Association of radiotherapy with secondary pelvic cancers in male patients with rectal cancer.

Purpose: We aimed to explore the risk of secondary prostate cancer (SPC) and secondary bladder cancer (SBC) in male rectal cancer (RC) patients after radiotherapy (RT) and to assess survival outcomes.

Methods: This large population-based study included men with RC from nine registries in the Surveillance, Epidemiology, and End Results (SEER) database between 1973 and 2015. Fine-Gray competing risks and Poisson regression were used to assess the RT-related risk of SPC and SBC in patients who received RT versus those who did not (NRT).

Results: After exclusion, 28,886 RC patients were included in further analysis, including 9763 RT-treated patients (33.8%) and 19,123 patients not treated with RT (66.2%). In competing risk regression analysis, RT was associated with a low risk of developing SPC (adjusted HR = 0.67; 95% CI = 0.64-0.82; P < 0.001) and with a high risk of developing SBC (adjusted HR = 1.44; 95% CI = 1.15-1.80; P = 0.001). In the survival analysis of SPC patients, the NRT group exhibited better 10-year OS and CSS than the RT group (OS: HR = 0.52; 95% CI = 0.43-0.64; P < 0.001; CSS: HR = 0.39; 95% CI = 0.26-0.56; P < 0.001).

Conclusion: Male rectal cancer patients receiving RT had a decreased risk of SPC and an increased risk of SBC, and the prognosis of SPC patients in the RT group was worse compared to that of the NRT group. Follow-up and monitoring of SBC and SPC should not be ignored.