxicity. Although bone marrow stem cells are generally felt to be less influenced by fractionation than are other normal tissues, preclinical studies of fractionated RIT indicate that larger doses of radiation can be administered with less marrow suppression than with single large doses. Clinical studies have confirmed these preclinical observations60 and have shown other advantages to fractionated RIT, including the opportunity to apply a patient-specific approach to radionuclide and radiation dosing and an increase in the duration of tumor responses. In addition to the well-documented benefits of fractionated EBRT, there is evidence of a need to fractionate radionuclide therapy. Experience over 50 years indicates that multiple doses of 131I-iodide are often required to eliminate differentiated thyroid cancer. Administration of multiple doses of 131I-iodide for thyroid cancer is based on observations that not all thyroid metastases compete equally for an individual dose of 131I-iodide. A subsequent dose of 131I-iodide often reveals metastases that were not apparent on images obtained after the earlier dose of 131I-iodide. Furthermore, other targeted radionuclide therapies involve the administration of multiple doses. This strategy has been used for radiolabeled peptides101 and, more recently, for radionuclide therapy for bone pain palliation.102

An important consideration in the use of dose fractionation of RIT is the stage of disease. Most trials to date have involved patients with late-stage disease, placing dose fractionation at a disadvantage. Patients with advanced cancer have usually received several prior chemotherapeutic regimens. These have most likely damaged the bone marrow and, thus, the MTD was most probably less. In patients with less advanced disease, longer time intervals that permit greater normal tissue recovery can be employed between doses of radiolabeled MAb; moreover, more dose fractions can be given.

In addition, the ability to employ dose fractionation of RIT could be facilitated through investigations of the following: 1) the immunogenicity of the MAb or Ig form used, and 2) the size of the Ig form. The majority of clinical trials with radiolabeled MAbs have employed whole mouse IgG. This is perhaps the worst form of Ig to use for dose fractionation because of the immunogenicity of the mouse constant regions. There are two major ways to reduce immunogenicity: altering the amino acid sequences of the Ig to make them less immunogenic, and altering the size of the Ig. Initial modification of mouse MAbs involved construction of chimeric MAbs. These molecules consist of mouse variable regions, i.e., complementarity determining regions (CDRs) and human constant regions (Fc). The next generation of molecules in which immunogenicity was decreased were the so-called “humanized” or CDR-grafted MAbs. In these molecules, the six CDRs (three of the heavy chain and three of the light chain) of the mouse MAb were grafted onto a human MAb so that the only mouse sequences were in the binding sites. It has since become apparent that even these CDR-grafted MAbs may elicit binding site host immune responses.103 Recent studies have shown that it is possible to identify specificity determining residues (SDRs) for a given MAb.104 These SDRs are, of course, in the hypervariable region of the MAb and are the most critical for antigen-MAb interactions. Indeed, studies have shown that all six CDRs are not critical to MAb binding to an antigen, and in some cases only three to five CDRs are essential. It is now possible to generate variants with minimal potential immunogenicity that still maintain antigen-binding capabilities, by modification at the single amino acid level within CDR regions. These constructs should be most useful in dose fractionation strategies involving multiple fractions over a long period of time.

Another approach to the efficient use of dose fractionation may well be the use of smaller Ig forms. In addition to less immunogenicity, these forms penetrate tumors more efficiently. Studies of the tumor penetration of different Ig forms, including whole IgG, F(ab′)2, Fab′ fragments, and single-chain sFv, have been conducted, using a human colon cancer xenograft in athymic mice.105 After systemic administration, quantitative autoradiographic analyses revealed that the whole IgG delivered to the tumor was concentrated in the region of, or immediately adjacent to, vessels, whereas the smaller sFv was more evenly distributed throughout the tumor. The distributions of the Fab′ and F(ab′)2 fragments showed intermediate penetration in a size-related manner. These findings have clear implications for dose fractionation. For larger tumors and IgG forms, one can use the analogy of peeling the skin of an onion with each dose fraction.

Fractionation of RIT has precedence; EBRT is routinely fractionated into 20–40 doses in an effort to ameliorate toxicity.106 Classically, fractionation of EBRT has been a means of increasing therapeutic gain by relatively sparing normal tissues compared with adjacent tumor. A rationale for fractionated RIT is based on irrefutable evidence for EBRT that the radiation dose to the tumor and the dose tolerated by normal tissues can be increased. Another advantage of fractionating RIT into multiple doses is better distribution of the microscopic radiation dose because of reduced heterogeneity of MAb targeting over several doses. It is important to appreciate the differences between RIT and EBRT. Whereas the latter represents high dose and high-dose-rate radiation, the former represents low dose and low-dose-rate radiation that seems to induce apoptosis, rather than reproductive cell death, as its primary mechanism of cytotoxicity. Tumor cells generally have greater propensity than normal cells for apoptosis, perhaps explaining the sometimes remarkable efficacy of RIT, particularly in lymphomas, known to have high levels of inherent apoptosis. Meyn107 demonstrated that low doses of radiation induced substantial apoptosis and that the dose response actually leveled off at doses higher than 7.5 Gy, suggesting that only a subset of cells in the tumor have the propensity for radiation-induced apoptosis at any discrete time. Meyn also showed that multiple smaller fractions of radiation produced a higher total of apoptotic cells than larger doses of radiation, suggesting that an apoptotic subpopulation of cells reemerged between doses in the fractionated protocols. He envisioned strategies (such as fractionated RIT) that capitalized on restoration of apoptotic propensity to radioresistant tumor cells for therapeutic benefit. There is empiric evidence from studies of mice45, 46, 108 and patients12–15 that fractionation is effective for RIT. Using a colon tumor xenograft model that mimics the heterogeneity of antigen and MAb distribution, Schlom et al.45 have shown the benefits of fractionated RIT in mice. Fractionation into three doses permitted dose escalation by 50% and greater therapeutic benefit; similar results have been reported by others.46, 108 Trials involving patients have also provided evidence that the fractionated dose strategy is effective, increasing the tolerated radiation dose.12–15, 60

Issues that need to be addressed if fractionated RIT is to be optimized include 1) the number of radionuclide doses; 2) the radionuclide dose amount, e.g., the use of multiple doses to achieve an MTD, or the use of multiple doses at the MTD dose level with adequate recovery interval between doses; 3) the interval between radionuclide doses; and 4) the optimal radionuclide physical half-time for a specific treatment interval. Finally, the dosing method is significant; in theory, it would seem that a radionuclide dose for each treatment based upon dose-limiting organ radiation dose would be preferable to an empirically determined radionuclide dose.

In summary, fractionation of RIT has been limited by the immunogenicity of MAbs, most of which have been of mouse origin thus far. Technologic developments and the increased availability of humanized MAbs or fragments of decreased immunogenicity now make fractionated RIT even more attractive. Trials involving humans should be designed with the goal of determining a fractionation interval for a nonimmunogenic, genetically engineered, or human MAb that reduces bone marrow toxicity while enhancing tumor uptake and distribution of each radiolabeled MAb dose. Improved efficacy is predicted for future RIT-treated malignancies because of the advantages of dose fractionation.


  1. Top of page
  2. Abstract
  3. Heterogeneity of Macromolecules in Tumors
  4. Preclinical Radioimmunotherapy Evidence
  5. Clinical Radioimmunotherapy Evidence
  6. Fractionated or Hyperfractionated Radiotherapy and Radioimmunotherapy
  7. Comparison of the Radiobiologic Aspects of Fractionation for External Beam Radiotherapy and Radioimmunotherapy
  8. Discussion
  9. Acknowledgements
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