INTRAMUSCULAR MYXOMA - A CLINICOPATHOLOGICAL STUDY OF 51 CASES WITH EMPHASIS ON HYPERCELLULAR AND HYPERVASCULAR VARIANTS

Intramuscular myxoma (IM) is a benign soft-tissue tumor that presents as a deeply seated mass confined to skeletal muscle. Surgical excision is virtually always curative. Recurrence, even after incomplete resec tion, is exceptional. Intramuscular myxoma is classically described as hypocellular and hypovascular, and is composed of cytologically bland stellate and bipolar fibroblasts separated by abundant extracellular myxoid matrix. What is underemphasized, however, is that IMs often sho w areas of increased cellularity and vascularity that can lead to a mi staken diagnosis of sarcoma, especially myxofibrosarcoma, low-grade fi bromyxoid sarcoma, and myxoid liposarcoma. In this report, we describe the clinicopathologic features of 51 IMs with special emphasis on tho se that exhibit these ''hypercellular regions.'' The patients included 35 women and 16 men who ranged in age from 27 to 89 (mean 52) years. The tumors measured from 2 to 15 (average 5.6) cm and all had a gelati nous, lobulated cut surface. Histologically, they all demonstrated cla ssic hypocellular, hypovascular regions. Thirty eight tumors contained areas of relative increased cellularity that occupied from 10 to 80% of the tumor. These foci had increased numbers of cells, more prominen t vascularity, and often increased collagen content. The hypercellular regions were not associated with cytologic atypia of the constituent cells, mitotic activity, or necrosis. Follow-up information was availa ble for 32 patients and ranged from 3 to 108 (average 30) months. No t umor recurred or metastasized. Areas of hypercellularity are common in IMs. Their recognition is important to avoid an erroneous diagnosis o f sarcoma.