Myoepithelioma/ Mixed Tumour of the Soft Tissue and Parachordoma

This rare group of soft tissue tumours show differentiation towards myoepithelial cells, either in pure form (myoepitheliomas) or in association with glandular structures (mixed tumors).

15-20% cases recur locally of which 5-10% metastasize. Some cases may start as carcinoma or sarcoma.

Age: All age groups (most common 50 years), 10% in young children.

Site: Limbs (especially hands & feet) are the most common site. Cases have been reported in the trunk and in the head and neck region.

Most cases are subcutaneous, 20% are intra-muscular or sub-fascial.

Gross: Most cases are grossly well circumscribed.

Microscopic features:

Tumours are usually lobulated.

i) 25-30% cases show presence of ductal components.

Myoepithelial cells are a) in the form of epitheloid cells arranged in nests, cords & ductules and/or b) nests of spindle cells within a hyalinized or chondromyxoid stroma.

ii) Proportion of myxoid/chondromyxoid & myoepithelial components are variable.

10-15% cases show chondro-osseous differentiation & some cases may show adipocytes differentiation.

Cytoplasmic hyaline inclusions may be present.

Mitotic activity is variable but generally low; atypical mitotic figures are usually not identified.

Classification: Tumors with benign cytomorphology or mild cytologic atypia (low-grade) are classified as myoepithelioma or mixed tumour, whereas tumors with moderate to severe atypia (high-grade) are classified as myoepithelial carcinoma (epithelioid or spindled cells with vesicular or coarse chromatin, prominent, often large nucleoli, or nuclear pleomorphism) or malignant mixed tumour (cytologically malignant cartilage or bone).

Immunohistochemistry:

1. Most cases are positive for cytokeratin and S-100 protein.

2. Some cases are positive for actin, GFAP or desmin

3.Consistent positive S-100 staining differentiates most metastatic carcinomas (excluding breast).

Differential diagnosis:

Ossifying fibromyxoid tumour of soft parts ; Extraskeletal myxoid chondrosarcoma . Myxoid chondrosarcoma is excluded by cytokeratin positivity ; Parachordoma.

Parachondroma : It is a very rare peripheral soft tissue tumour of unknown lineage. It is a circumscribed firm tumour, usually located in the deep soft tissue. They are usually found adjacent to the tendon, synovium & osseous structures in the extremities.

Microscopic features: i) the tumour has a pseudolobulated growth pattern ii) Small cellular aggregations are embedded within hyalinized & chondroid matrix. iii) The tumour consists of sheets, nests and cords of epithelioid cells, some with a physaliferous appearance. The epithelioid cells are immunohistochemically positive for vimentin, S-100 protein, neuron-specific enolase, keratin, carcinoembryonic antigen and epithelial membrane antigen, and negative for HMB45.

i)Parachordoma: a clinicopathologic and immunohistochemical study of four cases of an unusual soft tissue neoplasm.Ann Diagn Pathol. 1997 Oct;1(1):3-10.

ii)Parachordoma is immunohistochemically and cytogenetically distinct from axial chordoma and extraskeletal myxoid chondrosarcoma.Am J Surg Pathol. 1999 Sep;23(9):1059-67.

iii)Parachordoma is not distinguishable from axial chordoma using immunohistochemistry.Pathol Int. 2004 May;54(5):364-70.