Refractory testicular pure seminoma (PS) and non-seminomatous(NS) germ cell tumors (GCT): A comprehensive genomic profiling (CGP) study

565 Background: Although PSand NS testicular GCT have a favorable prognosis, on occasion these tumors can be refractory to conventional systemic treatments. Methods: FFPE tissues from 22 PS and 86 NSunderwent hybrid-capture based CGP to evaluate all classes of genomic alterations (GA). Tumor mutational burden (TMB) was determined on 1.1 Mbp of sequenced DNA and reported as mutations (mut) per megabase (Mb) and microsatellite instability (MSI) was determined on 114 loci. Results: PS patients were older than NS (P=0.007). The primary tumor was sequenced in 41% of PS and 18% of NS with a metastasis sample in 59% of PS and 82% of NS.Four (18%) of PS had syncytial trophoblast cells identified. The mean GA frequency at 4.1 mut/case for NS was higher than that seen in PS and this difference reached near significance (P=0.08). The KRAS, TP53, CCND2 and FGF6/23 GA frequencies were similar in both tumor types (Table). GA in KIT, PIK3CA/ MTOR, PTEN and BRCA2 were more frequent in PS than NS whereas BRAF and ERBB2 GA were more frequent in NS (Table). MSI-High status was absent in in PS (0%) and identified in 2% of NS. Higher levels of TMB were not encountered in PS (0% TMB ≥10 mut/Mb), but higher TMB levels were more frequent in NS (5% ≥ 10 mut/B and 1% ≥ 20 mut/Mb). Conclusions: The GA found in refractory PS and NS differ significantly. PS features a lower GA frequency with slightly higher potential for targeted therapies in kinase (KIT) and MTOR pathways but, has very low TMB predicting limited opportunities for immunotherapy for these patients. For NS targeted therapy biomarkers appear even more uncommon than seen in PS with only extremely rare kinase inhibitor opportunities. However, based on rare high TMB and MSI-High status, immunotherapies may be of benefit in a small subset of NS patients. Further study of genomic findings in relapsed and clinically aggressive PS and NS appears warranted. [Table: see text]