Histopathological, immunohistochemical and molecular spectrum of myoepithelial tumours of soft tissues.

Primary soft tissue myoepithelial tumours (METs) are rare. Recent studies have shown EWSR1 rearrangement in certain METs. We present clinicopathological, immunohistochemical and molecular features of 14 primary soft tissue METs. Fourteen tumours, five benign and nine malignant, occurred in 12 men and two women, with an age range of 18-60 years (mean, 39.2); in upper extremities, four (29 %); chest wall, three (21 %); paraspinal region, three (21 %); pelvis, two (14 %) and lower extremities, two (14 %). Tumour size varied from 2 to 21.6 cm (mean, 8.7). Microscopically, most tumours were at least focally circumscribed. Morphological heterogeneity was noted, commonest patterns being cord-like and diffuse arrangement of polygonal cells in a myxoid stroma. By immunohistochemistry, tumours were positive for epithelial membrane antigen (EMA) (10/12, 83 %), cytokeratin (CK)/MNF116 (3/12, 25 %), p63 (7/10, 70 %), CD10 (4/6, 67 %), calponin (6/6, 100 %), S-100P (11/13, 85 %), glial fibrillary acidic protein (GFAP) (6/12, 50 %), smooth muscle actin (SMA) (3/9, 33 %), INI1/SMARCB1 (6/10, 60 %), brachyury (0/11), CD34 (0/5) and vimentin (4/4, 100 %), implying 93 % positivity for at least one epithelial marker. EWSR1 gene rearrangement was detected in 3/6 (50 %) METs (one benign and two malignant) and in an eccrine porocarcinoma which was included for reasons of comparison. Outcome details were available for six patients all surgically treated; three tumours (two malignant and one benign) resected with unknown marginal status recurred; two patients died and a single patient with myoepithelial carcinoma, who underwent a wide excision, is disease-free. This study illustrates the wide morphological spectrum of soft tissue METs, including benign and malignant subtypes. EMA and S-100P are optimal markers that should be supplemented with broad spectrum keratins, such as AE1/AE3, along with p63, GFAP and calponin in case of need but the results must be correlated with morphological features. Brachyury is useful in separating parachordoma/myoepithelioma from chordoma. EWSR1 rearrangement mostly occurs in METs that are deep-seated, irrespective of benign or malignant behaviour. Most malignant METs are INI1 negative.