Inverted follicular keratosis and papillomavirus infection

This report describes a case of inverted follicular keratosis in a keratotic nodule found on the thigh of a 58-year-old man. The upper surface of the nodule was papillary, while the lateral aspect was smooth with columnar structures. Histologically, the lesion exhibited primarily exophytic growth, although there was some endophytic growth. There were multiple closely approximated columnar solid tumor nests. Many of these columnar tumor nests had a central funnel-shaped plug of dense parakeratotic keratinous material. The tumor nests were composed of peripheral smaller basaloid cells and central squamoid cells forming squamous eddies. Inflammatory lymphocytic cell infiltration and lytic change were found mainly at the base of the tumor. Several hair follicles transversed the tumor. Ballooning alteration of granular cells and vacuolated keratinocytes, especially within areas of hypergranulosis, were observed in each of the columnar tumor nests. Some cells with ballooning alteration were recognized by papillomavirus common antibody. Ultrastructurally, vacuolated keratinocytes exhibited cytoplasmic vacuoles containing keratohyaline composite granules. Compound structures of membrane-coating granules, which have been associated with outer root sheath keratinization, were noted. The case therefore represents an inverted follicular keratosis manifesting as a distinct hair follicle tumor related to prior infection by papillomavirus.