Malignant cutaneous mixed tumor with sebaceous differentiation.

Malignant cutaneous mixed tumor (CMT) is a very rare adnexal tumor with biphasic differentiation. In rare cases, a benign CMT (chondroid syringoma) undergoes malignant transformation. Sebaceous differentiation in a cutaneous malignant mixed tumor has not been previously reported. We present a malignant CMT with sebaceous differentiation, which occurred on the scalp of an 81-year-old man. The tumor showed epithelial elements composed of relatively small and bland-appearing ductal and cord-like structures lined by small, cuboidal-shaped adnexal cells, with a few large, dilated gland-like spaces lined by larger, apocrine-appearing cells with abundant eosinophilic-staining cytoplasm. However, the majority of the epithelial component was composed of nests and islands of markedly enlarged and atypical cells with pale÷clear to ground-glass cytoplasm. Focally, there was sebaceous differentiation identified, in the form of prominent multivacuolated cytoplasm, with nuclear indentations. The stroma showed a mixture of myxoid and hyalinized÷chondroid-appearing areas with focal calcifications. There was strong and diffuse staining of the sebaceous cells by cytokeratin (CK) 7, epithelial membrane antigen (EMA), and androgen receptor (AR). Mismatch repair proteins were investigated by immunohistochemistry, without evidence of loss of expression of MutS protein homolog 6 (MSH6), MutS protein homolog 2 (MSH2), MutL protein homolog 1 (MLH1), or postmeiotic segregation increased 2 (PMS2) in the sebaceous cells.