| Summary: | Background Previous studies have confirmed the higher risk of bladder cancer (BC) and rectal cancer (RC) development among prostate cancer (PCa) patients receiving radiotherapy. In this study, we intend to explore the long-term trend in second BC and RC incidence among PCa patients undergoing radiotherapy. Method We identified first primary PCa patients diagnosed between 1975 and 2014 from the Surveillance, Epidemiology, and End Results (SEER)-9 cancer registries. Standardized incidence ratios (SIRs) were calculated by calendar year of diagnosis among PCa patients receiving radiotherapy and not. P trends were evaluated using Poisson regression. 10-year cumulative incidence of BC and RC was calculated utilizing competing risk regression model. Result Of PCa patients treated with radiotherapy, SIRs of BC increased from .82 (95% CI: .35- 1.61) in 1980–1984 to 1.58 (95% CI: 1.48–1.68) in 2010-2014 ( Ptrend =.003). SIRs of RC increased from 1.01 (95% CI: .27-2.58) in 1980–1984 to 1.54 (95% CI: 1.31–1.81) in 2010-2014 ( Ptrend =.025). No statistically significant change in both BC and RC incidence was observed. The 10-year cumulative incidence of BC increased from 1975–1984 (.04%) to 2005–2014 (.15%) among PCa treated with radiotherapy. Simultaneously, the 10-year cumulative incidence of RC was demonstrated to range from 1975–1984 (.02%) to 2005–2014 (.11%). Conclusion we have observed an increasing trend in second BC and RC incidence in PCa patients receiving radiotherapy. There was no significant change in the incidence of second BC and RC in PCa without radiotherapy. These results reflect the increasing clinical burden of second malignant tumors in PCa patients undergoing radiotherapy.
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