Adenomas and carcinomas may be histologically detected in apparently normal colonic mucosa. A study of carcinogen-treated rats.

Clinical reports indicate that colonic neoplasias may be overlooked at colonoscopy. We investigated, in an experimental model, whether both gross and high power microscopical examinations could detect all induced colonic neoplasias. Forty-nine Sprague-Dawley rats received 1,2-dimethylhydrazine for 27 weeks. The colons were scrutinized grossly and at high-power microscopy complemented with chromoscopy. Biopsies were obtained from all mucosal alterations seen at gross and at high power microscopy. The gross examination of the colonic specimens in the 49 animals revealed 196 polyps or suspected polyps. Histology showed that 85.2% (167 out of 196) were adenomas or adenocarcinomas. The remnant polyp-free colons were subsequently examined by high-power stereo microscopy (46 magnifications). Fifty-five new lesions were detected and removed. Histology showed 48.9% (n = 24) additional neoplastic lesions in the 49 colon specimens. The remnant polyp-free colons were subsequently processed for histology. The sections showed 57 additional neoplastic lesions in the 49 colon specimens: 93.0% (n = 53) were adenomas (n = 35) and crypt dysplasias (n = 18), while the remaining 7.0% (n = 4) were invasive carcinomas. Experimentally-induced colonic neoplasias may be overlooked even after a careful scrutiny of the colonic mucosa using high-power optical devices.