• nonsmall cell lung cancer;
  • brain metastasis;
  • gamma knife radiosurgery;
  • circadian;
  • treatment time


  1. Top of page
  2. Abstract
  6. Acknowledgements


Circadian cell-cycle progression causes fluctuating radiosensitivity in many tissues, which could affect clinical outcomes. The purpose of this study was to determine whether outcomes of single-session gamma knife radiosurgery (GKRS) for metastatic nonsmall cell lung cancer (NSCLC) differ based on treatment time.


Fifty-eight patients received GKRS between 10:00 am and 12:30 pm and 39 patients received GKRS between 12:30 pm and 3:00 pm. The mean peripheral dose was 18.6 Gy. The mean tumor size was 7.3 cm3. Magnetic resonance imaging was used to score local control at 3 months. Cause of death (COD) was categorized as central nervous system (CNS)-related or systemic.


Demographic and disease characteristics of the 2 groups were similar. Local control at 3 months was achieved in 97% (35/36) of patients who underwent GKRS early in the day versus 67% (8/12) of patients who underwent GKRS later in the day (chi-square, P = .014). Early GKRS was associated with better survival (median 9.5 months) than late GKRS (median 5 months) (Kaplan-Meier log-rank test, P = .025). Factors contributing to better survival in a Cox regression model included early treatment time (P = .004) and recursive partition analysis class (P < .001). Cause of death in the early treatment group was CNS-related in 6% (3/47) of patients versus 24% (8/34) of patients in the late treatment group (chi-square test, P = .026).


GKRS for metastatic NSCLC had better local control, better survival, and a lower rate of CNS-related cause of death when given earlier in the day versus later in the day. These retrospective data should encourage future study in brain radiosurgery and non-CNS stereotactic body radiotherapy series. Cancer 2011. © 2010 American Cancer Society.

It is a well-accepted observation that cells vary in sensitivity to irradiation based on cell cycle phase, where those in G2/M phase are particularly vulnerable.1, 2 The radiation response in murine tissues like the intestinal crypts and bone marrow display varies in radiosensitivity throughout the day.3, 4 In other studies of murine bone marrow, the cycle of radiosensitivity can be shifted with changes in circadian light/dark schedule.5 These circadian-dependent changes in radiation response are found not only in rapidly proliferating tissues, but also in slowly proliferating tissues, such as the parotid gland.6 Although 1 study using an irradiation “top-up dose” model with murine skin did not demonstrate circadian variation in radiation response,7 the preponderance of animal data show circadian variation in radiation response.

One mechanism for the 24-hour variation in radiation response is based on the circadian structure of cellular progression through the cell cycle. Many benign and malignant tissues8-17 have demonstrated circadian changes in rates of cellular replication, which affects the profile of cell cycle states over the course of the day. For example, increased thymidine incorporation into DNA has been used to mark peaks in S phase of the cell cycle occurring during the daily activity phase.15 A similar circadian time structure has been shown with the immunohistochemical staining of the peak expression of G1, G2, and M checkpoint proteins found in serial biopsy specimens of the oral mucosa from multiple human subjects.17 In that study, the proteins cyclin A, E, and B1 peaked at successively later times in samples taken from midday until early evening.

Based on these and other measurements,18 a prospective randomized trial19 was designed to test the hypothesis that rates of mucositis toxicity would be affected by a nonlinear response to irradiation given in the morning (during G1 phase) versus the early evening (when cells would have progressed further into G2/M phase). In this trial, lower rates of mucositis and weight loss among smokers was associated with high-dose (>66 Gy) morning radiotherapy when compared with afternoon radiotherapy, thus providing evidence for circadian-related effects on clinical outcomes.

To our knowledge, no study has investigated circadian-related treatment effects of radiosurgery in metastatic cancer. Analysis of single-session radiosurgery offers an opportunity to investigate for differences in treatment time without the potential inconsistencies in daily timing of multiple fractions and eliminates possible confounding effects of cell cycle perturbation in surviving tumor cells (regenerative resistance) that could be present with fractionated irradiation. Currently, one of the most common applications of radiosurgery is brain metastasis of nonsmall cell lung cancer (NSCLC). There are approximately 165,000 new cases of NSCLC per year,20 and approximately 25%-40%21 will develop brain metastasis. This large population has a poor prognosis, with a median survival of 2-7 months after diagnosis22; therefore, methods for improving survival and quality of life could have wide implications. In the present study, we analyzed the effects of treatment time on post-GKRS survival in patients with NSCLC brain metastasis.


  1. Top of page
  2. Abstract
  6. Acknowledgements

Ninety-seven patients treated from 1989 to 2007 at the University of Virginia (UVA) Gamma Knife Center were reviewed after receiving institutional review board approval and informed consent from the patients. All had histologically proven primary NSCLC and radiological evidence of brain metastasis. Evaluations to rule out other malignancies included a clinical history, physical examination, chart review, and computed tomography imaging of the chest, abdomen, and pelvis. Patients with a previous malignancy were excluded. For all patients, treatment planning was conducted during the 60 minutes before beginning the treatment. All patients were treated during the normal operating hours of the UVA Gamma Knife Center (10:00 am to 3:00 pm). The start of treatment was considered the time at which the GKRS treatment plan was approved; the actual treatment began within approximately 10 minutes of approval. Scheduling was performed with consideration of each patient's preference and appointment availability but without consideration of each patient's clinical status.

All radiosurgical procedures were performed with the gamma knife model U from 1989 to 2001 and the model C from 2001 to 2007. Procedures were performed in a single session in all cases. The prescribed peripheral dose ranged from 10 to 24.5 Gy (mean 18.6 Gy, median 18 Gy), with 90 of the 97 patients having a peripheral dose between 16 and 24 Gy. The prescription isodose ranged from 30% to 91%, whereas the mean maximum dose was 49.8 Gy (range 26-90 Gy) The dose rates from these gamma knife sources were 3.66-1.59 and 3.56-2.31 Gy/min for model U from March 1989 to July 2001 (reloaded November 1995) and 3.88-1.98 Gy/min for model C (reloaded July 2006).

All patients were followed up at monthly intervals until mortality or the conclusion of the study. The primary outcomes measured were local control at 3 months post-GKRS and overall survival after GKRS. State death certificate records were used to verify date of death.

The survival data were first examined with individual survival curves for each hour of the day. This exploratory analysis showed clustering of the early treatment and late treatment data leading to 2 time brackets equally dividing the range of treatment times. The 2 groups used for analysis were patients treated between 10:00 am and 12:30 pm (n = 58) and patients treated between 12:30 pm and 3:00 pm (n = 39) and provide a convenient notation of risk for clinical and statistical applications..

The 2 groups were analyzed for relevant clinical variables (Tables 1 and 2) We used chi-square tests to compare categorical variables, including sex, recursive partition analysis (RPA) class, Karnofsky performance status, cystic appearance of tumor, control of primary tumor, extracranial metastasis, prior treatment with whole-brain radiotherapy, prior treatment with steroids, chemotherapy at any point during treatment, and prior lung radiotherapy. Wilcoxon rank sum tests were used to compare the following continuous variables, which were primarily nonnormally distributed: age, time from primary diagnosis to brain metastasis, time from diagnosis of brain metastasis to GKRS, average tumor volume, total number of metastasis, total volume of metastasis, volume of largest metastasis, number of metastasis treated with GKRS, peripheral dosage of treatment, maximum dosage of treatment, percent isodose of treatment, number of isocenters, and volume treated with GKRS. For patients with multiple metastases, the data for peripheral dosage, maximum dosage, and percent isodose were taken from the maximally treated metastasis.

Table 1. Patient Characteristics: Categorical Variables
CharacteristicsGKRS Before 12:30 pm (n = 58)GKRs After 12:30 pm (n = 39)Pa
  • All data are presented as the percentage (%) of patients.

  • RPA, recursive partition analysis; KPS, Karnofsky performance status; GKRS, gamma knife radiosurgery; WBRT, whole-brain radiotherapy; AC, adenocarcinoma; SCC, squamous cell carcinoma; LCC, large cell carcinoma; NOS NSCLC, not otherwise specified nonsmall cell lung cancer; SIR, score index for radiosurgery.

  • a

    Chi-square test.

Male sex4046.53
RPA class (1, 2, 3)24, 74, 223, 74, 3.90
KPS score (100, 90, 80, 70, 60)10, 21, 34, 31, 33, 18, 31, 46, 3.35
Cystic tumor145.17
Control of primary tumor at GKRS3651.14
Extracranial metastasis at GKRS2426.87
Prior WBRT6272.32
Steroid use7167.67
Lung radiotherapy7156.15
Histology (AC, SCC, LCC, and NOS NSCLC)45, 19, 9, 2851, 15, 5, 28.68
SIR class (1, 2, 3)3, 78, 190, 82, 18.49
Table 2. Patient Characteristics: Continuous Variables
CharacteristicsGKRS Before 12:30 pm (n = 58)GKRS After 12:30 pm (n = 39)Pa
  • All data are presented as the median unless noted otherwise.

  • GKRS, gamma knife radiosurgery.

  • a

    Wilcoxon rank sum test.

Age, y6064.18
Time from primary diagnosis to brain metastasis, mo14.28
Time from diagnosis of metastasis to GKRS, mo23.54
No. of tumors present at diagnosis12.31
No. of tumors treated12.17
Largest tumor size, cm34.34.5.69
Total tumor size, cm34.84.9.98
Total treatment size, cm36.28.1.51
Peripheral dose, Gy1818.83
Maximum dose, Gy51.450.10
Percent isodose40%40%.12
No. of isocenters3.55.40

Additionally, each patient was categorized by the score index for radiosurgery in brain metastasis.23 A point score from 0 to 2 was tallied for each of the following 5 variables: age in years (≥60 = 0, 51-59 = 1, ≤50 = 2), Karnofsky performance status (≤50 = 0, 60-70 = 1, ≥80 = 2), systemic disease status (progressive = 0, partial remission or stable disease = 1, complete remission or no evidence of disease = 2), size of largest lesion in cm3 (>13 = 0, 5-13 = 1, <5 = 2), and number of lesions (≥3 = 0, 2 = 1, 1 = 2). Patients were then assigned a class based on point total: class 1 was 1-3 points, class 2 was 4-7 points, and class 3 was 8-10 points. Higher class is associated with a better prognosis. Class assignments were compared between the 2 study groups to ensure that this categorical prognostic variable did not differ between groups (Table 1).

Statistical analysis of survival post-GKRS was performed using the Kaplan-Meier estimation and log rank test. Multivariable analysis using the Cox proportional hazards method was performed to assess the relationship between survival and treatment time while adjusting for relevant clinical factors, including sex, RPA class (I versus II or III), total tumor volume, prior whole-brain radiotherapy, and peripheral GK dose. All assumptions of the statistical tests were verified. Cause of death was recorded and categorized as central nervous system (CNS)-related (eg, neurologic disease progression, stroke, brain death) or systemic (eg, lung disease progression, extracranial disease progression).

Local control at 3 months post-GKRS was assessed by way of magnetic resonance imaging (MRI) when available. MRI scans were reviewed by certified neuroradiologists in the UVA Radiology Department who were blinded to patient characteristics. Dimensions of the treated lesions were measured on pretreatment MRI scans with the frame in place, as well as 3-month follow-up MRI scans. Cases were defined as local failure when there was both objective and subjective evidence of lesion progression. Objectively, cases of local failure were required to have lesion expansion >2 mm in any dimension, because this was felt to be the minimum interval that could be accurately measured as evidence of increased size. Subjectively, neuroradiologists defined local failure status as increased tumor size on T1 MRI scans with contrast to a degree greater than that which could be explained by posttreatment changes alone. A chi-square test was performed to assess local control in the early treatment and late treatment groups. The α level was set at.05 for each statistical test.


  1. Top of page
  2. Abstract
  6. Acknowledgements

Of the 97 patients in the study with survival data, 48 (49%) had 3-month follow-up MRI data (Table 3). Local control at 3 months was achieved in 97% (35/36) of patients treated before 12:30 pm compared with 67% (8/12) of those treated after 12:30 pm (chi-square test, P = .014). Figure 1 shows a patient who responded to GKRS and had local control at 3 months. For the group treated before 12:30 pm, MRI follow-up data were available for a range of 1-84 months (median 4 months); for the group treated after 12:30 pm, MRI follow-up data were available for a range of 0.25-8 months (median 2.25 months).

thumbnail image

Figure 1. Magnetic resonance imaging immediately before gamma knife radiosurgery (left images) and at follow-up (right images) in a patient with local control at 3 months.

Download figure to PowerPoint

Table 3. Local Control at 3 Months Post-GKRS Among Groups of Differing Treatment Time of Day
 GKRS Before 12:30 pm (n = 36)GKRS After 12:30 pm (n = 12)P
  • GKRS, gamma knife radiosurgery.

  • a

    P < .05 was considered statistically significant (chi-square test).

Local control, n (%)35 (97)8 (67).014a
Local failure, n (%)1 (3)4 (33) 

Data regarding post-GKRS survival as a function of treatment time of day are shown in the Kaplan-Meier plot in Figure 2. Follow-up time (from GKRS until mortality or conclusion of the study) ranged from 1 to 84 months (median 7 months). Overall, the evidence from the log rank analysis of the primary outcome of survival post-GKRS showed that those patients in the early treatment group had a longer survival than those treated later in the day (P = .025). The median survival of these groups was 9.5 months in the early treatment group and 5 months in the late treatment group.

thumbnail image

Figure 2. Survival curve for nonsmall cell lung cancer patients with brain metastasis after gamma knife (GK) radiosurgery as a function of time of day treated, with log-rank P value.

Download figure to PowerPoint

Overall, the patients in the 2 study groups did not differ significantly in distribution of the categorical or continuous clinical variables (Tables 1 and 2). The mean number of lesions treated in the early treatment group and late treatment group was similar (1.66 and 1.79, respectively; P = .17). The model U (used from 1989-2001) was used to treat 28 patients in the early treatment group and 14 patients in the late treatment group. The model C (used from 2001-2007) was used to treat 30 patients in the early group and 25 patients in the late group. There were no statistically significant differences in the distribution of GKRS treatment model (chi-square test, P = .319).

A Cox proportional hazards model (Table 4) showed that factors contributing to a better survival included treatment time before 12:30 pm (P = .004) and RPA class I (P < .001). In this model, survival was not significantly affected by sex (P = .11), tumor size (P = .56), prior whole-brain radiotherapy (P = .33), or peripheral GK dose (P = .28). The data on cause of death, which were available in 81 of 97 (84%) patients, are summarized in Table 5.

Table 4. Cox Regression Analysis of Post-GKRS Survival
 CoefficientSEHR (95% CI)P
  • SE, standard error, HR, hazard ratio; CI, confidence interval; RPA, recursive partition analysis; WBRT, whole-brain radiotherapy; GK, gamma knife.

  • a

    P < .05 was considered statistically significant (chi-square test).

RPA class1.070.302.92 (1.62-5.28)<0.001a
Time of day treated0.690.242.00 (1.25-3.17)0.004 a
Sex0.380.241.46 (0.92-2.33)0.11
Prior WBRT− (0.46-1.31)0.33
Total tumor size0.010.021.01 (0.98-1.05)0.56
Peripheral GK dose− (0.83-1.06)0.28
Table 5. Cause of Death Among Groups of Differing Treatment Time of Day
 GKRS Before 12:30 pm (n = 47)GKRS After 12:30 pm (n = 34)P
  • GKRS, gamma knife radiosurgery; CNS, central nervous system.

  • a

    P < .05 was considered statistically significant (chi-square test).

CNS-related, n (%)3 (6)8 (24).026a
Systemic, n (%)44 (94)26 (76) 


  1. Top of page
  2. Abstract
  6. Acknowledgements

This retrospective review shows that patients treated with GKRS for NSCLC metastasis have better local control, less frequent CNS-related cause of death, and longer survival when treated earlier in the day (before 12:30 pm) compared with similar patients treated later in the day (after 12:30 pm). An extensive list of risk factors for disease progression, patient characteristics, and treatment plan complexity were accounted for equally in these groups (Tables 1 and 2), making it unlikely that these factors influenced the differing outcomes in the 2 groups. In addition, the multivariate Cox regression analysis further supports our hypothesis by showing treatment time and RPA class to be the only independently significant factors regarding survival. The median post-GKRS survival for the early and late treatment groups in this study was 9.5 and 5 months, respectively, which is within the range of survival reported by previous studies of patients with brain metastasis.22, 23

We do not think the differences in outcome for the early and late treatment groups are related to the large GK dose per fraction associated with late vascular damage. Although there is no question that compared with the more common dose per fraction of 1.5-3.0 Gy, the median dose of 18 Gy used in our study probably caused some late vascular damage. However, the question is whether effects of circadian variation in treatment are observed at relatively high doses per fraction in short-term radiobiological assays. This can be found in the data from the jejunal crypt survival assay, where single doses as high as 14.5 Gy are needed to demonstrate epithelial cell damage associated with circadian time structure.24

Our data support the hypothesis that clinical response to irradiation is not linear across the day and, in this sense, is consistent with the concept of circadian differences in radiation response reported by Bjarnason et al.19 Based on the clinical response observed in this study, we hypothesize that the brain metastases were in a more radioresistant phase of the cell cycle at the later treatment time. GKRS in the early afternoon may have allowed the cancer cells in these metastatic NSCLC deposits to progress into S phase, thus accounting for a more radioresistant behavior. Our reasoning is based in part on the known similarity of relatively high α/β characteristics in rapidly proliferating malignant cells compared with rapidly dividing tissue, such as the oral mucosa.25, 26 Furthermore, the finding of more frequent local failure in patients treated later in the afternoon is consistent with a robust kinetic resistance.

In addition to the recognized circadian effects on the cell cycle checkpoint proteins, there are other molecular targets of human clock and clock-controlled genes that might play a role in the biorhythmicity of cell metabolism and function.27-30 Gene expression patterns in regenerating livers of mice have been associated with circadian expression of wee1 and cyclin B1-cdc2 kinase, which help mediate progression through the cell cycle.31 The clock genes period 1 and period 2 (per1 and per2) and clock-controlled genes have been shown to affect rates of cellular proliferation32-36 and apoptosis37 and to behave like tumor suppressors.37-39 Other work with the clock gene per1 shows its involvement with DNA damage-sensing pathways that could influence cell survival after irradiation.37 In this regard, a separate study examining the activity of nucleotide excision repair genes in normal mouse cerebral cortex has been shown to follow a circadian pattern.40 The activity of DNA repair genes was found to be 10-fold higher at different times of day, which could lead to periods of relative protection from DNA damage. These data underscore the need to investigate the role of circadian cell cycle structure in tumors that receive only single-fraction treatment. The treatment itself may induce perturbations in the cell cycle, which would have consequences for tumor behavior.

As with all retrospective studies, limitations exist with regard to the data available for the population selected. The local control data at 3 months appear to show an effect of treatment timing, but additional long-term data would be useful. To build on the concepts discussed in this study, future investigations could evaluate other malignancies, other fractionation schedules, and other treatment times, especially late afternoon or early evening. For instance, some cell populations have shown greater radiosensitivity late at night and early in the morning and less radiosensitivity at midday24; such findings represent a potential evolutionary adaptation to the genotoxic environmental insults more frequently encountered during periods of activity.27 Even larger differences in the effects of radiosurgery might be observed at treatment times later than those used in the present study (ie, after 3:00 pm). We are exploring some of these possibilities with ongoing retrospective analyses of the effects of radiosurgery timing on slowly proliferating neoplasms (eg, meningiomas and schwannomas) and on the treatment results of functional tumors of the pituitary.

In conclusion, we found a significant difference in the outcomes of patients with metastatic NSCLC based on their time of treatment. However, the possible benefit of this approach cannot be confirmed until a prospective randomized trial is performed comparing long-term local control and survival for different treatment times.


  1. Top of page
  2. Abstract
  6. Acknowledgements

We thank Greg Patterson, Marion Harding, and the rest of the University of Virginia Gamma Knife Center staff for their assistance.


  1. Top of page
  2. Abstract
  6. Acknowledgements
  • 1
    Kubota S, Sato S, Nakano T, et al. Prediction of radiosensitivity by DNA analysis. Gan No Rinsho. 1989; 35: 1572-1575.
  • 2
    Pawlik TM, Keyomarsi K. Role of cell cycle in mediating sensitivity to radiotherapy. Int J Radiat Oncol Biol Phys. 2004; 59: 928-942.
  • 3
    Ijiri K, Potten CS. Circadian rhythms in the incidence of apoptotic cells and number of clonogenic cells in intestinal crypts after radiation using normal and reversed light conditions. Int J Radiat Biol Relat Stud Phys Chem Med. 1988; 53: 717-727.
  • 4
    Newsome-Tabatabai R, Rushton PS. Daily variation in radiosensitivity of circulating blood cells and bone marrow cellularity of mice. Comp Biochem Physiol A. 1984; 78: 779-783.
  • 5
    Haus E, Halberg F, Loken MK. Circadian susceptibility-resistance cycle of bone marrow cells to whole body x-irradiation in Balb/c mice. Chronobiologia. 1974; 1974: 115-122.
  • 6
    El-Mofty SK, Hovenga TL, Russell JE, Simmons DJ. Parotid radiosensitivity changes: a temporal relation to glandular circadian rhythms. Int J Radiat Biol Relat Stud Phys Chem Med. 1982; 41: 335-342.
  • 7
    Hirn-Stadler B, Rojas A. Influence of circadian rhythms on radiosensitivity: single and fractionated dose studies in mouse skin. Int J Radiat Biol. 1991; 59: 185-193.
  • 8
    Drazen DL, Bilu D, Bilbo SD, Nelson RJ. Melatonin enhancement of splenocyte proliferation is attenuated by luzindole, a melatonin receptor antagonist. Am J Physiol Regul Integr Comp Physiol. 2001; 280: R1476-R1482.
  • 9
    Biederbick A, Elsasser H. Diurnal pattern of rat pancreatic acinar cell replication. Cell Tissue Res. 1998; 291: 277-283.
  • 10
    Smirnov SN, Zakharov VB, Mamontov SG. Diurnal dynamics of cell proliferation in rat liver during early postnatal ontogeny and effect of epidermal growth factor on hepatocyte proliferative activity. Bull Exp Biol Med. 2005; 139: 150-153.
  • 11
    Frentz G, Moller U, Holmich P, Christensen IJ. On circadian rhythms in human epidermal cell proliferation. Acta Derm Venereol. 1991; 71: 85-87.
  • 12
    Moller U, Larsen JK, Keiding N, Christensen IJ. Circadian-stage dependence of methotrexate in a keratinized epithelium. An in-vivo study using flow cytometry on the hamster cheek pouch epithelium. Cell Tissue Kinet. 1984; 17: 483-495.
  • 13
    Schell H, Hornstein OP, Egdmann W, Schwarz W. Evidence of diurnal variation of human epidermal cell proliferation. II. Duration of epidermal DNA synthesis. Arch Dermatol Res. 1981; 271: 49-53.
  • 14
    Smaaland R, Lote K, Sothern RB, Laerum OD. DNA synthesis and ploidy in non-Hodgkin's lymphomas demonstrate intrapatient variation depending on circadian stage of cell sampling. Cancer Res. 1993; 53: 3129-3138.
  • 15
    Buchi KN, Moore JG, Hrushesky WJ, Sothern RB, Rubin NH. Circadian rhythm of cellular proliferation in the human rectal mucosa. Gastroenterology. 1991; 101: 410-415.
  • 16
    Davidson AJ, Straume M, Block GD, Menaker M. Daily timed meals dissociate circadian rhythms in hepatoma and healthy host liver. Int J Cancer. 2006; 118: 1623-1627.
  • 17
    Bjarnason GA, Jordan RC, Sothern RB. Circadian variation in the expression of cell-cycle proteins in human oral epithelium. Am J Pathol. 1999; 154: 613-622.
  • 18
    Bjarnason GA, Jordan RC, Wood PA, et al. Circadian expression of clock genes in human oral mucosa and skin: association with specific cell-cycle phases. Am J Pathol. 2001; 158: 1793-1801.
  • 19
    Bjarnason GA, Mackenzie RG, Nabid A, et al. Comparison of toxicity associated with early morning versus late afternoon radiotherapy in patients with head-and-neck cancer: a prospective randomized trial of the National Cancer Institute of Canada Clinical Trials Group (HN3). Int J Radiat Oncol Biol Phys. 2009; 73: 166-172.
  • 20
    Jemal A, Siegel R, Ward E, Murray T, Xu J, Thun MJ. Cancer statistics, 2007. CA Cancer J Clin. 2007; 57: 43-66.
  • 21
    Mujoomdar A, Austin JH, Malhotra R, et al. Clinical predictors of metastatic disease to the brain from non-small cell lung carcinoma: primary tumor size, cell type, and lymph node metastases. Radiology. 2007; 242: 882-888.
  • 22
    Gaspar L, Scott C, Rotman M, et al. Recursive partitioning analysis (RPA) of prognostic factors in 3 Radiation Therapy Oncology Group (RTOG) brain metastases trials. Int J Radiat Oncol Biol Phys. 1997; 37: 745-751.
  • 23
    Weltman E, Salvajoli JV, Brandt RA, et al. Radiosurgery for brain metastases: a score index for predicting prognosis. Int J Radiat Oncol Biol Phys. 2000; 46: 1155-1161.
  • 24
    Hendry JH. Diurnal variations in radiosensitivity of mouse intestine. Br J Radiol. 1975; 48: 312-314.
  • 25
    Hall EJ, Giaccia AJ. Time, dose, and fractionation in radiotherapy. In: HallEJ, GiacciaAJ, eds. Radiobiology for the Radiologist. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2005: 378-397.
  • 26
    Thames HD, Hendry JH. Fractionation in Radiotherapy. 1st ed. London, UK: Taylor & Francis; 1987.
  • 27
    Shadan FF. Circadian tempo: a paradigm for genome stability? Med Hypotheses. 2007; 68: 883-891.
  • 28
    Lee CC. Tumor suppression by the mammalian Period genes. Cancer Causes Control. 2006; 17: 525-530.
  • 29
    Segall LA, Perrin JS, Walker CD, Stewart J, Amir S. Glucocorticoid rhythms control the rhythm of expression of the clock protein, Period2, in oval nucleus of the bed nucleus of the stria terminalis and central nucleus of the amygdala in rats. Neuroscience. 2006; 140: 753-757.
  • 30
    Koyama K, Krozowski Z. Modulation of 11 beta-hydroxysteroid dehydrogenase type 2 activity in Ishikawa cells is associated with changes in cellular proliferation. Mol Cell Endocrinol. 2001; 183: 165-170.
  • 31
    Matsuo T, Yamaguchi S, Mitsui S, Emi A, Shimoda F, Okamura H. Control mechanism of the circadian clock for timing of cell division in vivo. Science. 2003; 302: 255-259.
  • 32
    Hrushesky W, Bjarnason G. The application of chronobiology to cancer medicine. In: DeVitaV, HellmanS, RosenbergS, eds. Cancer: Principles and Practice of Oncology. Philadelphia, PA: J.B. Lippincott Co., 1992: 2666-2686.
  • 33
    Hrushesky WJ, Bjarnason GA. Circadian cancer therapy. J Clin Oncol. 1993; 11: 1403-1417.
  • 34
    Hrushesky W. Timing is everything. Sciences (New York). 1994; 34: 32-37.
  • 35
    Bjarnason G, Hrushesky W. Cancer chronotherapy. In: HrusheskyW, ed. Circadian Cancer Therapy. Boca Raton, FL: CRC Press; 1994: 240-263.
  • 36
    Bjarnason GA, Jordan R. Rhythms in human gastrointestinal mucosa and skin. Chronobiol Int. 2002; 19: 129-140.
  • 37
    Gery S, Komatsu N, Baldjyan L, Yu A, Koo D, Koeffler HP. The circadian gene per1 plays an important role in cell growth and DNA damage control in human cancer cells. Mol Cell. 2006; 22: 375-382.
  • 38
    Grechez-Cassiau A, Rayet B, Guillaumond F, Teboul M, Delaunay F. The circadian clock component BMAL1 is a critical regulator of p21WAF1/CIP1 expression and hepatocyte proliferation. J Biol Chem. 2008; 283: 4535-4542.
  • 39
    Kondratov RV, Antoch MP. Circadian proteins in the regulation of cell cycle and genotoxic stress responses. Trends Cell Biol. 2007; 17: 311-317.
  • 40
    Kang TH, Reardon JT, Kemp M, Sancar A. Circadian oscillation of nucleotide excision repair in mammalian brain. Proc Natl Acad Sci USA. 2009; 106: 2864-2867.