An unusual case of pilonidal p16 positive squamous cell carcinoma-a case report.

Basaloid squamous cell carcinoma (BSCC) is a rare and aggressive variant of squamous cell carcinoma. It is commonly seen in the oropharynx and strongly associated with p16-expressivity and high-risk human papilloma virus (HPV). We report the first case of primary cutaneous p16-positive BSCC in an elderly woman, with a background of chronic inverse psoriasis of the natal cleft. P16-expressivity is a common surrogate marker for oncogenic HPV16, routinely tested for oropharyngeal/anogenital squamous cell carcinoma. This is not routinely done for primary cutaneous disease. Pilonidal disease is uncommon in the elderly population, and malignant transformation is rarer still. Surgical resection is considered the mainstay of treatment for primary cutaneous BSCC, however due to this patient's broad distribution of cutaneous field change and p16-expressivity, she was effectively treated with primary radiotherapy. This is a unique case of malignant transformation of pilonidal disease in an atypical demographic, with a rare/aggressive disease variant.