This paper deals with the microsatellite instability in a patient with hereditary nonpolyposis color-ectal cancer (HNPCC). The patient was a 66-year-old woman. There were previous history of undergoing a hysterectomy for uterine cancer at the age of 416, and an extended right hemicolectomy for synchronous double cancer of the ascending and transverse colon in our hospital at the age of 58. Her family history disclosed that her father died of lung cancer, her mother died of uterine cancer, two of her sisters died of colon and gastric cancer respectively, and another two of her sisters had colon and uterine cancer respectively. After the last operation, she was diagnosed as having gastric and rectal cancer simultaneously, and we performed a distal gastrectomy and a Miles operation.
Histopathologically, both tumors were well differentiated adenocarcinoma. We found the genetic instability for the rectal specimen using a microsatellite assay. The patient and her family should be considered to have an increased risk of an additional primary cancer.