373 Characterizing Pseudoangiomatous Stromal Hyperplasia (PASH)

Introduction/Objective Pseudoangiomatous Stromal Hyperplasia (PASH) is a benign breast lesion that histologically mimics vascular proliferation. Methods/Case Report We examined 193 breast core biopsies with PASH from 183 patients (2017–2019) without prior or concurrent atypia or malignancy, to characterize associated lesions, follow-up diagnoses, and immunohistochemical (IHC) profiles. All cases were re-evaluated to confirm PASH. IHC was performed using a panel including ER, PR, SMA, BCL2, β-catenin, CD34, desmin, ERG, and TRPS1 Results Most cases (93.8%) showed ischemic-type fat necrosis and fibrocystic changes, with 84.5% exhibiting non-proliferative and 53.9% proliferative features. Hormone receptor–positive (versus hormone receptor-negative) PASH (46.7%) was significantly associated with non-proliferative fibrocystic changes (p = 0.047). Chart review of subsequent biopsies and resections revealed 4.9% of patients were diagnosed with atypia (3.3%), DCIS/pLCIS (0.5%), or invasive carcinoma (1.1%). All PASH cases expressed CD34 (diffuse, strong), with lower frequency expression of ER (29%), PR (33.7%), SMA (24.4%), BCL2 (34.2%), desmin (5.7%), and TRPS1 (1.0%). ERG and nuclear β-catenin were consistently negative. Conclusion These findings suggest that PASH often coexists with fibrocystic changes and ischemic-type fat necrosis. Compared to the published literature, this study provides a larger, contemporary systematic IHC review that will aid in distinguishing PASH from histologic mimics, improving diagnostic accuracy in benign breast disease.