Solitary keratoacanthoma: a self-healing proliferation that frequently becomes malignant.

To discuss the relation between solitary keratoacanthoma (KA) and crateriform squamous cell carcinoma (cSCC), the clinical and histologic features of cutaneous crateriform squamous cell proliferations were studied. Two hundred twenty cases of wholly excised crateriform squamous cell proliferations were studied both clinically (age, sex, location, and duration) and histologically (hematoxylin-eosin-stained sections). For comparison, we studied 100 consecutive cases of wholly excised noncrateriform squamous cell carcinoma (ncSCC). One hundred forty-four of the 220 cases of crateriform squamous cell proliferations were histologically classified as KA. In 47 other cases, a relatively large area of the KA showed frank histologic and cytologic malignant transformation (mKA); this event could happen during every stage of the KA. Twenty-nine lesions were cSCCs without remnants of KA. The patients in the KA group were significantly younger (p = 0.000) than those in the other three groups. The ages of the patients in these three groups were not significantly different (p = 1.0). More KAs (16%) were located in areas that are not usually exposed to the sun than was the case with the other groups of neoplasms considered (2%, 3%, and 3%, respectively), and this difference was statistically significant (p = 0.001). Regarding the duration of the lesion, only the differences between KA and cSCC, KA and ncSCC, and mKA and ncSCC were statistically significant. Not every cutaneous crateriform squamous cell proliferation is a KA; in KA, the crater must be multilocular, the "lips" must be perforated, and the cornified contents do not usually project out of the "mouth." At least a quarter of KAs undergo malignant transformation, which occurs more frequently in older patients and photoexposed areas. This transformation is a focal event and may happen at any stage of KA development. Consequently, a suspected KA must be wholly excised and studied in serial paraffin blocks so as to disclose any focus of malignant transformation.