The patient is a 65-year old Asian woman who presented with a growing mass on her left foot that has been growing for over 6 years. On examination it was slightly tender to palpation. An excisional biopsy was performed.
Histological sections reveal well circumscribed nodular lesion composed of nests and cords of epithelioid and plasmacytoid cells separated by a fibromyxoid stroma. The lesional cells show moderate amount of eosinophilic cytoplasm and eccentric round nuclei with mild nuclear atypia and focal hyperchromasia. There is no increase mitotic activity or necrosis present. Immunohistochemical stains for Cytokeratin (AE1/AE3) and S-100 are strongly and diffusely positive. SMA, CD34, Factor XIIIA, CD31 are negative.
FISH analysis using custom BAC probes showed the presence of a PLAG1 gene rearrangement, while no abnormalities were seen in the EWSR1, FUS or HMGA2 genes.
DIAGNOSIS: SOFT TISSUE MYOEPITHELIAL TUMOR
Myoepithelial tumor (MET), or myoepithelioma of soft tissue is a rare benign neoplasm that has been described relatively recently in 1995. Patients present with a mass on their extremities which involves either the subcutis or the deeper soft tissue. Cutaneous MET is a superficial type that presents as skin papules.
On gross examination, soft tissue myoepithelial tumors are well-circumscribed, but lack capsule. Histologically, this neoplasm is recognized by small aggregates of cells in a lobulated growth pattern that are embedded in a hyalinized, collagenous, myxoid or chondromyxoid stroma. Some METs do not show any stromal changes. The constituent cells in myoepithelioma are similar to those in the salivary gland lesions and include a wide range of morphology, such as epithelioid, spindled, and plasmacytoid. Rhabdoid, clear and small round cells may also be seen. The cellular arrangement varies from well-circumscribed lobules to reticular, trabecular, sheet-like or nested pattern with cords and ductules. Glandular structures may be seen in cases where the tumor displays differentiation toward a mixed tumor rather than a pure myoepithelioma. Osteoid production, chondroid and adipocytic differentiation may also be seen.
Immunohistochemically, METs co-express epithelial and myoepithelial markers, including pan-cytokeratin (AE1/AE3), S-100 protein, calponin (in almost all cases) and smooth muscle actin (in most cases). METs are also immunoreactive for CK8 and CK18, EMA, and may express GFAP in up to half of the cases; desmin is characteristically negative. Cytoplasmic and membranous staining for MIC2 (CD99) is seen in most cases. SOX10, a Schwannian and melanocytic marker, is expressed in the majority of benign METs and in up to a third of malignant tumors.
There is no clear-cut differentiation in the literature between myoepithelial tumor of the soft tissue and parachardoma, and the two likely exist on the spectrum. Microscopically, parachordoma is similar to the MET, but in addition may have abundant eosinophilic to vacuolated cytoplasm with morphologic resemblance to physaliphorous cells.
Because of its significant morphologic heterogeneity, several soft tissue tumors enter the differential diagnosis to include peripheral chordoma, chondroid lipoma,ossifying fibromyxoid tumor (OFMT) and extraskeletal myxoid chondrosarcoma (ESMC). A thorough immunohistochemical work-up may be required