Unlike screening for breast or prostate cancer, screening for colorectal cancer promises not only to find cancer early, but also to prevent it from occurring. In the 1960s, Gilbertsen first suggested that polypectomy could turn colorectal cancer into a preventable disease.1 Two decades later, Vogelstein envisioned the polyp-to-cancer progression as a stepwise process and detailed the genetic alterations that occur at each step.2 Colorectal cancer became widely viewed as having a long latency period — providing ample time for both early detection and prevention. Conditions were thus considered ideal for screening to reduce related mortality.

Nine randomized trials summarized in a Cochrane review provide empirical evidence of an effect of screening on both colorectal-cancer incidence and mortality.3 Four trials revealed a 14% reduction in colorectal-cancer mortality and a 5% reduction in colorectal-cancer incidence with fecal occult blood testing, suggesting that early cancer detection is primarily responsible for the reduction in mortality associated with that method. Five trials demonstrated a 28% reduction in colorectal-cancer mortality and an 18% reduction in colorectal-cancer incidence with sigmoidoscopy, suggesting that cancer prevention is the predominant mechanism for its effect. The trial data confirm colorectal cancer’s long latency period by revealing a substantial delay — on the order of a decade — between screening and reduced cancer incidence and mortality.