Cancer of left and right colon has a differing prevalence at varying ages, in high- and low-incidence nations, as well as in men and in women. There also is a difference in clinical presentation, in prognosis, and possibly in genetic and environmental epidemiology. This review proposes that cancers of proximal and distal colon are different tumors because of their embryologic origin, genetic changes, and biologic identity. These factors are important in understanding the ‘shift of tumors from more distal to more proximal sites in the colon’ and in evaluating potential suggestions for instituting advances in diagnosis and prevention.

Carcinogen induction of neoplasms in rodent colon has been used as a model for human colon cancer development and for evaluating chemopreventive regimens. We studied the regional distribution of small and large intestinal tumors in 229 rats given azoxymethane (AOM) once weekly for two weeks (15 mg/kg sc). The AOM regimen induced 63% more tumors in distal (DC) than in proximal colon (PC), although tumor volume was greater in PC. A high-fat (23% corn oil) diet increased tumors in PC and DC (p < 0.01). Caloric restriction of 10-30% of the ad libitum diet progressively reduced DC tumor formation but did not alter PC tumors. Tumor volume was unaffected by either regimen. Small intestinal tumors were concentrated in the proximal 15 cm of the intestine and were unaffected by dietary manipulation. This AOM model of colon tumor formation approximates human colon cancer distribution and is an appropriate model for rodent chemopreventive studies.