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

Follow up:

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 which was performed.

Discussion:

Cutaneous sweat gland tumors can present diagnostic challenges because they are uncommon and can have a variety of  histologic appearances that define different subtypes (which have many different names). The most important decision is benign vs. malignant.

Benign

Well-circumscribed, lobular growth

Two cell layers to ducts

Clear cell change, squamoid areas, papillary apocrine features

Fibrous stroma, thick capsule

Small dark nuclei, minimal pleomorphism

Rare mitoses, no necrosis

Malignant

Infiltrative edges

Tumor invades capsule

Mucin pools

Large, pleomorphic nuclei

Mitotic activity

Necrosis

Perineural invasion

Recurrence after incomplete excision