Importance of sessile serrated lesions in a patient with familial adenomatous polyposis.

A 28-year-old male visited hospital because his mother had been diagnosed with familial adenomatous polyposis (FAP) with a pathological variant of the APC gene. Total colonoscopy showed that he has more than 100 polyps distributed throughout the colorectum, and the APC gene variant was also detected. After he was diagnosed with FAP, he received information that surgery was currently the only way to prevent the development of colorectal cancer. However, he firmly declined to undergo surgical procedures and decided to have strict follow-up with frequent endoscopic polypectomy to prevent the development of colorectal cancer. At the first endoscopy, polypectomy was performed on 52 polyps. Histological analysis of the dissected polyps showed that they were all adenomas, but adenocarcinoma was not detected. The second endoscopic polypectomy was performed after 4 months later. We found a pale 20 mm wide flat, elevated type polyp in the ascending colon with an adherent mucus cap that was resistant to washing off. After endoscopic mucosal resection, histological analysis revealed that there were two lesions in the polyps, a sessile serrated lesion (SSL) and SSL with dysplasia. SSL is a high-risk lesion for colorectal cancer, but it was reported to be rare in patients with FAP, and the existence of SSL suggested another carcinogenesis pathway in patients with FAP in addition to the adenoma-carcinoma sequence. Our report may be significant not only in consideration of the pathogenesis of FAP but also useful to raise awareness of SSL for clinicians who perform endoscopic polypectomy to prevent the development of colorectal cancer in patients with FAP.