MIXED TUMORS AND MYOEPITHELIOMAS OF SOFT-TISSUE - A CLINICOPATHOLOGICAL STUDY OF 19 CASES WITH A UNIFYING CONCEPT

We report 19 unusual cases of mixed tumors and myoepitheliomas arising in soft tissues. The neoplasms occurred in 12 males and seven females . The age at diagnosis ranged from 2 to 83 years (mean 35, median 30). Eight tumors arose in the upper limb, six in the lower limb, three in the trunk, and two in the head and neck region. Three cases involved both dermis and subcutis; the remainder arose in subcutaneous (13 case s) or deep subfascial soft tissue (three cases). The most common prese nting complaint was a painless swelling, with duration ranging from 2 weeks to 1 year (median 2.5 months). Microscopically, the tumors were predominantly well circumscribed and lobulated. Six cases showed a foc ally infiltrative margin. Cardinal morphologic features included nests , cords, and ductules of epithelioid cells and/or nests of spindled ce lls within a hyalinized to chondromyxoid stroma. One tumor was predomi nantly composed of myoepithelial cells and devoid of epithelial differ entiation (i.e., ductules). Cytoplasmic hyaline inclusions were noted in two cases; squamous differentiation was seen in one case. Osteoid p roduction and/or metaplastic bone was observed in three tumors. Chondr oid differentiation (usually mature) was seen in four cases. Adipocyti c differentiation was seen in two tumors. Mitotic activity was variabl e but generally scant; atypical mitotic figures were not identified. B y immunohistochemistry, 16 of 16 cases expressed pan-keratin; 16 of 17 S-100 protein; six of 14 alpha smooth muscle actin (IA4); two of 10 m uscle specific actin (HHF-35); two of 10 desmin; three of 11 glial fib rillary acidic protein; and three of 16 epithelial membrane antigen. C linical follow-up was available in 10 patients and ranged from 6 month s to 20 years (mean 4.25 years, median 2 years). Two patients develope d local recurrence; metastasis to lung and lymph nodes were observed i n two additional patients, Both of the latter patients died, We believ e that these findings expand the concept of cutaneous mixed tumors to include neoplasms composed predominantly of myoepithelial cells and to include tumors arising in deeper subcutaneous and/or subfascial tissu es. The clinical behavior of such neoplasms, when arising in soft tiss ues, may be difficult to predict but is most often benign; however, a minority of lesions metastasize; Until larger studies with longer foll ow-up are available, treatment and prognostication are probably best b ased on criteria used in comparable salivary gland tumors.