Risk of second primary cancer after radiotherapy for localized prostate cancer: A single-center Korean cohort study

The association between radiotherapy (RT) for localized prostate cancer and second primary cancer (SPC) remains controversial. We evaluated the incidence and risk of SPC after RT in a Korean single-center cohort.

We retrospectively analyzed 1,464 patients with localized prostate cancer treated between 2007 and 2018. SPC incidence was compared between RT and non-RT groups using Cox proportional hazards and Fine-Gray competing risk regression models. Standardized incidence ratios (SIRs) were calculated using age- and calendar year-specific incidence rates from the Korea Central Cancer Registry. Propensity score matching (PSM) was performed as a sensitivity analysis.

During follow-up, 155 patients (10.6%) developed SPC. RT was associated with a higher risk of SPC compared with non-RT in multivariable Cox analysis (hazard ratio [HR] 3.43, 95% confidence interval [CI] 2.43-4.84) and Fine-Gray competing risk regression (subdistribution HR 3.32, 95% CI 2.36-4.67). SIR analysis demonstrated a markedly increased incidence of secondary bladder cancer in the RT group (SIR 11.51, 95% CI 6.83-18.20), whereas no increase was observed in the non-RT group. In contrast, colorectal cancer showed only a modest elevation after RT. In the PSM cohort, RT remained significantly associated with SPC in univariate analysis. Exploratory analyses according to RT modality and radiation field showed no significant differences.

RT for localized prostate cancer was associated with an increased occurrence of SPC, particularly bladder cancer. These findings should be interpreted cautiously given the retrospective design and potential residual confounding.