[A case of basaloid squamous cell carcinoma of rectosigmoid colon].

Basaloid squamous cell carcinoma is a rare and aggressive variant of squamous cell carcinoma, which mostly occurs in the upper aerodigestive tracts. Basaloid squamous cell carcinoma also typically arises in the anal canal, but is extremely rare in the lower gastrointestinal tract. A 70-year-old man presented with loose stool and intermittent hematochezia 2 months ago. Colonoscopy showed an ulceroinfiltrative mass on the rectosigmoid colon from 16 cm to 18 cm above the anal verge. Conventional colonoscope could not pass through the lesion but it was possible with pediatric colonoscope. Abdominal CT scan showed 1.6 cm sized wall thickening with circumferential luminal narrowing in the rectosigmoid colon and multiple ill-defined low density masses in both lobes of the liver. Therefore, colon cancer with liver metastasis was suspected. However, basaloid cells were noted on histologic examination, and they were weakly positive for synaptophysin on immunohistochemical study. After palliative lower anterior resection, histologic examination of the resected specimen revealed basaloid differentiation with keratin pearls, and tumor cells were positively stained with high molecular weighted cytokeratin (34BE12) and CK 5/6. Thus, the patient was finally diagnosed with basaloid squamous cell carcinoma of rectosigmoid colon with distant metastases.