Does radiotherapy increase the risk of colorectal cancer among prostate cancer patients? A large population-based study

Objective: The survival of prostate cancer (PC) patients after radiotherapy (RT) has improved over time, but it raises the debate of increased risk of secondary colorectal cancer (SCRC). This study aimed to assess whether RT for PC treatment increases the risk of SCRC in comparison with radical prostatectomy (RP). Methods: A population-based cohort of PC patients treated only with RT or only with RP between January 2007 and December 2015 was identified from the Taiwan Cancer Registry. The incidence rate of SCRC development was estimated using Cox regression model. Results: In this study, total 8,797 PC patients treated with either RT (n = 3,219) or RP (n =5,578). Patients subjected to RT were elder (higher percentage of 70years, p < 0.0001) and more advanced clinically (stage III: 22.90% vs. 11.87%; stage IV: 22.15% vs. 13.80%, p < 0.0001), compared to those subjected to RP. More patients subjected to RT had a much higher percentage of autoimmune disease (22.34% vs. 18.75%, p < 0.0001) and osteoarthritis and allied disorders (16.31% vs. 12.98%, p < 0.0001). Besides, RT patients had a higher percentage of underlying Crohn's disease (0.25% vs. 0.05%, p = 0.0230). Although almost all selected factors were not statistically significant, they presented the positive risk of SCRC for those under RP compared with those among RT. Besides, for PC patients in clinical stage I and II, patients with RP may have borderline significantly protective effects of SCRC compared with those under RT (stage I, HR: 0.14; 95% C.I.:0.01-1.39; p = 0.0929; stage II, HR: 1.92; 95% C.I.:0.93-3.95; p = 0.0775). Kaplan-Meier curves for a 3-year-period, which demonstrated no statistical difference in the risk of SCRC free between PC patients undergoing RT and RP (p = 0.9766). Conclusion: Whether or not pelvic RT for PC is associated with an increased risk for SCRC on a population-based level remains a matter of considerable debate. From a clinical perspective, these PC survivors should be counseled accordingly and received continued cancer surveillance with regular colonoscopy follow-up.