HPV-Negative Basaloid Squamous Cell Carcinoma of the Rectum: An Exceptionally Rare Entity.

Basaloid squamous cell carcinoma (BSCC) of the rectum is an exceptionally rare malignancy, distinct from other more common gastrointestinal cancers. Although human papillomavirus infection is strongly associated with squamous cell carcinomas at other anogenital sites, its role in rectal BSCC remains unclear. We report a 66-year-old woman who presented with fatigue and intermittent hematochezia. Colonoscopy revealed a distal rectal mass, and biopsy confirmed poorly differentiated BSCC. Immunohistochemistry was negative for human papillomavirus. Molecular profiling demonstrated high tumor mutational burden and elevated PD-L1 expression. This case underscores the extreme rarity of rectal BSCC and the diagnostic and therapeutic challenges it poses.