Increased risk of secondary bladder cancer after radiation therapy for endometrial cancer

To investigate the effect of radiation therapy (RT) after endometrial cancer (EC) diagnosis on the risk of occurring secondary bladder cancer (SBC) as well as on the survival outcome of those patients who suffered with SBC. Data was extracted from the Surveillance, Epidemiology, and End Results database between 1973 and 2015. Chi-squared test was utilized to compare clinicopathological characteristics among different groups. The Fine and Gray’s competing risk model was utilized to assess cumulative incidence and risk of occurring SBC in EC survivors. The Kaplan–Meier method and the Cox regression model were used for survival analysis. As a result, a total of 108,060 EC patients were included, among which 37,118 (34.3%) patients received RT while others did not. The incidence of SBC was 1.31%, 1.76% and 0.96% among patients who received prior brachytherapy, external-beam radiotherapy (EBRT) and others, respectively. Both of the EBRT (standardized incidence ratio (SIR) = 2.24, 95% CI [1.94–2.58]) and brachytherapy (SIR = 1.76, 95% CI [1.44–2.13]) group had a higher incidence of SBC than the general population in USA. The competing risk analysis demonstrated that receiving EBRT (HR = 1.97, 95% CI [1.64–2.36]) or brachytherapy (HR = 1.46, 95% CI [1.14–1.87]) were all independent risk factors for developing SBC. A survival detriment was only observed in SBC patients who received prior EBRT after EC diagnosis, but not for brachytherapy, when compared with those who did not undergo RT. Additionally, there were no significant survival differences between primary bladder cancer and SBC with or without prior RT history. Patients who underwent RT after EC had an increased risk of developing bladder cancer as secondary primary cancer. The prognosis of these SBC patients varied depending on types of RT that received after EC diagnosis.