Anatomic Pathology: Soft Tissue Pathology

859) The image represents an intramuscular myxoid neoplasm in the thigh of a 75-year-old man. Which one of the following is the MOST likely diagnosis?

• Intramuscular myxoma (IM) is a slow-growing benign neoplasm characterized by bland spindle cells embedded in abundant mucin with sparse vessels.

• It affects adults between the fourth and seventh decade of life and predilects females. It involves the musculature of the proximal extremities, and the thigh is the most commonly affected site.

• The majority of cases are solitary, but multiple lesions are seen in 5% to 10% of the cases. The association of multiple myxomas with fibrous dysplasia is known as Mazabraud syndrome. McCune-Albright syndrome is the association of multiple myxomas with cutaneous hyperpigmentation and endocrine abnormalities.

• Individual lesions are circumscribed but unencapsulated with some degree of muscle infiltration. They are composed of myxoid matrix with sparse vascularity in the form of delicate capillaries. The tumor cells are widely spaced and are spindle to stellate in shape. The cytoplasm is scant and the nuclei small, ovoid, and hyperchromatic. Mitoses are generally absent. IM is positive for vimentin and may be focally positive for smooth muscle actin (SMA) and CD34. Desmin and S100 protein are negative.

• A small subset of IM shows increased cellularity and vascularity (cellular IM).

• IM, solitary or multiple, is associated with a point mutation of the GNAS1 gene, the same point mutation seen in fibrous dysplasia of bone.

• Complete conservative excision is curative.

Miettinen M, Hockerstedt K, Reitamo J, Totterman S: Intramuscular myxoma: a clinicopathological study of 23 cases. Am J Clin Pathol 1985;84(3):265-272.

Nielsen GP, O’Connell JX, Rosenberg AE: Intramuscular myxoma: a clinicopathologic study of 51 cases with emphasis on hypercellular and hypervascular variants. Am J Surg Pathol 1998;22:1222-1227.

Szendroi M, Rahoty P, Antal I, Kiss J: Fibrous dysplasia associated with intramuscular myxoma (Mazabraud’s syndrome): a long term follow-up of three cases. J Cancer Res Clin Oncol 1998;124:401-406.

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