Pseudomyogenic (epithelioid sarcoma-like) hemangioendothelioma - a rare vascular neoplasm with deceptive morphology and distinctive immunophenotype.

Pseudomyogenic (epithelioid sarcoma-like) hemangioendothelioma is a rare, low grade vascular (endothelial) neoplasm typically presenting as multicentric, superficial to deep nodules in extremities with a slight tendency of affecting young adult males. We report a case of pseudomyogenic hemangioendothelioma in a 15-year-old boy presenting initially with a 1 cm right thigh painless cutaneous lump. The lump was excised with the clinical impression of a sebaceous cyst. On microscopy, a poorly circumscribed, mild to moderately atypical spindle cell lesion in fascicular and storiform patterns with strikingly myoid-like eosinophilic cytoplasm was identified. The spindle cells were highlighted by pancytokeratin AE1/AE3, CD31, and ERG with retained INI-1, while being negative for MNF116, S100, CD34, EMA, desmin, SMA, caldesmon, myogenin, MyoD1, HHV-8 and CD163. Following the first diagnostic report, a positron emission tomography-computed tomography (PET-CT) scan revealed another 4 cm ill-defined nodule accompanied by a smaller adjacent 0.7 cm ipsilateral satellite nodule within the right psoas muscle that displayed similar morphology and immunophenotype as the cutaneous lump, supporting the multicentric feature of this unique entity. It is an uncommon yet increasingly recognised neoplasm of endothelial origin possessing a misleading myoid morphology and distinctive immunophenotype worth notifying.