Myxoid variant of so-called angiomatoid "malignant fibrous histiocytoma": clinicopathologic characterization in a series of 21 cases.

Angiomatoid "malignant fibrous histiocytoma" (AMFH) is a tumor of intermediate malignancy and undefined lineage, mostly arising in the extremities of young patients. Examples with a prominent myxoid matrix are very uncommon. Twenty-one cases of myxoid AMFH (among a total of 414) identified in consult files are described, including clinicopathologic features, fluorescence in situ hybridization analysis in a subset of cases, and follow-up. Thirteen patients were female and 8 male, ranging in age from 2 to 51 y (median 17 y). These circumscribed tumors arose in subcutaneous or deep somatic soft tissue, with a median size of 2.5 cm (range, 1 to 8 cm), being located in the extremities (14/21), trunk (4/21), and upper limb girdle (3/21). Characteristic features included a fibrous pseudocapsule (20/21), peritumoral lymphoplasmacytic infiltrates (20/21), blood-filled cystic spaces (17/21), and prominent myxoid morphology comprising 60% to 100% of the tumor surface area examined. Histiocytoid or spindled tumor cells exhibited vesicular nuclei, inconspicuous nucleoli, palely eosinophilic cytoplasm, and multinodular growth without necrosis. Mucin pools and scattered multinucleate giant cells were observed in a subset of cases. Mild to moderate atypia was observed in 4 cases; 1 tumor showed a pseudochondroid matrix. Immunohistochemically, 14/21 cases expressed desmin, 12/21 expressed EMA, and 4/7 exhibited EWSR1 rearrangement. Follow-up, available for 11 patients (median, 43 mo), revealed that 3 developed local recurrence after 2, 7, and 48 months, respectively. All patients were alive without metastases. AMFH may present with prominent myxoid features making diagnosis difficult and causing possible confusion with other myxoid tumors including low-grade fibromyxoid sarcoma, extraskeletal myxoid chondrosarcoma, and myxoid liposarcoma.