Pseudomyogenic hemangioendothelioma of the scalp with diffuse nuclear FOSB immunostaining

Summary: Pseudomyogenic hemangioendothelioma (PHE) is a rare vascular neoplasm of intermediate biologic potential that has been infrequently reported to occur on the head and neck. The histopathologic differential is broad and includes epithelioid hemangioendothelioma and epithelioid sarcoma. FOSB gene fusion is an important diagnostic feature. We present a 37-year-old woman with a 5-mm erythematous cerebriform papule on her right scalp. Histopathologic evaluation showed an ill-defined, dermal tumor composed of short, intersecting fascicles and small sheets of spindled to epithelioid cells with pleomorphic nuclei, vesicular chromatin, prominent nucleoli, and glassy, eosinophilic cytoplasm. Other morphologic features characteristic of PHE, rhabdoid-appearing cells and scattered neutrophils, were noted. Mitotic figures were readily identified, 5 per 10 high-power fields. A broad immunohistochemical (IHC) workup demonstrated expression of pankeratin AE1/3, CAM5.2, and ERG. Negative immunostains included: p40, p63, S100, CD34, CD31, myogenin, myoD1, desmin, SMA, caldesmon, CD68, CD163, INI-1 (intact), BRG (intact), CAMTA1, and TFE3. Given the morphologic features and positive keratin and ERG, PHE was considered. FOSB IHC showed diffusely positive nuclear expression, supporting the diagnosis. Next-generation sequencing detected a SERPINE1:FOSB fusion, confirming the diagnosis of PHE. Dermatopathologists may utilize FOSB IHC as an initial diagnostic marker for PHE prior to confirming with molecular studies.