Infiltrating eccrine carcinoma arising in hidradenoma

    A 78 year-old woman presented with a 3 X 2 cm dermal nodule on the scalp. The clinical diffrential diagnosis was cylindroma vs plexiform neurofibroma vs angiofibroma. A punch biopsy in the middle of the lesion was performed.

    Histologic analysis showed a nodular proliferation within the deep dermis that extended to peripheral and deep edges of the biopsy. No connection to the overlying epidermis was seen. The lesion was composed of cellular islands and cords of epithelial cells with a fibrous stroma. On high power, there were 2 populations of cells, one with round dark nuclei and eosinophilic cytoplasm and a second population of more elongated cells with clear cytoplasm. Focally there was lumen formation. The following diagnosis was rendered: "Epithelial tumor with eccrine differentiation in deep dermis, extending to specimen margins, see note. Note: The lesion is positive for keratin and negative for SMA consistent with an appendageal tumor, most suggestive of nodular hidradenoma. No necrosis, significant cytologic atypia or mitoses are seen, however the lateral circumscription, infiltrative nature or potential behavior cannot be determined on this incisional biopsy." A complete excision was recommended for full evaluation, which was performed.