Combined inverted follicular keratosis and adnexal tumors: a series of 12 cases.

Inverted follicular keratosis (IFK) is an uncommon benign skin epithelial neoplasm that microscopically consists of a superficial, sharply circumscribed tumor composed of bland keratinocytes associated with squamous eddies. Only a few examples of combined tumors associating IFK and either trichoblastoma or a sebaceous component have been reported in the literature. Herein, we report the clinicopathological and molecular characterization of 12 cases of combined tumor comprising IFK associated with an adnexal neoplasm, including trichoblastoma (n = 7), sebaceoma (n = 4), and tubular adenoma (n = 1). The age at presentation ranged from 26 to 79 years, with a mean of 55 years. The majority of the tumors were located on the head (n = 8) with a median size of 6 mm (Range: 3-14 mm). Microscopically, all cases showed a biphasic appearance with a prominent IFK component superficially, associated either with trichoblastoma, sebaceoma or tubular adenoma at the deeper aspect, with an abrupt transition between the two tumor components. There was no evidence of nevus sebaceus in the surrounding tissue. Molecular investigation revealed pathogenic HRAS or KRAS mutations in 3 of the 4 cases harboring a sebaceous component and also in the specimen with combined IFK and tubular adenoma. No mutations were detected in any of the 5 tested IFK-TB cases. To conclude, our series expands the spectrum of combined tumors associated with IFK. Genetic characterization suggests that pathogenic mutations of RAS might drive a subset of these cases, particularly when associated with sebaceoma, even in the absence of pre-existing nevus sebaceus.