Clinical and molecular characterization of urothelial cancer developing after pelvic radiation.

518Background: The development of urothelial cancer (UC) after pelvic radiotherapy (RT) is an increasingly recognized phenomenon. Numerous studies show a relationship between ionizing radiation and urothelial dysplasia and DNA damage. It is not known if UC that develops after pelvic RT is associated with a distinct molecular signature, which could have treatment implications for this subset of UC. Methods: The University of North Carolina (UNC) Genitourinary Oncology Database was queried for cases of UC between 01/2012 and 06/2016 in patients with prior pelvic RT. Demographics, clinical stage, and details of prior RT were collected. Patients with available archival FFPE tumor tissue underwent next generation targeted exome sequencing per the UNCSeq pipeline. Somatic mutations and copy number alterations were analyzed. Total mutation burden (TMB) (mutations per total Megabase region of 30X coverage, mut/Mb) and insertion-to-deletion (indel) ratio was calculated. Wilcoxon rank sum was used to compare TMB an...