Solid carcinoma revisited: a possible variant of microcystic adnexal carcinoma

Primary malignant apocrine and eccrine skin neoplasms are rare and their nosology is still evolving. First described in 1997, solid carcinoma is now considered a discrete entity by some, although this remains controversial. Apocrine differentiation was postulated. A single case since then was the first to include immunohistochemistry findings. The authors report an additional case of solid carcinoma, together with its immunohistochemical profile. A 63-year-old man presented with a firm nodule 3 cm in diameter on the posterior scalp. On punch biopsy, the lesion was interpreted as an adnexal neoplasm of indeterminate malignant potential. The subsequently excised tumor was initially interpreted as microcystic adnexal carcinoma with perineural invasion involving the surgical margins. Re-excision yielded clear surgical margins. After review of all the histologic material, a final diagnosis of solid carcinoma was rendered. Histologically, innumerable solid aggregations of neoplastic epithelial cells filled the dermis and extended into the subcutis. The aggregations varied in size and ranged from round or ovoid nests to elongated columns or cords embedded within a fibrotic stroma. The neoplastic cells were round and uniform in size with small nuclei and pale or clear cytoplasm. Tubular structures and mitoses were absent. Solid carcinoma is a rare skin neoplasm. Histologically, it closely resembles and may be indistinguishable from the solid areas of microcystic adnexal carcinoma. Biologically, solid carcinoma, like microcystic adnexal carcinoma, is an indolent but relentless locally destructive neoplasm that must be removed completely. The clinical, histologic, and biologic similarities suggest that solid carcinoma may be a variant of microcystic adnexal carcinoma.