Effects of Radiation Therapy on the Incidence of Bladder Cancer and Rectal Cancer in Patients with Prostate Cancer: A Nationwide Nested Case-Control Study in Korean Population.

Purpose: This study aimed to assess the risk of secondary bladder and colorectal cancers following radiotherapy for prostate cancer using a nationwide nested case-control design based on Korea's Health Insurance Review and Assessment database.

Materials and methods: We conducted a nationwide nested case-control study using database, including men newly diagnosed with prostate cancer between 2011 and 2020. Patients were classified into radiotherapy and non-radiotherapy groups and matched 1:1 based on age, comorbidity index, and follow-up duration. Conditional logistic regression was used to assess the association between radiotherapy and secondary bladder and colorectal cancer incidences.

Results: In the crude cohort, radiotherapy was not significantly associated with secondary bladder or colorectal cancer. Kaplan-Meier analysis showed no significant differences in the cumulative incidence between radiotherapy and non-radiotherapy groups. However, nested case-control analysis revealed a significantly increased risk of secondary bladder cancer in the radiotherapy group (adjusted hazard ratio, 1.38; 95% confidence interval, 1.02 to 1.86), while no association was found for colorectal cancer (adjusted hazard ratio, 0.96; 95% confidence interval, 0.72 to 1.27).

Conclusions: Radiotherapy was linked to an increased bladder cancer risk but not colorectal cancer risk, underscoring the need for targeted long-term surveillance.