S3979 Metastatic Basaloid Squamous Cell Carcinoma of the Rectum: A Rare and Aggressive Malignancy

Introduction: Basaloid squamous cell carcinoma (BSCC) is a rare subtype of squamous cell carcinoma typically found in the upper aerodigestive tract and anal canal. Involvement of the rectum is exceedingly uncommon and poses diagnostic and therapeutic challenges. BSCC is histologically aggressive with high metastatic potential, and there are no established treatment guidelines. Case Description/Methods: Sixty-five-year-old woman with a history of hyperlipidemia, diabetes, and vitamin D deficiency, presenting for a routine evaluation. She experienced daily diarrhea, urinary frequency, decreased appetite, lower extremity musculoskeletal pain, and suspected sinus and urinary tract infections. Notably, she reported a 20-pound weight loss over the past year, chronic constipation, occasional blood in stool, dysphagia, heartburn, nausea, and vomiting. The patient, a heavy smoker with a 108 pack-year history, expressed concern over a cyst on her right buttock that recently drained. An abscess culture returned positive for E. coli, treated with a 7-day course of trimethoprim/sulfamethoxazole. Anemia was noted with a hemoglobin level of 10.8. Comprehensive evaluations included computed tomography imaging revealing a 9 mm liver lesion, a pelvic mass suggestive of rectal carcinoma, and a retroperitoneal mass. EGD showed severe candidal esophagitis, while colonoscopy identified a large, ulcerated mass in the distal rectum, with biopsies confirming invasive, poorly differentiated squamous cell carcinoma. A follow-up PET scan indicated metastatic disease, leading to the initiation of carboplatin and paclitaxel treatment for metastatic anal cancer. Discussion: Rectal BSCC is a rare, aggressive malignancy with fewer than 200 cases reported. It can mimic poorly differentiated carcinomas and neuroendocrine tumors. Immunohistochemistry aids in diagnosis, with characteristic positivity for CK5/6, EMA, and p53. The disease carries a poor prognosis due to high recurrence and metastatic rates. There is no standardized treatment; regimens are extrapolated from anal SCC protocols and include chemoradiation or systemic chemotherapy. Rectal BSCC should be considered in elderly patients with rectal masses and anemia. Early recognition and histological confirmation are critical. This case highlights the need for a multidisciplinary approach and contributes to the limited literature on this rare malignancy. More studies are needed to define optimal treatment strategies.