Myolipoma of Soft Tissue: Clinicopathologic Analysis of 34 Cases.

Myolipoma of soft tissue, which was first described by Meis and Enzinger (1991), is a rare benign neoplasm characterized by the admixture of mature adipocytes and well-differentiated smooth muscle cells. Recently, cytogenetic alteration of the HMGA2 gene has been reported in 2 myolipomas. We present the clinicopathologic features of 34 cases of myolipoma of soft tissue, study immunoreactivity for HMGA2, and review the previous literature. In our series, there were 32 women and 2 men, with age at presentation ranging from 35 to 94 years (median, 55 y). The most frequently affected site was retroperitoneum (47%), followed by pelvis (15%), abdominal wall (12%), and intra-abdominal sites (9%). Follow-up information was available for 17 patients (50%), ranging from 1 to 202 months (mean, 41 mo). None has developed local recurrence or metastasis. Grossly, tumors were well circumscribed, and the cut surface showed an admixture of yellowish adipose tissue and tan-whitish nodules. The size ranged from 2.4 to 60 cm (median 10.5 cm). Histologically, the tumors were composed of an intimate admixture of mature fat cells and bland spindle-shaped cells with brightly eosinophilic cytoplasm, arranged in fascicles. Some cases showed the following unusual features focally: hypercellular fascicular pattern (N=2), degenerative nuclear atypia (N=1), round cell morphology (N=1), hemosiderin deposition (N=1), metaplastic cartilage (N=1), metaplastic bone (N=1), and eosinophil infiltrates (N=1). Immunohistochemically, spindle cells showed strong and diffuse positivity for desmin (26/26 cases), SMA (20/21), and ER (13/15). Nuclear positivity for HMGA2 was identified in 15 of 25 cases (60%). MDM2 and CDK4 were usually negative (14/15, 8/9, respectively). In summary, myolipoma of soft tissue is a distinctive benign tumor composed of mature fat cells and smooth muscle cells and arises most commonly in deep-seated locations of middle-aged women. In our study, 60% of cases showed nuclear staining for HMGA2 by immunohistochemistry, which supports the possibility that these tumors harbor aberration of the HMGA2 gene, as seen in lipomas and leiomyomas elsewhere.