'Pure' spindle cell variant of angiomatoid fibrous histiocytoma, lacking classic histologic features

Angiomatoid fibrous histiocytoma (AFH) is a soft tissue tumor of intermediate biologic potential and uncertain differentiation that most frequently occurs in the superficial extremities of children and young adults. It is histologically typified by nodules of ovoid to spindle cells with pseudoangiomatoid spaces and a surrounding dense lymphoplasmacytic infiltrate, desmin expression in about 50%, and association with EWSR1-CREB1, EWSR1-ATF1 or FUS-ATF1 gene fusions. The diagnosis still poses a challenge because AFH may not display all classic features, can show a variety of unusual histologic findings and lacks a specific immunoprofile. We describe a case of 'pure' spindle cell AFH arising in the forearm musculature of a 19 year-old female, which harbored EWSR1-CREB1 fusion transcripts by reverse transcription-polymerase chain reaction. The neoplasm was composed entirely of highly cellular fascicles of spindled cells architecturally resembling spindle cell sarcoma, and lacked obvious pseudoangiomatoid spaces or a lymphoid cuff. This purely spindle cell variant adds to the significant morphologic spectrum of AFH, and emphasizes that even when occuring at a typical site, AFH may be difficult to recognize when showing non-classical morphology. This is of clinical relevance, as AFH with this morphology could be potentially misdiagnosed as a high-grade sarcoma, with the patient subject to more radical therapeutic approaches.