This lesion on the leg came from a 69-year-old female. It was submitted to rule out actinic keratosis and squamous cell carcinoma. Porokeratosis comes in several clinical varieties. In this case the patient had a solitary lesion. These lesions have a characteristic peripheral fine scale which corresponds to the cornoid lamella (spire of parakeratosis). The histologic differential diagnosis is often an actinic keratosis, but atypia is minimal-to-absent in porokeratosis.
Histologic sections of skin show an alteration in the stratum corneum. The lesion fit completely on the slide, but two images had to be apposed for visualization. The characteristic feature is the cornoid lamella, or angulated parakeratotic spires. This lesion shows a mixed inflammatory response at the base. Cytologic atypia is absent.