Keratinocytic, Adnexal, and Other Epithelial Neoplasms
Keratinocytic tumors
Actinic keratosis shows basal atypia with parakeratosis; full-thickness atypia is squamous carcinoma in situ, and invasion is defined by dermal extension, not adnexal involvement. Keratoacanthoma is a crateriform, symmetric proliferation with glassy cytoplasm and a neutrophilic crust; it can be indistinguishable from well-differentiated squamous carcinoma, so report margins and state the uncertainty. Basal cell carcinoma subtypes: nodular (palisading, clefting, mucin), superficial (multifocal buds), morpheaform (cords in fibrous stroma, perineural risk), and basosquamous. Immunostains help: basal cell carcinoma is BerEP4 positive with CD10 in tumor cells, whereas trichoepithelioma shows CD34-positive stroma and no clefting.
Adnexal and other epithelial neoplasms
Sebaceous adenoma, sebaceoma, and sebaceous carcinoma raise Muir-Torre syndrome, so recommend mismatch repair immunohistochemistry. Trichilemmoma points to Cowden syndrome. Pilomatricoma shows shadow cells and nuclear beta-catenin. Microcystic adnexal carcinoma is deeply infiltrative and perineural, and is easily underdiagnosed on superficial biopsy. Extramammary Paget disease shows intraepidermal large cells positive for CK7, CAM5.2, and mucin, and must be separated from melanoma in situ and squamous carcinoma in situ.
Traps
Superficial biopsies underdiagnose morpheaform basal cell carcinoma and microcystic adnexal carcinoma. Do not call a crateriform lesion squamous carcinoma without noting keratoacanthoma-like architecture. A sebaceous proliferation in a young patient should prompt syndrome workup.
Rule of thumb
Basal cell carcinoma palisades and clefts; trichoepithelioma has a CD34-positive stroma and no clefts. A nasal biopsy with basaloid cords, perineural invasion, and dense fibrous stroma is morpheaform basal cell carcinoma, requiring re-excision with margin control.