[Flat neoplasia of the colon and rectum: definitions and illustrations]

Thanks to recent progresses in colonoscopy non-polypoid lesions, even very small, can be detected and removed per endoscopy. They are defined endoscopically as flat polyps or flat lesions and correspond microscopically to neoplasia in more than 30% cases. This review presents the definitions and different types of flat adenomas and flat early adenocarcinomas as established by Japanese authors. The use of appropriate definitions and classifications and the need of a perfect method in preparing the samples before microscopic examination are underlined. Flat adenocarcinomas tend to invade the submucosa at a smaller size than carcinomas arising in polypoid adenomas. They result either of the malignant transformation of flat adenomas or of a de novo type histogenesis. Flat early adenocarcinomas invading the submucosa and measuring less than 1 cm belong to the latter category.