Abstract

Inverted follicular keratosis (IFK) is a benign cutaneous epithelial tumor which typically presents as a small papule on the head and neck. We have occasionally encountered deeply endophytic tumors on genital skin with some characteristics of IFK but also atypical features that raised consideration of squamous cell carcinoma (SCC). Herein we describe 4 such cases to better characterize and classify these unusual lesions. All patients were young women (age 22-36) who presented with a solitary 0.5 to 1 cm lesion on the vulva or perineum. One lesion was described as cauliflower-like. Microscopically, each showed an epithelial proliferation arising from the epidermis, with an endophytic growth that was deep, bulbous, and/or expansile but not infiltrative. An exophytic component and a slightly verrucous surface were noted in 2 (50%) and 3 (75%) cases, respectively. The tumors were composed of closely packed squamous lobules with numerous squamous eddies. Another consistent feature was small irregular spaces within the squamous lobules, likely due to spongiosis or myxoid degeneration. Dyskeratotic cells were readily found in 3 (75%) cases. Nuclear atypia was seen in only 1 (25%) case and was mild. Mitotic activity was not brisk. No koilocyte or lichen sclerosus was identified. Evidence of trauma and irritation was observed in all cases. By immunohistochemistry, all demonstrated wild-type p53 expression and lack of p16 block positivity. In-situ hybridizations for low- and high-risk human papillomavirus were negative. In summary, we describe a unique series of endophytic tumors on genital skin of young women. Despite some atypical features, the absence of infiltrative growth or high-grade cytologic atypia, the presence of numerous squamous eddies, and the reassuring immunohistochemical profile support a variant of IFK with robust reactive changes. We propose the term “proliferating IFK” to account for the florid squamous proliferation and reactive atypia in these lesions.

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