OBJECTIVE: Although it is well known that radiotherapy for prostate cancer 
increases comorbid rate of secondary bladder cancer, the effect of aging and 
smoking with radiotherapy on incidence rate of secondary bladder cancer remains 
unknown. Then, this study investigated the combinational effect of external beam 
radiotherapy for prostate cancer and aging or smoking on comorbid rate of 
secondary bladder cancer.
METHODS: This study included 754 Japanese patients with prostate cancer treated 
with radiotherapy (n = 319) and radical prostatectomy (n = 435) from 2000 
through 2013. The relationship between therapeutic modality for prostate cancer 
as well as age or smoking status and comorbid rate of secondary bladder cancer 
was examined.
RESULTS: During the median follow-up period of 4.3 and 3.1 years, secondary 
bladder cancer occurred in 11 (3.4%) and 5 (1.1%) of patients with prostate 
cancer treated with external beam radiotherapy and radical prostatectomy, 
respectively. The 5-year bladder cancer-free survival rate was 97.3% in the 
external beam radiotherapy group and 99.4% in the radical prostatectomy group. 
Age (hazard ratio = 1.15, P = 0.027) and ever smoking (hazard ratio = 5.65, P 
= 0.011) were significant predictive factors of secondary bladder cancer 
incidence in the external beam radiotherapy cohort, but not in the radical 
prostatectomy cohort. Inversely, among men with ever smoking, but not among 
older men, external beam radiotherapy (hazard ratio = 9.64, P = 0.0052) was a 
significant risk factor of secondary bladder cancer.
CONCLUSIONS: Taken together, these findings suggest that smoking history might 
be one of criteria to choose radical prostatectomy than external beam 
radiotherapy for prostate cancer, and that age would not be a criterion for 
therapeutic selection in terms of secondary bladder cancer.