We observed seborrheic keratoses with many basilar clear cells, creating a microscopic pattern that mimicked a seborrheic keratosis involved by melanoma in situ.

We sought to report a series of these seborrheic keratoses and the immunohistochemical stains used to reach a proper diagnosis.

We reviewed 9 cases of seborrheic keratosis that had a distinctive pattern of basal clear cells with ample cytoplasm. All cases were evaluated by conventional microscopy, and Melan-A, S-100, and high molecular weight keratin 903 immunostains.

The basal clear cells failed to react with Melan-A and S-100 protein antisera. In contrast, these cells labeled with an antikeratin antibody in all cases. In all, 7/9 (78%) showed immunopositivity only at the peripheries of cells, creating a pattern that could be mistaken for a negative stain if not examined at high magnification.

This is a retrospective review of cases limited to a large referral dermatopathology service.

We describe a previously uncharacterized pattern of seborrheic keratosis that can microscopically mimic melanoma in situ. Careful conventional microscopy coupled with a panel of immunostains can allow the proper diagnosis to be reached.