e17002 Background: Radiation therapy (RT) is a backbone of treatment for patients with prostate cancer (PCa). However, locally recurrent disease after definitive RT (i.e. radiorecurrent PCa) is not uncommon and is associated with a higher risk of distant metastases and death from PCa. While the genomic landscape of primary PCa is well-characterized, little is known regarding the genomic landscape of radiorecurrent PCa or how this compares to that of primary PCa. We hypothesized that the genomic landscape of radiorecurrent PCa differs significantly from primary PCa and that these differences have clinical relevance. We examined this hypothesis by performing whole exome sequencing (WES) of radiorecurrent PCa. Methods: We identified 25 patients with radiorecurrent PCa with available post-RT tissue obtained from biopsy or radical prostatectomy, as well as germline tissue. The tumor and germline tissue for 19 patients successfully underwent WES. We identified genomic variants including single nucleotide variants (SNVs), insertions/deletions, and copy number alterations. Furthermore, we estimated the tumor mutational burden (TMB; number of nonsynonymous mutations per megabase Mb) and contribution of individual mutational signatures. We compared our samples to a publicly available large cohort of primary PCa (n = 680) to define genomic alterations unique to radiorecurrent PCa. Results: In the overall cohort of 25 patients, the RT modality included external beam RT (56%), brachytherapy (36%), and combination of both (8%). 40% of patients received upfront androgen deprivation therapy with RT. The median time to local recurrence was 6.5 years. For the 19 radiorecurrent patients with WES data, the median TMB was 2.7 mutations/Mb, which was significantly higher than the median TMB of 0.7 mutation/Mb for primary PCa ( P= 0.002 after multivariable adjustment). Radiorecurrent PCa demonstrated an enrichment of short deletions, with a significantly higher deletion/SNV ratio compared to primary PCa ( P= 0.006). TP53 was the most frequently mutated gene in radiorecurrent PCa (n = 6), and the TP53 mutation prevalence was significantly higher compared to primary PCa (32% vs 10%, P= 0.016 by Fisher’s exact test). TP53 was also determined to be recurrently mutated using MutSigCV (Q = 0.0003). Additionally, 3 samples demonstrated evidence of whole genome doubling. Conclusions: Radiorecurrent PCa has a distinct genomic profile compared to primary PCa, characterized by a higher TMB with an enrichment of short deletions as part of the mutational composition, which may be a scar of nonhomologous end joining subsequent to RT-induced DNA double-stranded breaks. In addition, TP53 mutations may be of functional consequence in radiorecurrent PCa. Further efforts are underway to examine other genomic features apparent in WES data, as well as perform whole transcriptome sequencing to provide complementary insights into radiorecurrent PCa.