Extensive Perianal Basal Cell Carcinoma Presenting With Severe Anorectal Dysfunction.

Perianal basal cell carcinoma is an exceedingly rare malignancy that may mimic benign anorectal conditions or anal squamous cell carcinoma. We report an 84-year-old man with a several-year history of a progressively enlarging circumferential perianal lesion, complicated by constipation, chronic fecal incontinence, bleeding, and significant weight loss. Examination revealed a 15 cm ulcerated tumor involving the entire anal verge, with biopsy confirming basal cell carcinoma and sparing of the anal canal mucosa. Imaging showed fecal impaction and perianal soft tissue thickening without metastasis. Given extensive involvement, vismodegib was initiated. This case highlights the importance of early biopsy of atypical perianal lesions.