Association between radiotherapy laterality and cardiovascular disease mortality in male breast cancer patients receiving postoperative radiotherapy: a competing risks analysis and standardized mortality ratio assessment using SEER data

Background The association between radiotherapy (RT) laterality and cardiovascular disease (CVD) mortality in male breast cancer (MBC) remains unknown. We evaluated whether left-sided RT confers higher CVD mortality than right-sided RT in MBC patients, and quantified excess CVD mortality relative to the general U.S. male population. Methods This retrospective cohort study included 2,152 MBC patients receiving postoperative RT from the SEER database (2000–2020). Fine-Gray competing risks regression estimated subdistribution hazard ratios (sHRs) for CVD death comparing left- vs. right-sided RT. Multiple sensitivity analyses were performed, including propensity score matching (PSM). Standardized mortality ratios (SMRs) were calculated using age-specific CVD mortality rates from CDC WONDER. Results Over a median follow-up of 5.6 years, no significant difference in CVD mortality was observed between left- and right-sided RT (fully adjusted sHR = 1.03; 95% CI: 0.71–1.49; P = 0.887). Results remained consistent across all sensitivity analyses (all P > 0.750). Compared with the general U.S. male population, irradiated MBC patients had a significantly elevated risk of CVD death (SMR = 1.36; 95% CI: 1.15–1.61). Age-stratified analysis showed the numerically highest point estimate in the 45–54 age group (SMR = 2.08; 95% CI: 0.84–4.29), though this did not reach statistical significance. Conclusions In MBC patients receiving modern RT, left- and right-sided treatment confer comparable risks of CVD mortality. Nevertheless, overall CVD mortality remains significantly higher than in the general U.S. male population, warranting comprehensive cardiovascular risk assessment and management for all RT recipients.