A 4 mm white-yellow submucosal tumor-like lesion was detected in the sigmoid colon of an asymptomatic 52-yr-old Japanese man. Because the lesion was unexpectedly suspicious for adenocarcinoma by pathological examination of the biopsy specimen, it was treated by endoscopic mucosal resection. The specimen obtained demonstrated well-differentiated adenocarcinoma without any adenomatous element, and was located principally in the submucosal layer with a maximum depth of 1600 mum from the muscularis mucosae. The cancer exposed to the luminal surface was pathologically concluded to be diminutive. Intriguingly, aggregation of lymphocytes was found beneath the mucosal layer, which might have compromised the integrity of the muscularis mucosae. Because of deep submucosal infiltration and the latent aggressive nature of de novo cancer, the patient underwent an additional partial sigmoidcolectomy, which demonstrated no residual cancer and no regional lymph node metastasis. The lesion in this patient exhibited a previously undescribed appearance of de novo colon cancer as submucosal tumor in an early phase of growth.
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