in patients with obstructive colorectal cancer , where conventional colonoscopy can not traverse obstruction , clearance of the proximal colon remains a problem . a 58-year - old woman presented to her physician with rectal bleeding and intermittent diarrhea . conventional colonoscopy revealed a bulky tumor which was diagnosed as rectal cancer . slim optical colonoscopy ( pcf - p240al / i , olympus , tokyo , japan ) showed obstructive cancer in the rectosigmoid junction ( fig . 1 , arrow a ) and no information of the proximal side of the obstruction . the patient then underwent multidetector - row computed tomographic ( ct ) colonography ( virtual colonoscopy ) for further evaluation of the proximal colon . three - dimensional endoluminal  fly - through images revealed another protruded lesion ( fig . diagnosis of synchronous double cancer was made by ct air - contrast enema image ( fig . this ct data was not only used to create three - dimensional images but also to decide on a preoperative clinical staging . laparoscopy - assisted high anterior resection was performed and t3 rectal cancer and t1 sigmoid colon cancer were confirmed in the resected specimen ( fig . between 4.6 and 11% of patients with colorectal cancer have a second synchronous cancer [ 1 , 2 , 3 ] . preoperative evaluation of the whole colon and rectum in patients with colorectal cancer is widely recommended . optical colonoscopy is the current gold standard method as it examines the whole colon and rectum . however , preoperative examination by optical colonoscopy may be hampered by occlusive colorectal cancer , and failure to find the presence of a synchronous cancer will change the surgical approach from circumscribed resection to resection of the involved segments . ct colonography might be offered in preference to barium enema for incomplete colonoscopy [ 5 , 6 ] . ct colonography has recently become a popular preoperative examination tool with significant improvement in quality of image due to a rapid progress in computer technology [ 7 , 8 ] . ct colonography correctly showed synchronous double cancer in our case and provided crucial information for planning surgery . we recommend that ct colonography should be used for evaluating the proximal side of obstructive colorectal cancer , as it is well tolerated and less invasive [ 6 , 9 ] .