Objective To explore the impact of perioperative external beam radiation therapy(EBRT)on the risk and prognosis of second primary bladder cancer(SPBC)in rectal cancer(RC)patients.Methods We retrospectively reviewed 51,649 RC patients diagnosed by pathology from January 1st,1973 to December 31st,2015,utilizing the SEER database.Standardized incidence ratio(SIR)was calculated using Poisson regression and SEER*Stat software.The cumulative incidence of SPBC in RC patients was assessed through Fine-Gray competitive risk regression analysis using R software.Finally,the propensity score matching Kaplan-Meier method was employed to evaluate the survival outcomes of SPBC patients.Results Among the 51 649 RC patients,15 612 underwent both surgery and EBRT,while 36 037 underwent surgery alone.After 30 years of follow-up,the cu-mulative incidence of SPBC was 4.50%in the radiotherapy(RT)group and 2.20%in the non-radio-therapy(NRT)group.The SIR in the RT group exceeded that of the general population(SIR=1.43,95%CI:1.24-1.64,P<0.050).Competitive risk regression analysis indicated that RT was associated with a higher risk of SPBC(HR=1.49,95%CI:1.13-1.95,P=0.004).Survival anal-ysis of SPBC patients combined after propensity score matching between the RT group and NRT group showed a statistically significant difference in 10-year overall survival between the two groups(62.8%vs 72.7%,P=0.010).Additionally,the 10-year overall survival in patients with SPBC,both with(27.7%vs 40.4%,P=0.005)and without(24.2%vs 35.1%,P<0.001)RT,was significantly lower than that in patients diagnosed with only primary bladder cancer(OPBC).Conclusions RT emerges as a pivotal risk factor in SPBC for RC patients with long-term survival,potentially contributing to a poorer prognosis in patients with SPBC.
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