Myoepithelial tumors in skin and soft tissue are uncommon but have been increasingly characterized over the past decade. Men and women are equally affected across all age groups and lesions arise most frequently on the extremities and limb girdles. Approximately 20 % of cases occur in pediatric patients, in whom they are frequently malignant. Similar to their salivary gland counterparts,
myoepithelial tumors of soft tissue demonstrate heterogeneous morphologic and immunophenotypic features.
Tumors are classified as mixed
tumor/
chondroid syringoma,
myoepithelioma, and myoepithelial
carcinoma; in soft tissue,
tumors having at least moderate cytologic atypia are classified as malignant. Mixed
tumor and
myoepithelioma show a benign
clinical course, with recurrence in up to 20 % (typically secondary to incomplete excision), and do not metastasize. In contrast, myoepithelial
carcinoma shows more aggressive behavior with recurrence and
metastasis in up to 40-50 % of cases. The majority of myoepithelial
neoplasms typically coexpress epithelial
antigens (
cytokeratin and/or EMA) and
S-100 protein; GFAP and p63 are frequently positive and a subset of
malignant neoplasms lose INI1 expression. Up to 45 % of
myoepitheliomas and myoepithelial
carcinomas harbor EWSR1 gene rearrangements, unlike mixed
tumor/
chondroid syringoma which is characterized by PLAG1 gene rearrangement. While mixed
tumor/
chondroid syringoma are likely related to primary salivary
myoepithelial tumors, soft tissue
myoepithelioma and myoepithelial
carcinoma appear to be pathologically distinct
neoplasms.