BACKGROUND: Flat-type colorectal cancer is frequently reported in Japan and Europe, but its clinical features remain obscure. Thus, we investigated the clinical features of flat-type early colorectal cancer with respect to tumor location and patient age and compared them with those of polypoid-type early and advanced cancer. METHODS: Between January 1999 and June 2001, total colonoscopy was performed in 6,178 patients (mean age, 61 years; 4,290 males and 1,888 females). Of these patients, 402 patients with 429 colorectal cancers were found: 202 at advanced stage (invading beyond muscularis propria) and 227 at early stage (carcinoma in situ or invading within submucosa). Early-stage cancer was classified into two macroscopic subgroups: flat-type and polypoid-type. RESULTS: Out of 227 early cancers, 44 were flat type and 183 were polypoid. Flat-type early cancer was more frequently located in the right colon (57 percent, 25/44) than polypoid-type cancer (19 percent, 35/183; P < 0.001). Adenomatous component in flat-type early cancer was less frequent than in polypoid-type cancer (23 percent vs. 92 percent, P < 0.001). The proportion of right-sided colon in flat-type early cancer increased with age (33 percent in patients less than or equal to 59 years, 50 percent in patients between 60 and 69 years, and 72 percent in patients greater than or equal to 70 years), whereas polypoid-type early cancer showed minimal change (16 percent, 18 percent, and 25 percent, respectively). An increase in the proportion of right-sided colon with age was also found in advanced cancer (20 percent, 38 percent, and 52 percent, respectively). CONCLUSION: The incidence of flat-type early cancer in right-sided colon increased with age, similar to the pattern of advanced cancer. This suggests that flat-type early cancer may be a precursor of advanced cancer in the right colon, especially in older people.
