Purpose: No prospective observations of "flat" tumors of the colon have yet been reported. The purpose of this study was to investigate the progression of small flat tumors by prospective observations using colonoscopy.
Methods: The shapes of flat tumors were classified into three types, including slightly elevated lesions with a flat top (flat-top type), slightly elevated lesions with a wide depression in the center (wide-depressed type), and slightly elevated lesions with a slit-like depression in the center (slit-depressed type). A total of 14 flat tumors of the colon in 13 patients (2 women; median age, 58; range, 46-72 years) were examined in this project. All cases were followed up prospectively using colonoscopy, and all underwent a resection using the submucosal saline injection and snaring technique.
Results: Because two patients were eventually dropped from the follow-up, only 12 lesions were studied. The observation period ranged from 11 to 26 (median, 19) months. At the time the observations started, the diameter of the tumors varied from 2 to 6 (median, 4) mm, and the shapes were flat-top type in five lesions, wide-depressed type in three lesions, and slit-depressed type in four lesions. Of the 12 flat tumors, 8 showed various changes in their shape. However, only two lesions demonstrated an increase in diameter of the tumor from 2 to 4 mm. In the other ten tumors any change in size was less than 2 mm. No lesions were carcinomas according to the final histologic diagnosis, but all were adenomas.
Conclusions: Flat tumors of the colon did not rapidly progress when they measured approximately 5 mm in diameter. Such flat tumors did tend to change their shapes; however, such changes did not indicate invasion to the submucosal layer.