The Impact of Histologic Subtypes on Clinical Outcomes After Radiation-Based Therapy for Muscle-Invasive Bladder Cancer.

Outcomes of radiation-based therapy (RT) for muscle-invasive bladder cancer (MIBC) with histologic subtypes of urothelial cancer (HS-UC) are lacking. Our objective was to compare survival outcomes of pure urothelial carcinoma (PUC) to HS-UC after RT. A multicenter retrospective study of 864 patients with MIBC who underwent curative-intent RT to the bladder for MIBC (clinical T2-T4aN0-2M0) between 2001 and 2018 was conducted. Regression models were used to test the association between HS-UC and complete response (CR) and survival outcomes after RT. In total, 122 patients (14%) had HS-UC. Seventy-five (61%) had HS-UC with squamous and/or glandular differentiation. A CR was confirmed in 69% of patients with PUC and 63% with HS-UC. There were 207 (28%) and 31 (25%) patients who died of metastatic bladder cancer in the PUC and HS-UC groups, respectively. There were 361 (49%) and 58 (48%) patients who died of any cause in the PUC and HS-UC groups, respectively. Survival outcomes were not statistically different between the groups. The HS-UC status was not associated with survival outcomes in multivariable Cox regression analyses. In our study, HS-UC responded to RT with no significant difference in CR and survival outcomes compared to PUC. The presence of HS-UC in MIBC does not seem to confer resistance to RT, and patients should not be withheld from bladder preservation therapy options. Due to low numbers, definitive conclusions cannot be drawn for particular histologic subtypes.