From the Departments of Pathology (Drs Palmer, North, and Parham), Orthopedics (Dr Nicholas), and Radiology (Dr Allison), University of Arkansas for Medical Sciences and Arkansas Children's Hospital, Little Rock.
AN 11-YEAR-OLD African American boy presented to his local physician with a complaint of painless swelling in the right calf. The lesion subsequently increased in size, and 2 months later he was limping and complaining of pain in the leg. Magnetic resonance imaging of the knee revealed a lobulated heterogeneous soft tissue mass in the lateral head of the gastrocnemius muscle with associated edema in the surrounding musculature (Figure 1). A deep-seated, tennis ball–sized mass was subsequently resected 2 weeks later. The resection included the lateral head of the gastrocnemius muscle, underlying soleus muscle, and surrounding soft tissues. The closest margin was along the neurovascular bundle in the popliteal space. There was no attachment of tumor to the periosteum or synovium.
Grossly, the fairly well-circumscribed, vaguely lobulated, fusiform tumor measured 6.0 cm in greatest diameter and consisted of soft to firm, tan-white tissue with a focally gelatinous cut surface. Focal areas of cystic hemorrhage were present (Figure 2). Microscopically, the tumor was composed of strands of epithelioid to spindle-shaped cells with uniform, round to oval, hyperchromatic nuclei and variably clear to eosinophilic cytoplasm embedded in a fibromyxoid matrix. Densely cellular areas were separated by thick bands of fibrous stroma, and loosely cellular clusters of cells were embedded in a more abundant myxoid matrix (Figure 3) that was reactive with Alcian blue (pH 2.5) in the presence or absence of hyaluronidase. Well-differentiated hyaline cartilage was not identified. The mitotic activity was generally low. No necrosis was observed. The tumor cells were reactive immunohistochemically with vimentin and epithelial membrane antigen in a focal pattern. Results of staining for S100 protein and cytokeratins were negative.
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