Genetic variants and normal tissue toxicity after radiotherapy: a systematic review

Genetic variants associated with predisposition to prostate cancer and potential clinical implications

Prostate cancer is the commonest cancer in the developed world. There is an inherited component to this disease as shown in familial and twin studies. However, the discovery of these variants has been difficult. The emergence of genome-wide association studies has led to the identification of over 46 susceptibility loci. Their clinical utility to predict risk, response to treatment, or treatment toxicity, remains undefined. Large consortia are needed to achieve adequate statistical power to answer these genetic-clinical and genetic-epidemiological questions. International collaborations are currently underway to link genetic with clinical/epidemiological data to develop risk prediction models, which could direct screening and treatment programs.

Standardized Total Average Toxicity score: a scale- and grade-independent measure of late radiotherapy toxicity to facilitate pooling of data from different studies

PURPOSE

The search for clinical and biologic biomarkers associated with late radiotherapy toxicity is hindered by the use of multiple and different endpoints from a variety of scoring systems, hampering comparisons across studies and pooling of data. We propose a novel metric, the Standardized Total Average Toxicity (STAT) score, to try to overcome these difficulties.

METHODS AND MATERIALS

STAT scores were derived for 1010 patients from the Cambridge breast intensity-modulated radiotherapy trial and 493 women from the University Hospitals of Leicester. The sensitivity of the STAT score to detect differences between patient groups, stratified by factors known to influence late toxicity, was compared with that of individual endpoints. Analysis of residuals was used to quantify the effect of these covariates.

RESULTS

In the Cambridge cohort, STAT scores detected differences (p < 0.00005) between patients attributable to breast volume, surgical specimen weight, dosimetry, acute toxicity, radiation boost to tumor bed, postoperative infection, and smoking (p < 0.0002), with no loss of sensitivity over individual toxicity endpoints. Diabetes (p = 0.017), poor postoperative surgical cosmesis (p = 0.0036), use of chemotherapy (p = 0.0054), and increasing age (p = 0.041) were also associated with increased STAT score. When the Cambridge and Leicester datasets were combined, STAT was associated with smoking status (p < 0.00005), diabetes (p = 0.041), chemotherapy (p = 0.0008), and radiotherapy boost (p = 0.0001). STAT was independent of the toxicity scale used and was able to deal with missing data. There were correlations between residuals of the STAT score obtained using different toxicity scales (r > 0.86, p < 0.00005 for both datasets).

CONCLUSIONS

The STAT score may be used to facilitate the analysis of overall late radiation toxicity, from multiple trials or centers, in studies of possible genetic and nongenetic determinants of radiotherapy toxicity.

Biological complexities in radiation carcinogenesis and cancer radiotherapy: impact of new biological paradigms

Although radiation carcinogenesis has been shown both experimentally and epidemiologically, the use of ionizing radiation is also one of the major modalities in cancer treatment. Various known cellular and molecular events are involved in carcinogenesis. Apart from the known phenomena, there could be implications for carcinogenesis and cancer prevention due to other biological processes such as the bystander effect, the abscopal effect, intrinsic radiosensitivity and radioadaptation. Bystander effects have consequences for mutation initiated cancer paradigms of radiation carcinogenesis, which provide the mechanistic justification for low-dose risk estimates. The abscopal effect is potentially important for tumor control and is mediated through cytokines and/or the immune system (mainly cell-mediated immunity). It results from loss of growth and stimulatory and/or immunosuppressive factors from the tumor. Intrinsic radiosensitivity is a feature of some cancer prone chromosomal breakage syndromes such as ataxia telangectiasia. Radiosensitivity is manifested as higher chromosomal aberrations and DNA repair impairment is now known as a good biomarker for breast cancer screening and prediction of prognosis. However, it is not yet known whether this effect is good or bad for those receiving radiation or radiomimetic agents for treatment. Radiation hormesis is another major concern for carcinogenesis. This process which protects cells from higher doses of radiation or radio mimic chemicals, may lead to the escape of cells from mitotic death or apoptosis and put cells with a lower amount of damage into the process of cancer induction. Therefore, any of these biological phenomena could have impact on another process giving rise to genome instability of cells which are not in the field of radiation but still receiving a lower amount of radiation. For prevention of radiation induced carcinogenesis or risk assessment as well as for successful radiation therapy, all these phenomena should be taken into account.

Prediction of normal tissue toxicity as part of the individualized treatment with radiotherapy in oncology patients

Normal tissue toxicity caused by radiotherapy conditions the success of the treatment and the quality of life of patients. Radiotherapy is combined with surgery in both the preoperative or postoperative setting for the treatment of most localized solid tumour types. Furthermore, radical radiotherapy is an alternative to surgery in several tumour locations. The possibility of predicting such radiation-induced toxicity would make possible a better treatment schedule for the individual patient. Radiation-induced toxicity is, at least in part, genetically determined. From decades, several predictive tests have been proposed to know the individual sensitivity of patients to the radiotherapy schedules. Among them, initial DNA damage, radiation-induced apoptosis, gene expression profiles, and gene polymorphisms have been proposed. We report here an overview of the main studies regarding to this field. Radiation-induced apoptosis in peripheral blood lymphocytes seem to be the most promising assay tested in prospective clinical trials, although they have to be validated in large clinical studies. Other promising assays, as those related with single nucleotide polymorphisms, need to be validated as well.

Biologically effective dose and breast cancer conservative treatment: is duration of radiation therapy really important?

To evaluate if biologically effective dose (BED), and in particular the duration of radiation treatment, has an effect on local relapse risk. Between January 2000 and December 2008 a total of 762 patients with T1-2 N0/+ breast cancer was treated with breast-conserving surgery and radiotherapy, with and without hormone therapy and chemotherapy. Adjuvant radiation therapy was administered to a total dose of 60-66 Gy in 30-33 fractions. The computed BEDs were divided in four groups: <43.1, 43.1-44.9, 45.0-46.1, and >46.1 Gy (A-D, respectively). Kaplan-Meier method was used to calculate local relapse rates. Cox regression method was used to identify prognostic factors of local relapse. Evaluated variables were age, tumor histology, tumor size, surgical margin status, axillary nodal status, tumor grading, adjuvant therapies, adjuvant chemotherapy alone, adjuvant hormone therapy alone, adjuvant anthracyclines, and BEDs values. 8-year local relapse rates were 18.0% for group A, 8.5% for group B, 4.6% for group C, and 2.7% for group D (P=0.008). Multivariate Cox regression analysis showed that BEDs values were associated with higher local relapse risk (P=0.001). In our study, a prolongation of radiotherapy treatment, intended as a lower BED value, after breast-conserving surgery is associated with an increased risk of local relapse. Considering the wide range of results published in other studies, hypofractionation for breast cancer should be considered, at the moment, feasible in selected patients.

[Radiosensitivity assays of normal tissues]

Radiotherapy allows locoregional control with systemic impact in some indications. Technologic advances decrease the dose received by normal tissues leading to a low crude number of late side effects near to 5%. Intrinsic radiosensitivity are still of interest in this context of high level of technology and optimized treatments. Assays of radiosensitivity are detailed in this article arguing the negative results but also the perspectives.

Genetics and genomics of radiotherapy toxicity: towards prediction

Radiotherapy is involved in many curative treatments of cancer; millions of survivors live with the consequences of treatment, and toxicity in a minority limits the radiation doses that can be safely prescribed to the majority. Radiogenomics is the whole genome application of radiogenetics, which studies the influence of genetic variation on radiation response. Work in the area focuses on uncovering the underlying genetic causes of individual variation in sensitivity to radiation, which is important for effective, safe treatment. In this review, we highlight recent advances in radiotherapy and discuss results from four genome-wide studies of radiotoxicity.

Transcriptional modulation induced by ionizing radiation: p53 remains a central player

The cellular response to DNA damage is vital for maintaining genomic stability and preventing undue cell death or cancer formation. The DNA damage response (DDR), most robustly mobilized by double-strand breaks (DSBs), rapidly activates an extensive signaling network that affects numerous cellular systems, leading to cell survival or programmed cell death. A major component of the DDR is the widespread modulation of gene expression. We analyzed together six datasets that probed transcriptional responses to ionizing radiation (IR) - our novel experimental data and 5 published datasets - to elucidate the scope of this response and identify its gene targets. According to the mRNA expression profiles we recorded from 5 cancerous and non-cancerous human cell lines after exposure to 5 Gy of IR, most of the responses were cell line-specific. Computational analysis identified significant enrichment for p53 target genes and cell cycle-related pathways among groups of up-regulated and down-regulated genes, respectively. Computational promoter analysis of the six datasets disclosed that a statistically significant number of the induced genes contained p53 binding site signatures. p53-mediated regulation had previously been documented for subsets of these gene groups, making our lists a source of novel potential p53 targets. Real-time qPCR and chromatin immunoprecipitation (ChIP) assays validated the IR-induced p53-dependent induction and p53 binding to the respective promoters of 11 selected genes. Our results demonstrate the power of a combined computational and experimental approach to identify new transcriptional targets in the DNA damage response network.

Association between single nucleotide polymorphisms in the XRCC1 and RAD51 genes and clinical radiosensitivity in head and neck cancer

PURPOSE

Individual variability in radiosensitivity is large in cancer patients. Single nucleotide polymorphisms (SNPs) in genes involved in DNA repair and in protection against reactive oxygen species (ROS) could be responsible for such cases of radiosensitivity. We investigated the association between the occurrence of acute reactions in 101 patients with squamous cell carcinoma of the head and neck (SCCHN) after radiotherapy (RT) and five genetic polymorphisms: XRCC1 c.1196A>G, XRCC3 c.722C>T, RAD51 (c.-3429G>C, c.-3392G>T), and GSTP1 c.313A>G.

MATERIALS AND METHODS

Genetic polymorphisms were detected by high resolution melting analysis (HRMA). The development of acute reactions (oral mucositis, skin erythema and dysphagia) associated with genetic polymorphisms was modeled using Cox proportional hazards, accounting for biologically effective dose (BED).

RESULTS

Development of grade ≥2 mucositis was increased in all patients (chemo-radiotherapy and radiotherapy alone) with XRCC1-399Gln allele (HR=1.72). The likelihood of developing grade ≥2 dysphagia was higher in carriers of RAD51 c.-3429 CC/GC genotypes (HR=4.00). The presence of at least one SNP or the co-presence of both SNPs in XRCC1 p.Gln399Arg /RAD51 c.-3429 G>C status were associated to higher likelihood of occurrence of acute toxicities (HR=2.03).

CONCLUSIONS

Our findings showed an association between genetic polymorphisms, XRCC1 c.1196A>G and RAD51 c.-3429 G>C, and the development of radiation-induced toxicities in SCCHN patients.

Identification of SNPs associated with susceptibility for development of adverse reactions to radiotherapy

Although cancer treatment with radiation can produce high cure rates, adverse effects often result from radiotherapy. These toxicities are manifested as damage to normal tissues and organs in the radiation field. In recognition of the substantial variation in the intrinsic response of individuals to radiation, an effort began approximately 10 years ago to discover the genetic markers, primarily SNPs, which are associated with susceptibility for the development of these adverse responses to radiation therapy. The goal of this research is to identify the SNPs that could serve as the basis of an assay to predict which cancer patients are most likely to develop complications resulting from radiotherapy. This would permit personalization and optimization of the treatment plan for each cancer patient.

Association of XRCC1 variants with acute skin reaction after radiotherapy in breast cancer patients

After irradiation results in cytotoxic effects by DNA damage, base excision repair (BER) pathway is involved in the repair of single-strand breaks and nonhomologous end joining and homologous repair of double-strand breaks caused by radiotherapy. Alterations in the function of BER DNA repair genes may affect DNA repair proficiency and influence the response of patients with cancer to radiotherapy. The association of single nucleotide polymorphisms of BER DNA repair X-ray repair cross-complementing group 1 protein (XRCC1) and risk of radiotherapy-induced ≥grade 2 acute skin reaction in patients with breast cancer was examined. It was found that the risk of ≥grade 2 acute skin toxicity after radiotherapy could be increased by 2.86-fold in patients carrying the XRCC1 -77TC and CC genotypes (p = 0.016). However, the other three coding XRCC1 variants did not influence the risk of ≥grade 2 acute skin toxicity for patients with breast cancer after radiotherapy. Our results suggested that the XRCC1 polymorphism is associated with increased risk of radiation-induced acute skin reaction in a Chinese population.

Radiotherapy-induced myelosuppression in dogs: 103 cases (2002-2006)

Definitive radiotherapy refers to delivery of large doses, typically 48-62 Gray, of ionizing radiation over several weeks using a daily or alternate-day fractionation schedule. The impact of definitive radiotherapy alone on haematopoiesis in tumour-bearing dogs is unknown. Medical records from 103 dogs receiving definitive (60) Cobalt teletherapy for cancer over a 5-year period were reviewed for signalment, tumour type and location, total radiotherapy dose and fractionation scheme. Complete blood count data were collected before, halfway through, and at the end of radiation treatment, and analysed for changes associated with patient variables. The results demonstrate significant reductions in haematocrit, total white blood cell count, neutrophils, eosinophils, monocytes, lymphocytes and platelets occurred during definitive radiotherapy but remained within laboratory reference intervals. These data are important for anticipation of toxicity associated with combinations of radiotherapy and chemotherapy in dogs but do not support the routine monitoring of haematology parameters during definitive radiotherapy.

Association between single nucleotide polymorphisms in the gene for XRCC1 and radiation-induced late toxicity in prostate cancer patients

BACKGROUND AND PURPOSE

Polymorphisms in genes responsible for DNA damage signaling and repair might modulate DNA repair capacity and, therefore, affect cell and tissue response to radiation and influence individual radiosensitivity. The purpose of the present prospective investigation was to evaluate the association of single nucleotide polymorphisms in XRCC1 with radiation-induced late side effects in prostate cancer patients treated with radiotherapy.

MATERIAL AND METHODS

To analyze the role of XRCC1 polymorphisms for late toxicity 603 participants from the Austrian PROCAGENE study treated with three-dimensional conformal radiotherapy were included in the present investigation. Three non-synonymous candidate polymorphisms in the X-ray repair cross-complementing group 1 (XRCC1) gene (Arg194Trp; Arg280His; Arg399Gln) were selected and determined by 5´-nuclease (TaqMan) assays.

RESULTS

Within a median follow-up time of 35 months, 91 patients (15.7%) developed high-grade late toxicities (defined as late bladder and/or rectal toxicity RTOG≥2). In a Kaplan-Meier analysis, carriers of the XRCC1 Arg280His polymorphism were at decreased risk of high-grade late toxicity (p=0.022), in multivariate analysis including clinical and dosimetric parameters as potential confounders the XRCC1 Arg280His polymorphism remained a significant predictor for high-grade late toxicity (HR=0.221, 95% CI 0.051-0.956; p=0.043). No significant associations were found for the remaining polymorphisms.

CONCLUSIONS

We conclude that the XRCC1 Arg280His polymorphism may be protective against the development of high-grade late toxicity after radiotherapy in prostate cancer patients.

Test of association between variant tgβ1 alleles and late adverse effects of breast radiotherapy

PURPOSE

To test for association between single nucleotide polymorphisms at the TGFβ1 locus and the risk of late normal tissue injury following whole breast radiotherapy.

METHODS

A retrospective study compared the number of variant alleles at -509 and codons 10 and 25 of the TGFβ1 locus in women followed up in two prospective clinical trials who developed either marked radiotherapy adverse effects or no adverse effects after matching on fractionation schedule, breast size, surgical deficit, chemotherapy and length of follow up.

RESULTS

Median follow up in the two trials was 7.4 (maximum 15) years and 5.3 (maximum 5.3) years. 1237/1716 (72%) women with photographic assessments of radiotherapy adverse effects were alive and well, and 147/1237 (12%) potential cases with the most marked change in photographic change in breast appearance were matched to potential controls recording no change. In an unmatched analysis of 82 cases and 108 controls, no significant difference in the number of genetic variants was observed.

CONCLUSIONS

No association was detected between sequence variations at the TGFβ1 locus and the risk of late adverse effects of breast radiotherapy.

Single nucleotide polymorphisms of DNA repair genes as predictors of radioresponse

Radiation therapy is a key modality in the treatment of cancer. Substantial progress has been made in unraveling the molecular events which underpin the responses of malignant and surrounding normal tissues to ionizing radiation. An understanding of the genes involved in processes such as DNA double-strand break repair, DNA damage response, cell-cycle control, apoptosis, cellular antioxidant defenses, and cytokine production, has evolved toward examination of how genetic variants, most often, single nucleotide polymorphisms (SNPs), may influence interindividual radioresponse. Experimental approaches, such as candidate SNP-association studies, genome-wide association studies, and massively parallel sequencing are being proposed to address these questions. We present a focused review of the evidence supporting an association between SNPs in DNA repair genes and radioresponse in normal tissues and tumors. Although preliminary results indicate possible associations, there are methodological weaknesses in many of the studies, and independent validation of SNPs as biomarkers of radioresponse in much larger cohorts will likely require research cooperation through international consortia.

Genome-wide association study to identify single nucleotide polymorphisms (SNPs) associated with the development of erectile dysfunction in African-American men after radiotherapy for prostate cancer

PURPOSE

To identify single nucleotide polymorphisms (SNPs) associated with erectile dysfunction (ED) among African-American prostate cancer patients treated with external beam radiation therapy.

METHODS AND MATERIALS

A cohort of African-American prostate cancer patients treated with external beam radiation therapy was observed for the development of ED by use of the five-item Sexual Health Inventory for Men (SHIM) questionnaire. Final analysis included 27 cases (post-treatment SHIM score ≤7) and 52 control subjects (post-treatment SHIM score ≥16). A genome-wide association study was performed using approximately 909,000 SNPs genotyped on Affymetrix 6.0 arrays (Affymetrix, Santa Clara, CA).

RESULTS

We identified SNP rs2268363, located in the follicle-stimulating hormone receptor (FSHR) gene, as significantly associated with ED after correcting for multiple comparisons (unadjusted p = 5.46 × 10(-8), Bonferroni p = 0.028). We identified four additional SNPs that tended toward a significant association with an unadjusted p value < 10(-6). Inference of population substructure showed that cases had a higher proportion of African ancestry than control subjects (77% vs. 60%, p = 0.005). A multivariate logistic regression model that incorporated estimated ancestry and four of the top-ranked SNPs was a more accurate classifier of ED than a model that included only clinical variables.

CONCLUSIONS

To our knowledge, this is the first genome-wide association study to identify SNPs associated with adverse effects resulting from radiotherapy. It is important to note that the SNP that proved to be significantly associated with ED is located within a gene whose encoded product plays a role in male gonad development and function. Another key finding of this project is that the four SNPs most strongly associated with ED were specific to persons of African ancestry and would therefore not have been identified had a cohort of European ancestry been screened. This study demonstrates the feasibility of a genome-wide approach to investigate genetic predisposition to radiation injury.

Genetic variants of NPAT-ATM and AURKA are associated with an early adverse reaction in the gastrointestinal tract of patients with cervical cancer treated with pelvic radiation therapy

PURPOSE

This study sought to associate polymorphisms in genes related to cell cycle regulation or genome maintenance with radiotherapy (RT)-induced an early adverse reaction (EAR) in patients with cervical cancer.

METHODS AND MATERIALS

This study enrolled 243 cervical cancer patients who were treated with pelvic RT. An early gastrointestinal reaction was graded using the National Cancer Institute Common Toxicity Criteria, version 2. Clinical factors of the enrolled patients were analyzed, and 208 patients were grouped for genetic analysis according to their EAR (Grade ≤1, n = 150; Grade ≥2, n = 58). Genomic DNA was genotyped, and association with the risk of EAR for 44 functional single-nucleotide polymorphisms (SNPs) of 19 candidate genes was assessed by single-locus, haplotype, and multilocus analyses.

RESULTS

Our analysis revealed two haplotypes to be associated with an increased risk of EAR. The first, comprising rs625120C, rs189037T, rs228589A, and rs183460G, is located between the 5' ends of NPAT and ATM (OR = 1.86; 95% CI, 1.21-2.87), whereas the second is located in the AURKA gene and comprises rs2273535A and rs1047972G (OR = 1.75; 95% CI, 1.10-2.78). A third haplotype, rs2273535T and rs1047972A in AURKA, was associated with a reduced EAR risk (OR = 0.42; 95% CI, 0.20-0.89). The risk of EAR was significantly higher among patients with both risk diplotypes than in those possessing the other diplotypes (OR = 3.24; 95% CI, 1.52-6.92).

CONCLUSIONS

Individual radiosensitivity of intestine may be determined by haplotypes in the NPAT-ATM and AURKA genes. These variants should be explored in larger association studies in cervical cancer patients.

Radiation pharmacogenomics: a genome-wide association approach to identify radiation response biomarkers using human lymphoblastoid cell lines

Radiation therapy is used to treat half of all cancer patients. Response to radiation therapy varies widely among patients. Therefore, we performed a genome-wide association study (GWAS) to identify biomarkers to help predict radiation response using 277 ethnically defined human lymphoblastoid cell lines (LCLs). Basal gene expression levels and 1.3 million genome-wide single nucleotide polymorphism (SNP) markers from both Affymetrix and Illumina platforms were assayed for all 277 human LCLs. MTS [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium] assays for radiation cytotoxicity were also performed to obtain area under the curve (AUC) as a radiation response phenotype for use in the association studies. Functional validation of candidate genes, selected from an integrated analysis that used SNP, expression, and AUC data, was performed with multiple cancer cell lines using specific siRNA knockdown, followed by MTS and colony-forming assays. A total of 27 loci, each containing at least two SNPs within 50 kb with P-values less than 10(-4) were associated with radiation AUC. A total of 270 expression probe sets were associated with radiation AUC with P < 10(-3). The integrated analysis identified 50 SNPs in 14 of the 27 loci that were associated with both AUC and the expression of 39 genes, which were also associated with radiation AUC (P < 10(-3)). Functional validation using siRNA knockdown in multiple tumor cell lines showed that C13orf34, MAD2L1, PLK4, TPD52, and DEPDC1B each significantly altered radiation sensitivity in at least two cancer cell lines. Studies performed with LCLs can help to identify novel biomarkers that might contribute to variation in response to radiation therapy and enhance our understanding of mechanisms underlying that variation.

Genome wide screen identifies microsatellite markers associated with acute adverse effects following radiotherapy in cancer patients

Background

The response of normal tissues in cancer patients undergoing radiotherapy varies, possibly due to genetic differences underlying variation in radiosensitivity.

Methods

Cancer patients (n = 360) were selected retrospectively from the RadGenomics project. Adverse effects within 3 months of radiotherapy completion were graded using the National Cancer Institute Common Toxicity Criteria; high grade group were grade 3 or more (n = 180), low grade group were grade 1 or less (n = 180). Pooled genomic DNA (gDNA) (n = 90 from each group) was screened using 23,244 microsatellites. Markers with different inter-group frequencies (Fisher exact test P < 0.05) were analyzed using the remaining pooled gDNA. Silencing RNA treatment was performed in cultured normal human skin fibroblasts.

Results

Forty-seven markers had positive association values; including one in the SEMA3A promoter region (P = 1.24 × 10^-5). SEMA3A knockdown enhanced radiation resistance.

Conclusions

This study identified 47 putative radiosensitivity markers, and suggested a role for SEMA3A in radiosensitivity.

Excessive Soft Tissue Reaction after Stereotactic Body Radiation Therapy in a Woman with Four Different Cancer Diagnoses

Patients experiencing several cancers can be a challenge, as optimal treatment options for the different cancers might interfere with each other. In this case report, we present a woman diagnosed with 4 different types of cancer. She was treated with surgery, chemotherapy and radiotherapy. Her performance status was generally good, and she tolerated the treatment very well, except some troublesome side effects in the thoracic soft tissue after stereotactic body radiotherapy.

No association between SNPs regulating TGF-β1 secretion and late radiotherapy toxicity to the breast: results from the RAPPER study

BACKGROUND AND PURPOSE

Several small studies have reported associations between TGFB1 single nucleotide polymorphisms (SNPs), considered to increase secretion of TGF-β1, and greater than 3-fold increases in incidence of fibrosis - an indicator of late toxicity after radiotherapy in breast cancer patients.

MATERIALS AND METHODS

Two SNPs in TGFB1, C-509T (rs1800469) and L10P (rs1800470), were genotyped in 778 breast cancer patients who had received radiotherapy to the breast. Late radiotherapy toxicity was assessed two years after radiotherapy using a validated photographic technique, clinical assessment and patient questionnaires.

RESULTS

On photographic assessment, 210 (27%) patients showed some degree of breast shrinkage, whilst 45 (6%) patients showed marked breast shrinkage. There was no significant association of genotype at either of the TGFB1 SNPs with any measure of late radiation toxicity.

CONCLUSION

This adequately powered trial failed to confirm previously reported increases in fibrosis with TGFB1 genotype - any increase greater than 1.36 can be excluded with 95% confidence. Similar frequent failures to replicate associations with candidate genes have been resolved using genome-wide association scans: this methodology detects common, low risk alleles but requires even larger patient numbers for adequate statistical power.