Secondary bladder and rectal cancer risk estimates following standard fractionated and moderately hypofractionated VMAT for prostate carcinoma.

Abstract

To estimate the risk for bladder and rectal cancer induction due to standard fractionated (SF) and moderately hypofractionated (HF) volumetric modulated arc therapy (VMAT) for prostate carcinoma. Twelve patients with low or intermediate-risk of prostate cancer referred for external-beam radiotherapy were included in this study. Three computed tomography-based VMAT plans were created for each study participant. The first plan was generated by assuming patient's irradiation with SF-VMAT (78Gy in 39 fractions). The second and third plans were created on the basis of two different HF schedules (HF-VMAT1 : 70Gy in 30 fractions, HF:VMAT2 : 60Gy in 20 fractions). Data from differential dose-volume histograms obtained by the above treatment plans were employed to calculate the organ equivalent dose (OED) of the bladder and rectum with the aid of a nonlinear model accounting for fractionation and proliferation effects. The calculated OED values were used to estimate the average lifetime attributable risk (LARav ) for the appearance of radiotherapy-induced secondary bladder and rectal malignancies. The lifetime risk of radiation carcinogenesis was compared with the respective organ-, and age-dependent lifetime intrinsic risk (LIR) of cancer development for unexposed males. The average OED of the rectum from SF-VMAT, HF-VMAT1 and HF-VMAT2 for prostate cancer was 972.0, 900.2, and 815.7cGy, respectively. The corresponding values for bladder were 73.4, 72.3, and 71.0cGy. The LARav for rectal cancer induction varied from 0.06% to 0.4% by the fractionation schedule used for irradiation and by the age of the patient at the time of treatment. The corresponding risk range related to the development of secondary bladder malignancies was 0.06-0.33%. The SF-VMAT, HF-VMAT1 and HF-VMAT2 led to an increase of the lifetime rectal cancer risk with respect to LIR by 2.2-9.8%, 2.0-9.1% and 1.8-8.2%, respectively, depending upon the patient's age. The corresponding elevation for bladder cancer induction was up to 8.0%, 7.9% and 7.7%. The use of VMAT for prostate carcinoma leads to a noteworthy increase of the lifetime risk for bladder and rectal cancer induction compared to that of unexposed people irrespective of the patient's age at the time of treatment and the applied fractionation scheme. The cancer risk data presented in this study may be taken into account by radiation oncologists and medical physicists in the selection of the optimal radiation therapy plan.