Introduction: Intramural metastasis (IM) is common in esophageal cancer, and it is an important factor in determining the resection area and tumor malignancy. However, IM is rare in rectal cancer; therefore, little is known about IM in rectal cancer, and the clinical significance remains unclear. Case Presentation: We describe a case of rectal cancer with distally spreading IM. A 58-year-old man consulted a primary care physician, with a chief complaint of constipation; tests revealed a high carcinoembryonic antigen value. A colonoscopy revealed a type 2, advanced rectal tumor, which covered two-thirds of the circumference of the upper rectum. In addition, 3 protruding lesions were observed under the normal mucosa on the anal side of the primary tumor. A laparoscopic low anterior resection was performed. Pathologic findings showed that the primary tumor was a moderate-to-well-differentiated adenocarcinoma with a cribriform structure. The 3 lesions on the anal side found under normal mucosa were separate from the primary tumor, but morphologically similar to the primary adenocarcinoma; therefore, these were diagnosed as IMs. Based on a review of previous case reports, rectal carcinomas were often accompanied by vascular invasions and lymph-node metastases. Moreover, in rectal cancer, tumors with IMs often show vascular invasion. Therefore, we assumed that IM could be a marker of poor prognosis. Conclusion: This study revealed that, in surgery, detection of a distally spreading IM is an important finding for determining the optimal surgical resection margin.
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