An Unusual Case of Angiomatoid Fibrous Histiocytoma

Summary: A 21-year-old female patient presented to clinic with a 1-year history of a soft 2-centimeter subcutaneous mass located on the volar right forearm. The mass was mobile, nontender, and without epidermal changes. It was excised by General Surgery who clinically believed the mass to be a sebaceous cyst. Histopathology revealed an atypical spindle cell lesion with surrounding dense lymphoplasmacytic inflammatory cell infiltrate, consistent with angiomatoid fibrous histiocytoma. The superficial subcutaneous fat showed well-circumscribed proliferation of atypical spindle cells with nuclear pleomorphism, occasional prominent nucleoli, and scattered mitotic figures. There is surrounding dense lymphoplasmacytic infiltrate with associated germinal center formation. Immunohistochemistry revealed negative S100, SOX10, panCK, desmin, CD34, and CD31, and weak positivity for SMA and EMA. CD45 and CD68 highlight the inflammatory component. This case is particularly unusual because it shows a solid-type architectural pattern, and the morphology shows diffuse pleomorphism – both uncommon in angiomatoid fibrous histiocytoma. A high degree of suspicion is needed as angiomatoid fibrous histiocytoma is rare, however, considering its metastatic potential, it should still be considered in younger populations presenting with painless, slow-growing masses.