Skin, Keratoacanthoma

This is a case of involuting keratoacanthoma, in the background of severe solar elastosis. Keratoacanthoma is considered by some to be a form of squamous cell carcinoma (SCC); it may represent proliferation of a hair follicle (infundibular portion). One can certainly appreciate its resemblance to (well-differentiated) SCC, although some argue that SCC and keratoacanthoma are different entities, based on gene expression and IHC profiles.

Keratoacanthomas can be described to be in different phases. They can involute/regress to form the keratin-filled crater, as shown in this case.

While excision is generally sufficient, it can exhibit an aggressive growth pattern.