Background: Colorectal cancer associated with Crohn's disease (CD) is increasing in proportion to the number of patients with CD in Japan. There are two subtypes of colorectal cancer with CD: sporadic cancer and colitis-associated cancer. Early diagnosis of colitis-associated cancer is sometimes difficult; when colorectal cancer is found in patients with CD, both colitis-associated cancer and sporadic cancer should be kept in mind. Here, we describe a case of metachronous, colitis-associated rectal cancer that developed after the complete resection of an adenoma that became a sporadic adenocarcinoma in a patient with longstanding CD. To the best of our knowledge, this is the first report of colitis-associated cancer in a patient with CD after removal of a sporadic cancer.Case presentation: We describe a 51-year old man with CD who had difficulty in defecation. A rectal polyp was detected and a transanal resection of the polyp was performed. A histopathological examination showed an adenoma with sporadic adenocarcinoma. After three years, a follow-up colonoscopy revealed a reddish, elevated lesion in the patient's rectum. A colonoscopic biopsy showed a signet ring cell carcinoma. We performed an abdominoperineal resection of the rectum and a bilateral pelvic lymph node dissection. A histopathological examination revealed a mucinous adenocarcinoma with signet ring cell carcinoma and lymph node metastasis. The patient received adjuvant chemotherapy with oral uracil 224 mg combined with tegafur 100 mg plus leucovorin. No signs of recurrence were noted at a follow-up 18 months after the third surgery and 60 months after the second surgery.
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