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Abstract

We report a rare case of a proliferative verrucous cyst, an entity not previously described, initially presumed to be a pilonidal cyst on clinical grounds. A 23-year-old man underwent excision of a 6 × 6 cm indurated chronically draining top of sacrum lesion clinically diagnosed as a pilonidal cyst with associated abscess. The patient had no history of cutaneous warts, immunosuppression, or prior local trauma. Histopathologic examination revealed a cyst lined by markedly acanthotic squamous epithelium with papillomatous projections, hyper-granulosis, parakeratosis, orthokeratosis, and coarse keratohyalin granules. Mild nuclear atypia was present, without increased mitotic activity. Epidermal papillae projected into the lumen, and the surrounding stroma showed a dense mixed inflammatory infiltrate with abscess formation. Immunohistochemically, p16 expression was patchy and weak, while p53 highlighted basal keratinocytes. Chromogenic in situ hybridization for low- and high-risk HPV types was negative; however, the probe set used does not cover all HPV subtypes, so the possibility of an undetected HPV type remains. PCR-based testing could further clarify HPV involvement. Verrucous cysts are rare, and proliferative verrucous cyst are even rarer non-plantar HPV-associated epidermoid cyst variants that are clinically indistinguishable from epidermal inclusion cysts and require histopathologic confirmation. This case underscores the importance of considering proliferative verrucous cyst in the differential diagnosis of cystic squamous lesions with verrucous architecture, even in atypical locations, and highlights the limitations of standard CISH testing in excluding HPV.