Abstract 1123: Genomic characterization of radiotherapy-associated bladder cancer

Abstract Background: Bladder cancer (BCa) is the 4th most common cancer diagnosed in men, with 82,290 new cases predicted in 2023. As BCa frequently occurs at an advanced patient age, its diagnosis may follow radiotherapy (RT) for the treatment of prostate cancer. Currently, data is limited regarding the influence of RT on the clinical course and genomic landscape of subsequent BCa, mainly due to a paucity of samples. From prior studies, pelvic radiation confers an increased risk of BCa, with resulting tumors that are largely genomically similar to non-RT tumors with the exception of an increase rate of genomic insertions. Our current study looks to expand on these findings by characterizing the genomes of the largest cohort of RT-associated bladder cancers to date. Methods: We identified BCa cases from patients with a prior history of pelvic radiation for prostate cancer, and tumor only targeted sequencing was performed using Foundation Medicine (FMI) on FFPE tissue (n=41). For control cases (CTRL), patients with no prior record of pelvic radiation and who had previously underwent standard of care FMI sequencing were included (n=41). Sequencing data were re-aligned to hg38 and mutation calling was performed on the newly aligned sequences. Results: CTRL and RT cohorts had similar distributions of patients based on race and smoking status. While cT stage was similar between the two cohorts, the CTRL cohort was biased toward higher cN and cM stages with no difference in overall survival (median OS: CTRL = 20m, RT = 23m), though the advanced stage of the CTRL cohort could mask a worse prognosis in the RT cohort. Latency specific analysis revealed short latency tumors (<10 years from RT) had a significantly shorter overall survival than long latency tumors (p<0.01, HR=5.59, CI=1.81-17.3), and a numerically, but not significantly, shorter survival than CTRL patients (p=0.17, HR=1.55, CI=0.82-2.94). Long latency tumor patients had a significantly longer overall survival than CTRL patients (p<0.01, HR = 0.22, CI=0.07-0.63). In line with prior studies, RT samples had increased numbers of insertions (p<0.01). In addition, we also detected an increase in SNVs in the RT samples (p<0.01). Of note, KDM6A alterations were enriched within the RT cohort (CTRL = 10/41, RT = 30/41, p<0.001), with series of splice site mutations exclusive to RT samples (p<0.001). Conclusion: Patients diagnosed with BCa within a short interval following pelvic radiation have a worse prognosis. Genomic analysis showed RT-associated tumors having an increased number of insertions, SNVs, and KDM6A splice site mutations compared to non-RT tumors. Overall, the spectrum of recurrent alterations within the cohorts were consistent with previous studies, with the exception of the KDM6A mutations. To our knowledge, this is the first report of a RT-associated alteration in a specific gene in bladder cancer and additional studies will be needed to validate and understand the consequences of this finding. Citation Format: N. Ari Wijetunga, Kathryn H. Gessner, Krishna Kanchi, Sara E. Wobker, David Corcoran, Matthew D. Galsky, Matthew I. Milowsky, William Y. Kim, Tracy L. Rose, Jeffrey S. Damrauer. Genomic characterization of radiotherapy-associated bladder cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1123.