• Open Access

Sequential Low-Dose Rate Half-Body Irradiation and Chemotherapy for the Treatment of Canine Multicentric Lymphoma

Authors

  • D.M. Lurie,

    1. Veterinary Medical Teaching Hospital, University of California, Davis, CA
    2. Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL
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  • I.K. Gordon,

    1. the Department of Surgical and Radiological Sciences, University of California, Davis, CA
    2. Radiation Oncology Branch, National Cancer Institute, National Institutes of Health, Bethesda, MD
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  • A.P. Théon,

    1. the Department of Surgical and Radiological Sciences, University of California, Davis, CA
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  • C.O. Rodriguez,

    1. the Department of Surgical and Radiological Sciences, University of California, Davis, CA
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  • S.E. Suter,

    1. Veterinary Medical Teaching Hospital, University of California, Davis, CA
    2. Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC
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  • M.S. Kent

    1. the Department of Surgical and Radiological Sciences, University of California, Davis, CA
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  • The work was done at the University of California, Davis, CA. This work has not been presented at a meeting.

Corresponding author: Dr David Lurie, Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Room V-160, 2015 SW, 16th Avenue, Gainesville, FL 32610; e-mail: luried@vetmed.ufl.edu.

Abstract

Background: Sequential half-body irradiation (HBI) combined with chemotherapy is feasible in treating canine lymphoma, but prolonged interradiation intervals may affect efficacy. A 2-week interradiation interval is possible in most dogs receiving low-dose rate irradiation (LDRI) protocols at 6 Gy dose levels.

Hypothesis: LDRI incorporated into a cyclophosphamide, doxorubicin, vincritine, and prednisone (CHOP)-based chemotherapy protocol is effective for the treatment of lymphoma in dogs.

Animals: Thirty-eight client-owned animals diagnosed with multicentric lymphoma.

Methods: Retrospective study evaluating the efficacy and prognostic factors for the treatment of canine lymphoma with sequential HBI and chemotherapy.

Results: The median 1st remission was 410 days (95% confidence interval [CI] 241–803 days). The 1-, 2-, and 3-year 1st remission rates were 54, 42, and 31%. The median overall survival was 684 days (95% CI 334–1,223 days). The 1-, 2-, and 3-year survival rates were 66, 47, and 44%.

Conclusions and Clinical Relevance: Results of this study suggest that treatment intensification by a 2-week interradiation treatment interval coupled with interradiation chemotherapy is an effective treatment for dogs with lymphoma.

Abbreviations:
CdRT

caudal half-body irradiation

CrRT

cranial half-body irradiation

HBI

half-body irradiation

LDRI

low-dose rate irradiation

Multidrug chemotherapy protocols currently comprise the standard of care for the treatment of canine lymphoma. Manipulation of standard cyclophosphamide, doxorubicin, vincritine, and prednisone (CHOP)-based treatment protocols by changes in the chemotherapy agents used, their dose scheduling, or both has failed to result in a clinically relevant improvement in 1st remission rate (70–88%) or 1st remission duration (168–270 days).1–5 As a result, additional therapies such as total body irradiation, in which most protocols utilize a sequential half-body irradiation (HBI) approach, are being investigated.6–8 Neoplastic lymphocytes exhibit exquisite radiosensitivity and ionizing radiation lacks cross-resistance with the commonly used chemotherapy agents.9 Consequently, the use of concurrent chemo-radiotherapy protocols has improved remission durations and survival times in people with non-Hodgkin's lymphoma.10,11 Similarly, chemo-radiotherapy has been effective in treating canine lymphoma,6,7,12 but modest improvements in outcome have been offset by increased cost of treatment. Toxicity to dose-limiting organs such as the bone marrow and gastrointestinal tract has impacted protocol design in previous veterinary studies, resulting in the use of prolonged 3–4-week interradiation treatment intervals.6–8 This approach may inadvertently affect efficacy by allowing repopulation of the irradiated half by neoplastic cells from the nonirradiated half. We have recently reported on the toxicity profile of an alternative low-dose rate irradiation (LDRI) protocol, which allowed treatment intensification by shortening the interradiation interval to 2 weeks in the majority of dogs.13 This protocol proved to be well tolerated at the 6 Gy dose level and preliminary efficacy data were very promising. The purpose of this study was to extend these initial results by evaluating the efficacy of this protocol in a larger group of dogs with lymphoma.

Materials and Methods

The electronic medical record database at the University of California, Davis, was searched from February 2002 through May 2006 for dogs diagnosed with lymphoma that were scheduled to receive sequential HBI as part of their induction treatment protocol.

Information including signalment, presenting stage, and outcome were extracted from the medical record. When outcome information was not in the medical record, additional information was obtained by contact with the owner or local veterinarian.

Entry Criteria

Dogs included in the analysis had stage 3, 4, or 5 multicentric lymphoma, substage a or b. Immunophenotype was not a criterion for inclusion nor was remission status at the time of the 1st scheduled HBI treatment. Dogs were excluded if induction chemotherapy treatments deviated in any way from the previously outlined protocol13 or if they were treated with radiation doses other than 6 Gy. Thirty-eight dogs with multicentric lymphoma were included for analysis. Twenty-nine dogs had chemotherapy treatment initiated at UC Davis and 9 had treatment initiated in private practice with referral to UC Davis for the radiation treatments. Some of these patients returned to their referring veterinarians for ongoing chemotherapy in accordance with the established protocol, but close follow-up was maintained in each instance.

Diagnosis and Staging

Dogs were diagnosed with lymphoma by either cytologic or histopathologic evaluation of a lymph node. Staging was done according the World Health Organization criteria.14 Where possible, staging diagnostics included a CBC, serum biochemistry, urinalysis, thoracic radiographs, abdominal ultrasound examination, BM aspirate evaluation, and tumor immunophenotype (CD3,a Pan T cell and CD79a,b Pan B cell) as described previously.15

Chemo-Radiotherapy Protocol Design

All dogs were treated with 6 Gy LDR (8–14 cGy/min) sequential HBI incorporated into a standard chemotherapy protocol as described previously (Table 1).13 Five dogs were treated with an abbreviated chemotherapy protocol in which an actinomycin D-based maintenance regimen was not used (Table 1). All dogs received prednisone treatment on a tapering dose schedule (starting at 2 mg/kg) for the 1st 4 weeks of induction chemotherapy. Furosemide was used concurrently with cyclophosphamide in this protocol at the discretion of the attending medical oncologist. Radiation dose was calculated at the mid-plane and given through bilateral opposed portals at extended source-to-skin distance (180–185 cm). Dogs were placed in lateral recumbency and cranial and caudal fields were separated by a transverse line drawn between the dorsal spinous process of the 13th thoracic vertebra and the xiphisternum as described previously.16 These landmarks were used to abut the cranial and caudal fields at the 50% isodose line. All parts of the body were included within the treatment field, allowing for at least a 2 cm beam fall-off at the edge of the patient. Bolus material was not used and dose corrections were not made for tissue heterogeneity. Source-to-axis distance dose calculations along the central axis were done with a computerized treatment planning system, Prowess 2000,a into which hand measurements of patient thickness and extended source-to-skin distances were entered.

Table 1.   Chemo-radiation treatment protocols (long versus short).
ProtocolWeek
123456789101112141618202224262831343740
  1. •, Treatment given; HBI, half-body irradiation.

Long
 l-asparaginase (400 IU/kg)                       
 Vincristine (0.5 mg/m2 IV)                     
 Vincristine (0.7 mg/m2 IV)                
 Cyclophosphamide (200 mg/m2 IV)                   
 Doxorubicin (30 mg/m2 IV)                    
 Actinomycin D (0.7 mg/m2 IV)                       
 Cranial HBI (6 Gy)                       
 Caudal HBI (6 Gy)                       
 Repeat weeks 31–40 at 3 week intervals until week 52, then at 4 week intervals until week 104.
Short
 l-asparaginase (400 IU/kg)                       
 Vincristine (0.5 mg/m2 IV)                     
 Vincristine (0.7 mg/m2 IV)                  
 Cyclophosphamide (200 mg/m2 IV)                    
 Doxorubicin (30 mg/m2 IV)                    
 Cranial HBI (6 Gy)                       
 Caudal HBI (6 Gy)                       

Criteria for patients to receive caudal half-body irradiation (CdRT) have been outlined.13 Patients were divided into 2 groups based on the duration of their maintenance chemotherapy protocol. Group 1 (n = 33) received treatments until 104 weeks or death, whereas group 2 (n = 5) had chemotherapy discontinued after 28 weeks.

Assessment of Response

Complete response (CR) was diagnosed when complete regression (100%) of all palpable lymph nodes occurred, partial response (PR) occurred when >50% but <100% regression of all palpable lymph nodes was seen, and no response was defined as <50% regression in size of lymph nodes or progression of the disease as described previously.3

Assessment of Toxicity/Chemotherapy Delays Post-CdRT

All dogs were evaluated weekly during induction and toxicity was graded according to Veterinary Co-operative Oncology Group (VCOG) guidelines17; grades 1–5 being mild, moderate, severe, life-threatening, and death, respectively. Complete toxicity data for 6/38 dogs in this study has been presented in a previous study evaluating the toxicity profile of this combination LDRI/chemotherapy protocol.13 Given that the protocol was generally well tolerated in the 6 Gy HBI group in that study, BM aspirates after CdRT, serum biochemistry, and bile acid analysis pre- and post-HBI were not routinely conducted in the remaining 32 dogs in this study. In this study, toxicity assessment relied largely on the analysis of CBC. CBC were done on samples collected pre-cranial half-body irradiation (pre-CrRT) and at 1 and 2 weeks post-CrRT and post-CdRT, respectively. In addition, patient records were reviewed to determine the cause and frequency of chemotherapy delays within the 1st 4 weeks (1 chemotherapy cycle) after CdRT. Neutropenia (<2,500/μL), thrombocytopenia (<25,000/μL), or both resulted in a treatment delay of 1 week.

Statistical Analysis

To evaluate for changes in hematological data, an ANOVA test for repeated measures was done. Survival and 1st remission duration were estimated by the Kaplan-Meier product limit method. The date of induction of chemotherapy was counted as day 0 for both analyses. Cases were censored if dogs were still alive for survival analysis or still in remission for 1st remission duration at the time of analysis or if dogs were confirmed to have died without evidence of lymphoma on necropsy. One dog that died of chemotherapy-related sepsis while in remission was not censored for 1st remission duration or overall survival (OS). The following variables were tested for their effect on 1st remission duration and survival by a log-rank test: sex, stage, stage 5, substage, chemotherapy delays within the 1st month after radiation therapy, immunophenotype, chemotherapy protocol used, remission status at the time of RT, and whether or not the dog was anemic at the time of starting RT. A P-value <.05 was considered statistically significant. A commercially available software programb was used to perform statistical analysis.

Results

Patient Characteristics

Thirty-eight cases were identified that met the inclusion criteria. Thirty-three of 38 dogs were in group 1 and scheduled to receive the extended (104 weeks) chemotherapy protocol and 5/38 were in group 2, scheduled to receive the shortened (28 weeks) chemotherapy protocol. There were 28 purebred dogs and 10 mixed breed dogs. Purebred dogs included 4 German Shepherd Dogs, 3 each of Golden Retrievers and Labrador Retrievers; 2 each of Bernese Mountain Dogs, Pembroke Welsh Corgi, and Tibetan Terriers, and 1 each of Afghan Hound, Australian Cattle Dog, Bouvier des Flandres, Boxer, Cocker Spaniel, Great Dane, German Short Haired Pointer, Irish Setter, Maltese, Weimeraner, West Highland White Terrier, and Wirehaired Fox Terrier. There were 21 males (3 intact) and 17 females (all neutered). The mean and median age of all dogs was 7.0 and 6.0 years (range, 2.4–14.7). The mean and median weight for all dogs was 29.7 and 30.9 kg (range, 4.8–61).

Clinical Staging

Thirty-four of 38 dogs were completely staged. Tumor stages included 3a (n = 2); 4a (n = 7); 4b (n = 12); 5a (n = 6); and 5b (n = 7). Of the 4 dogs not completely staged, 3 were substage a, and 1 was substage b. Immunophenotype was available for 30/38 dogs. Twenty-five of 30 were B-cell and 5/30 were T-cell lymphomas.

First Remission Rate

Thirty-three of 38 (87%) dogs were in CR at the time of CrRT. After CdRT, 34/38 (89%) dogs were in CR. Of the 5 dogs not in CR at the time of CrRT, 1 dog (Tibetan Terrier) had advanced stage 5b disease with central nervous system involvement and this dog died of disease (57 days) before receiving CdRT. Two dogs (Great Dane and Mixed Breed) failed to achieve CR after CdRT but did after continued chemotherapy. Both dogs were euthanized for problems other than lymphoma (1 dog bit the owner) at 351 days and (chemotherapy sepsis) at 231 days. The other 2 dogs (German Shepherd and Tibetan Terrier) did achieve CR after CdRT. One of these died of metastatic osteosarcoma (986 days) whereas the other remains alive and in remission (771 days) at the time of writing. In addition, 1 dog (mixed breed) in CR at the time of CrRT lost remission in the caudal half body before CdRT. This dog died of progressive disease at 147 days.

First Remission Duration

Twenty-four of 38 dogs lost remission, 6 dogs were euthanized while still in complete 1st remission for causes unrelated to lymphoma, and 8 dogs remain alive and in complete 1st remission at a median of 1,228 days at the time of writing (range, 574–1,980 days). The median 1st remission for all 38 dogs was 410 days (95% confidence interval [CI] 241–803 days; Fig 1). The 1-, 2-, and 3-year 1st remission rates were 54% (95% CI 37–69%), 42% (95% CI 25–58%), and 31% (95% CI 16–47%), respectively. Stage (P= .26), chemotherapy protocol used (long versus short) (P= .69), chemotherapy delay during the 4 weeks post-CdRT (P= .18), anemia (P= .77), diagnosis at stage 5 (P= .12), sex (P= .87), and remission status at time of CrRT (P= .46) were not prognostic for loss of 1st remission. However, substage (P= .02) and immunophenotype (T versus B) (P= .03) were both prognostic for 1st remission duration (Fig 2).

Figure 1.

 Kaplan-Meier median 1st remission duration curve for 38 dogs with lymphoma treated with sequential LDR HBI and chemotherapy. LDR, Low dose-rate; HBI, half-body irradiation. Tick marks indicate censored cases.

Figure 2.

 Kaplan-Meier median 1st-remission duration curves for (a) substage (a versus b) and (b) immunophenotype (B versus T cell). Substage b and T-cell phenotype represented by dashed lines in curves a and b, respectively. Tick marks indicate censored cases.

Second Remission Duration

Of the 24 dogs that lost 1st remission, 14 were known to be reinduced, 6 dogs had no reinduction information recorded as to specific protocol and 4 dogs had reinduction declined by their owners. Of the 14 dogs reinduced, 3 were reinduced with CCNU alone, 3 with the same CHOP-based protocol, and 8 dogs were reinduced with different combinations of CCNU, doxorubicin, cyclophosphamide, vinblastine, and l-aspariginase. The median 2nd remission time was 109 days (95% CI 58–334 days) with a range of 7–695 days.

Overall Survival

The median OS for all 38 dogs was 684 days (95% CI 334–1,223 days; Fig 3). The 1-, 2-, and 3-year survival rates were 66% (95% CI 48–79%), 47% (95% CI 30–63%), and 44% (95% CI 27–60%), respectively. In this group, 6 dogs were euthanized in remission for reasons unrelated to lymphoma and 10 dogs remain alive and in remission at the time of writing (range, 574–1,980 days). Of these 10 dogs, 8 are in their 1st remission and 2 were reinduced with lomustine-based protocols achieving durable 2nd remissions of 545 and 695 days, respectively. Stage (P= .23), substage (P= .06), immunophenotype (P= .31), chemotherapy protocol (long versus short) (P= .95), chemotherapy delay within 4 weeks of post-CdRT (P= .20), anemia (P= .69), diagnosed at stage 5 (P= .10), sex (P= .83), and remission status at time of CrRT (P= .87) were not prognostic for survival.

Figure 3.

 Kaplan-Meier survival curve for 38 dogs with lymphoma treated with sequential LDR HBI and chemotherapy. LDR, low dose rate; HBI, half-body irradiation. Tick marks indicate censored cases.

Assessment of Toxicity

CBC results were retrieved for time points corresponding to pre-CrRT (n = 34), 1 week post-CrRT (n = 25), 2 weeks post-CrRT (n = 36), 1 week post-CdRT (n = 28), and 2 weeks post-CdRT (n = 27). The mean values (± SD) for hematocrit, neutrophil, and platelet counts at all 5 time points are presented in Figures 4–6. Analysis of these data indicated that a significant increase in hematocrit occurred 2 weeks post-CrRT (P= < .001) and 2 weeks post-CdRT (P= .001) with respect to pre-CrRT hematocrit. Neutrophil data analysis showed that a significant decrease in the neutrophil count with respect to pre-CrRT results was observed at 1 (P= < .001) and 2 weeks post-CrRT (P= < .001) and at 1 (P= < .001) and 2 weeks post-CdRT (P= < .001). Significant decreases in platelet count with respect to pre-CrRT counts were also observed at 1 (P= < .001) and 2 weeks post-CrRT (P= < .001) and at 1 (P= < .001) and 2 weeks post-CdRT (P= < .001). In addition, a significant decrease in platelet count was noted between the 1 week post-CrRT time point and time points 2 weeks post-CrRT (P= < .001) and 1 (P= < .001) and 2 weeks post-CdRT (P= .002) as well as between the 2 weeks post-CrRT time point and those at 1 (P= < .001) and 2 weeks post-CdRT (P= .003). The severity of hematologic toxicity (neutropenia and thrombocytopenia) as graded by VCOG guidelines is presented in Table 2.

Figure 4.

 Mean hematocrit (±SD) pre-RT and at 1 and 2 weeks post-CrRT and post-CdRT, respectively. Normal reference range 40–55%. *Statistically significant increases in hematocrit occurred 2 weeks post-CrRT (P= <.001) and 2 weeks post-CdRT (P= .001) with respect to pre-CrRT hematocrit. CrRT, cranial half-body irradiation; CdRT, caudal half-body irradiation; HBI, half-body irradiation.

Figure 5.

 Mean neutrophil count (±SD) pre-RT and at 1 and 2 weeks post-CrRT and post-CdRT. Normal reference range 3,000–10,500. *Neutrophil data analysis showed that a significant decrease in the neutrophil count with respect to pre-CrRT values was observed at 1 (P= <.001) and 2 weeks post-CrRT (P= <.001) and at 1 (P= <.001) and 2 weeks post-CdRT (P= <.001). CrRT, cranial half-body irradiation; CdRT, caudal half-body irradiation. HBI, half-body irradiation.

Figure 6.

 Mean platelet count (±SD) pre-RT and at 1 and 2 weeks post-CrRT and post-CdRT, respectively. Normal reference range 150,000–400,000. *Significant decreases in platelet counts with respect to pre-CrRT levels were also observed at 1 (P= <.001) and 2 weeks post-CrRT (P= <.001) and at 1 (P= <.001) and 2 weeks post-CdRT (P= <.001), respectively. In addition, a significant decrease in platelet count was noted between the 1 week post-CrRT time point and time points at 2 weeks post-CrRT (P= <.001) and 1 (P= <.001) and 2 weeks post-CdRT (P= .002) as indicated by a +, as well as between the 2 weeks post-CrRT time point and those at 1 (P= <.001) and 2 week post-CdRT (P= .003) indicated by a @. CrRT, cranial half-body irradiation; CdRT, caudal half-body irradiation HBI, half-body irradiation.

Table 2.   Hematologic toxicity (VCOG guidelines).
Time Point# DogsToxicityGrade
01234
  1. CrRT, cranial half-body irradiation; CdRT, caudal half-body irradiation; VCOG, Veterinary Co-operative Oncology Group.

Pre-CrRT35Neutropenia323000
 Thrombocytopenia341000
1 week post-CrRT25Neutropenia214000
 Thrombocytopenia250000
2 weeks post-CrRT36Neutropenia278100
 Thrombocytopenia315000
1 week post-CdRT28Neutropenia1412200
 Thrombocytopenia115921
2 weeks post-CdRT27Neutropenia205200
 Thrombocytopenia149400

Thirty-six of 38 dogs received irradiation to both body halves. One dog lost remission in the cranial half as evidenced by lymphoma cells found in a BM sample taken from the humerus 2 weeks after receiving radiation to the cranial half. This dog did not have lymphoma infiltrates in the BM of its humerus at the time of pretreatment staging. The other dog never achieved CR and died before receiving CdRT. Thirty of 36 dogs received both HBI treatments within 16 days of each other. The remaining 6 dogs received both treatments within a range of 19–31 days of each other. These 6 treatment delays were a result of having a BM aspirate analysis that indicated inadequate recovery of all cell lines in the cranial half or <10% cellularity. Although 1st remission times and OS were shorter in dogs with interradiation intervals >16 days, this difference was not significant with 1st remission times of 220 days (95% CI 69–1,126 days) versus 550 days (95% CI 285–1,223 days; P= .08) and OS of 334 days (95% CI 57–1,131 days) versus 752 days (95% CI 385–1,533 days; P= .09). After CdRT, 19 dogs had a delay in their scheduled chemotherapy protocol because of a neutropenia, arising at various time points within the next 4 weeks (1st ensuing chemotherapy cycle). One of these dogs also had grade 4 thrombocytopenia (<25,000/μL). Chemotherapy delays were associated with longer 1st remission times of 476 days (95% CI 350—upper limit not reached) versus 241 days (95% CI 177–1,126 days; P= .18) and OS of 1130 days (95% CI 403—upper limit not reached) versus 556 days (95% CI 231–1,223 days; P= .20), but these differences were not significant.

Discussion

The primary goal of this study was to report on 1st remission duration and OS time in dogs with lymphoma treated with sequential LDR HBI (6 Gy) incorporated into a CHOP-based chemotherapy protocol. The study showed that dogs with lymphoma that were treated with this protocol had 2- and 3-year survival rates of 47 and 44% respectively.

We have previously reported on the toxicity profile of an intensified combination LDRI and chemotherapy protocol for the treatment of canine lymphoma.13 This protocol attempts to enhance the therapeutic gain of a combined chemo-radiation protocol by shortening the interradiation interval. A shorter interradiation interval can be achieved because of the decreased tissue toxicity associated with LDRI. Classically, this results from the repair of sublethal damage that occurs during prolonged radiation exposure.18 As dose rate is lowered and treatment time lengthened, greater cellular recovery of radiation damage occurs during exposure.18

This protocol was designed to integrate radiation therapy early in the induction phase of treatment and to shorten the interradiation interval to 2 weeks to minimize the possibility of tumor repopulation in the interval between irradiation of cranial and caudal body halves. In this study, remission status at the time of CrRT was not an exclusion factor. Only 5 dogs had not achieved CR by the time of CrRT and remission status did not prove to be predictive of outcome with respect to 1st remission duration or OS in this group. This may reflect the fact that 4 of these 5 dogs were in PR after 1 cycle of chemotherapy with relatively low disease burdens. Only 1 dog with advanced stage (5b) and large tumor burden did poorly. These data should be viewed cautiously given the small number of dogs and given that 2 of the 5 dogs were censored from analysis because they were euthanized for unrelated problems. Radiation therapy effectiveness is generally achieved in direct proportion with the tumor burden treated.19 This lends support to the use of HBI in either a consolidation or induction setting in which a clinical remission has been achieved and minimal residual tumor burden remains.

All dogs that had treatment initiated at UC Davis were staged completely, whereas data for dogs having treatment initiated elsewhere generally was incomplete with respect to immunophenotype and BM evaluation. As such, some of these dogs may have been classified with a less-advanced stage.

This combination chemo-radiotherapy protocol initially used a prolonged maintenance CHOP protocol (routinely used in our clinic) to allow comparison with a previously treated cohort of dogs in our clinic receiving chemotherapy alone.20 Toward the end of the study, a shorter chemotherapy regimen was used based on studies that reported no survival advantage for prolonged treatment and as such a shorter treatment time could save time and money for owners.2,21–23 These findings were similar to the chemotherapy alone studies in that no significant difference in either 1st remission duration or OS was noted for either group (long versus short chemotherapy) of dogs. These results should be viewed with caution given that only 5/38 dogs were treated with the short chemotherapy protocol.

The median 1st remission duration of 410 days in this study compares favorably to previously reported median 1st remission durations of 219–360 days for several studies evaluating chemotherapy alone.8,24–26 These remission and survival data are also favorable in comparison with previous combination chemo-radiotherapy protocols with reported median remission durations ranging from 209 to 455 days and median OS times ranging from 384 to 560 days, although direct comparisons cannot be made.6–8 The remission and survival times reported, however, are not as good as those recently reported in another study evaluating the same protocol.13 This may be a result of the larger number of dogs evaluated in this study as well as a larger number of dogs having T-cell and higher stage lymphomas. Given the retrospective nature of this study, it may be more useful to discuss the 1-, 2-, and 3-year survival rates in these dogs. Conventional CHOP-based protocols reportedly have 2-year survival rates of only 20–25%.27 In this study, 2- and 3-year survival rates were doubled at 47 and 44%, respectively. This suggests that a subpopulation of dogs will benefit substantially by the addition of HBI, although we cannot identify specifically which dogs will benefit.

In this study, 84% of the patients were irradiated as planned (<16 days between CrRT and CdRT). An interradiation interval of >16 days was not significantly associated with shorter 1st remission times (P= .08) or shorter OS (P= .09). A major concern of prolonged interradiation intervals is tumor repopulation; however, the fact that neither 1st remission times nor survival was impacted may be the result of treatment intensification and successful reinduction of patients that had lost remission, respectively. Another reason could be due to the fact that only 6 dogs had prolonged interradiation intervals, and this data should be interpreted cautiously. After CdRT, 19 dogs (50%) had a delay in their scheduled chemotherapy protocol because of neutropenia arising at various time points within the next 4 weeks (1st ensuing chemotherapy cycle). Chemotherapy delays were not associated with significant decreases in 1st remission or OS. This may be a result of the fact that these delays were because of neutropenia, which has previously been associated with improved remission duration.28

Hematologic toxicity in this study was similar to that reported in previous chemo-radiotherapy studies.8,29 In this study, the incidence and severity of both neutropenia and thrombocytopenia increased steadily from 1 week post-CrRT to 1 week post-CdRT (3 weeks from initiation of CrRT) and then showed signs of resolution by 2 weeks post-CdRT. The majority of hematologic toxicities (neutropenia and thrombocytopenia) noted were considered to be mild (grades 1 or 2) with only 3 dogs experiencing toxicity of grade 3 or higher (2 with grade 3 thrombocytopenia and 1 with grade 4 thrombocytopenia). Interestingly, the higher grade toxicities all occurred 1 week post-CdRT. This time point was also noted to have more frequent episodes of both neutropenia and thrombocytopenia and may reflect the kinetics of individual cell lineages. In general, however, dogs tolerated the protocol well and could continue with chemotherapy as planned.

The results of this study show that treatment intensification using a short 2-week interradiation interval is feasible in dogs treated with 6 Gy LDRI and these data suggest that substantial improvements in 1st remission duration and survival time can be obtained. One important limitation is the retrospective nature of this study, and prospectively designed studies are needed to directly compare this protocol with other combination chemo-radiotherapy protocols as well as conventional CHOP-based chemotherapy.

Footnotes

aProwess Inc, Chico, CA

bStata version 10.0, StataCorp LP, College Station, TX

Acknowledgments

The authors acknowledge Drs Cheryl London, Gabriella Sfiligoi, Iain Grant, Erin Romansik, and Suzanne Waltman, who were the medical oncologists who managed the chemotherapy treatments for these cases. This work was not supported by a grant.

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