An early stage small bowel adenocarcinoma with microsatellite instability phenotype in a case of hereditary nonpolyposis colorectal cancer

Background: Hereditary nonpolyposis colorectal cancer (HNPCC) is an autosomal dominantly inherited disease characterized by onset at a relatively early age, excess of synchronous and metachronous tumors, and a variety of extracolorectal malignancies. Small bowel carcinoma is included in the tumor spectrum of HNPCC, but the frequency of occurrence of this tumor in HNPCC patients is comparatively rare. Case presentation: We report the case of a 55-year-old woman who had a history of multiple colon cancers at 33 years of age, sigmoid colon cancer at 47, and endometrial carcinoma at 51. This case fulfills the Amsterdam criteria for HNPCC and followed from the patient's age of 47 at our institute. Surveillance colonoscopy showed a sessile polyp in the ileum that was 9 mm in diameter and located about 10 cm proximal to the ileorectal anastomosis, and that was resected by endoscopic mucosal resection. Histopathological studies showed an adenoma with well-differentiated adenocarcinoma in the mucosa. A molecular analysis of the adenoma component of this polyp was performed, and microsatellite instability was found in four of the nine analyzed loci. The patient was a mutation carrier in hMSH2, one of the mismatch repair genes responsible for HNPCC. Conclusion: Reports of early-stage carcinoma of the small bowel in HNPCC are very rare, and an adenoma-carcinoma sequence was present in the small bowel tumor of this patient. The molecular findings of this tumor are discussed.