D ISCUSSION

This study was performed to analyze the influence of nongenetic and genetic factors on the development of acute radiation-induced nocturia. Significant associations were found between the TGFB1 -509 C>T and codon 10 T>C variant alleles and the development of RT-induced acute nocturia in prostate cancer patients. Also, primary IMRT (as opposed to postoperative IMRT) and the presence of pretreatment nocturia contribute to the variability in radiation toxicity.

This study is the first that examines genetic markers in TGFB1 and their association with acute nocturia following IMRT for prostate cancer patients. The polymorphisms in TGFB1 have been extensively examined in normal tissue radiobiology. Much research has been performed to find associations between SNPs in the TGFB1 gene and several RT-induced late adverse events (13). These results show no consistency, however. Currently, the radiogenomics consortium is working on a meta-analysis of published and unpublished data to confirm or refute the relationship between TGFB1 SNPs and late radiotoxicity (20). Only 2 studies have reported on the correlation of SNPs in the TGFB1

gene and acute radiation toxicity. Whereas Suga et al (16) could not find an association between -509 C>T and early skin reactions in Japanese breast cancer patients, Zhang et al (17) were able to demonstrate an association of the variant allele of -509 C>T with an increased risk of acute radiation-induced

oesophageal toxicity in Chinese lung cancer patients. Our study demonstrates a significant association between the variant alleles of TGFB1 -509 C>T and codon 10 T>C and the occurrence of radiation-induced acute nocturia in prostate cancer patients.

In this study, 1 specific radiation-induced side effect –nocturia- was considered. Former studies (13) used a single grade that resulted of combining multiple symptoms such as haematuria, dysuria, urgency, nocturia, and increased frequency. Using this approach in our study, 105 instead of 82 patients would have been categorized as having RT-induced acute GU toxicity. Subdividing the patients in this manner leads to the loss of the significant associations between the TGFB1 polymorphisms (-509 C>T and codon 10 T>C) and the general radiation-induced GU injury (data not shown). This illustrates that combining multiple symptoms can mask significant effects and confound the analysis.

In this study, pretreatment nocturia was strongly linked to the development of acute RT-induced nocturia. This was also found in the study of Peeters et al (6). Registering pretreatment symptoms data is mandatory, but omitting the registration implicates that the pretreatment symptom cannot be included as a confounding factor, leading to an inaccurate quantification of RT-induced toxicity. Jereczek-Fossa et al (7) showed that the development of acute genitourinary side effects is strongly correlated with the development of late events. This is also the case for nocturia in our study. Of the patients developing acute grade ≥2 nocturia, 51% developed at least late grade 1 nocturia (ie, 2-3 times/night), and 38% developed late grade ≥2 nocturia (>4 times/night or medication needing).

Our study shows that the incidence of acute nocturia was 3 times lower in patients treated with postoperative IMRT compared with patients treated with definitive IMRT. This is in accordance with Zelefsky et al (11). Therefore, we hypothesize that edema of the prostatic urethra is the predominant factor contributing to acute urinary complications. This might also explain the excellent response to α-blocker therapy (1). The lack of a significant response of nonsteroidal anti-inflammatory drugs (NSAIDs) suggests that the inflammatory component is clinically less important (11). Nevertheless, the current study indicates that TGFB1, a mediator of inflammation, is one of the many factors

involved in the pathogenesis of RT-induced nocturia. Because evidence exists that acute GU toxicity originates from damage to the urethra, we introduced a surrogate for the urethral dose, the CTVmedian. The difference between patients with and without acute nocturia was found to be clinically irrelevant. This raises the question of whether the parameter chosen is an adequate surrogate for the urethral dose. Nevertheless, in future work, it will be necessary to study the complete dose-volume histogram of the urethra. This will be difficult because it is not easy to delineate the urethra.

A strength of our investigation is the relatively large number of patients enrolled. We were also able to build a nearly complete data set with few missing values (with the exception of hypercholesterolemia status). Furthermore, patients were from an ethnically homogeneous population, and patient recruitment as well as clinical outcome data collection were carried out prospectively. We investigated 1 endpoint of RT-induced GU injury and were able to include a large number of clinical and treatment variables, including the presence of pretreatment symptoms. Nevertheless, a major issue in genetic association studies is the increasing risk for false-positive findings. We anticipated this problem by controlling the false discovery rate by means of the Benjamini-Hochberg procedure. Although the associations hold after correcting for multiple testing, the results of this study should be validated in an independent study.

In conclusion, radical prostatectomy and the presence of mild nocturia before therapy and the variant alleles of TGFB1 -509 C>T and codon 10 T>C are identified as factors involved in the development of acute radiation-induced nocturia. Because a specific symptom was investigated, this study is unique and is also a call for standardization of radiation toxicity assessment in RT treatment of prostate cancer. The results of this study may be useful in research focusing on prediction of late severe nocturia after IMRT for prostate cancer.