Secondary bladder cancer after permanent iodine-125 brachytherapy for prostate cancer

Bladder cancer has been observed in patients after permanent iodine-125 brachytherapy （hereafter referred to as brachytherapy） for prostate cancer. We investigated secondary bladder cancer after brachytherapy. From January 2005 to March 2021, a total of 1,478 patients underwent brachytherapy for localized prostate cancer at Showa University Hospital and Showa University Koto Toyosu Hospital. A standardized incidence rate was calculated from the incidence of secondary bladder cancer after brachytherapy and compared with the national cancer registry general incidence data. The median follow-up was 4.7 years. Secondary bladder cancer occurred in 15 of 1478 patients （median age: 71 years）. The incidence was 198 cases/100,000 persons/year. The incidence was 2.84 times higher than that of Japan’s national cancer registry. In addition, as a characteristic of secondary bladder cancer, the malignancy tended to be high regardless of the depth of invasion. The relative risk between cases with a secondary bladder cancer period until occurrence of ＞5 years and cases with a secondary bladder cancer period until occurrence of ≤5 years was 1.61 times （95％ CI: 0.57-4.50）, but the difference was not significant. In this study, we found that the incidence of bladder cancer was 2.84 times higher than that of the general population of Japan, suggesting that brachytherapy increases the risk of developing bladder cancer. To our knowledge, this study is the first to compare the incidence of brachytherapy and general bladder cancer in Japan. The results of this study suggest that although brachytherapy is effective for prostate cancer, attention should be paid to the early detection of bladder cancer because of the risk of secondary cancer development.