Extra-mammary Paget’s disease (EMPD) of the skin: A comprehensive genomic profiling (CGP) study

9591 Background: EMPD is an intraepidermal adenocarcinoma which frequently progresses into invasive/metastatic disease. Methods: 29 EMPD underwent 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 mut/Mb. Microsatellite instability (MSI) was determined on 114 loci. PD-L1 expression was determined by IHC (Dako 22C3 antibody). Results: All patients had locally recurrent and/or metastatic disease. The 20 (69%) male and 9 (31%) female patients had a median age of 70 years (range 45 to 101 years). The primary EMPD site, known in 90% of cases and involved the scrotum (6 cases), groin (6 cases), vulva (5 cases), anus (3 cases), perineum (2 cases), abdomen (2 cases), penis (1 case) and buttock (1 case). was used for sequencing in 15 (52%) cases and a metastatic site 14 (48%) cases. The GA/tumor frequency was 5.0. The most frequent currently untargetable GA involved TP53 (48%), CDKN2A (38%), CDKN2B (31%), KMT2C (28%), and MYC (14%). The most frequent potentially targetable GA were identified in ERBB2 (35%), ERBB3 (17%) and PTEN (14%) and rarely in PDGFRA, CDK4/6, NF1/2, and ROS1 (fusion) all at 3%. The ERBB2 GA included short variant (SV) activating mutations (21%), amplifications (10%) and multiple ERBB2 GA in 3%. This finding is in contrast with mammary Paget’s disease which classically features > 90% of cases with ERBB2 amplification. No EMPD cases were either MSI-High or stained positively for PD-L1 expression. The median and mean TMB were 5.2 mut/Mb and 6.3 mut/Mb, respectively; 4 (14%) had TMB ≥ 10 mut/Mb and 1 (3%) had TMB ≥ 20 mut/Mb. Conclusions: EMPD is a rare source of relapsed/metastatic adenocarcinoma which features GA that are distinct from the more common disease originating on the nipple of the breast including a lower ERBB2 GA frequency and lower relative frequency of ERBB2 amplification versus SV mutations. A variety of MTOR pathway, cell cycle and kinase targets are also identified when EMPD undergoes CGP. However, the low TMB and absence of both MSI-High status and PD-L1 expression in EMPD cohort indicates a likely lack of benefit for immunotherapies for patients suffering from this rare form of malignancy.