Definition

Intramuscular myxoma is a benign neoplasm currently classified among soft tissue tumors of uncertain differentiation.

Epidemiology and Presentation

It occurs mainly in adult or elderly patients (more frequently females) as a painless intramuscular mass. The combination of intramuscular myxoma with bone fibrous dysplasia defines the Mazabraud’s syndrome. Magnetic resonance imaging shows a poorly vascularized tumor (which can measure up to 20 cm in diameter) hyperintense on T2-weighted images.

Pathology

Macroscopically, the mass – which may show undefined margins – displays a gelatinous lobulated cut surface and may present fluid-filled cystic spaces.

Miscroscopically, intramuscular myxoma is composed of uniform spindled and stellated cells separated by abundant extracellular myxoid stroma composed of glycosaminoglycans (similar to low-grade myxofibrosarcoma, → see dedicated section). Mitoses, pleomorphism, and necrosis are not present even in the most cellular areas. Intramuscular myxoma with more cellularity is also called cellular myxoma. Differential diagnosis may be needed with the following: chondrosarcoma (bone or soft tissue tumor mimicking chordoma with rows of cuboidal cells separated by myxoid background; stains positive for S100 and vimentin, negative for cytokeratins); myxoid liposarcoma (mitotic figures, lipoblasts, positive for FUS-DDIT3 fusion gene); and myxoid leiomyosarcoma (invasive, highly myxomatous; see typical smooth muscle cells alternating with mesenchymal cells); intramuscular cellular myxoma may need to be differentiated from low-grade myxofibrosarcoma (no GNAS mutations).

Biomarkers

At immunohistochemistry, the cells stain variably positive for CD34 and desmin and negative for S100. From the genetics viewpoint, point mutations of the GNAS geneFootnote 1 are commonly observed (about 60% of all cases).

Prognosis

Intramuscular myxoma is a benign tumor, although cellular myxomas may recur.

Therapy

Surgical excision is the treatment of choice.