Primary Rectal Squamous Cell Carcinoma: A Rare Gastrointestinal Malignancy Presenting As Hematochezia.

Primary rectal squamous cell carcinoma (SCC) is an extremely rare malignancy. We describe a 67-year-old male with a history of paroxysmal atrial fibrillation who presented with abdominal cramping, lightheadedness, fatigue, and new-onset hematochezia. Colonoscopy revealed a submucosal, ulcerated, partially obstructing mass in the distal rectum. Endoscopic ultrasound demonstrated a hypoechoic mass arising from the muscularis propria with associated lymphadenopathy on MRI of the pelvis. Biopsies confirmed moderately differentiated invasive SCC. The patient was treated with chemoradiation using 5-fluorouracil, mitomycin C, and radiation therapy. This case highlights the importance of considering rectal SCC in the differential diagnosis of rectal masses and hematochezia, as its management has shifted toward definitive chemoradiation with improved outcomes compared to traditional surgical approaches.