Overview
Why Screening Can Prevent Cancer
Colorectal cancer usually develops through a sequence of molecular changes that takes years.
Colorectal cancer usually develops through a sequence of molecular changes that takes years. In the conventional adenoma–carcinoma pathway, early loss of APC regulation allows an adenoma to form; later KRAS activation promotes growth, and TP53 loss contributes to malignant transformation and chromosomal instability. Roughly 10–20% of colorectal cancers arise through a serrated pathway involving BRAF or KRAS mutation, CpG island methylation, and often microsatellite instability. That long preclinical phase creates two opportunities. A screening test can detect cancer before symptoms appear, and colonoscopy can remove a precancerous polyp before it becomes cancer. Screening therefore prevents some cancers; it is not merely a way to find established disease earlier. The benefit depends on the entire pathway: the right patient is offered an appropriate test, the patient completes it, an abnormal result leads to colonoscopy, and the colonoscopy is complete and adequately prepared. A test ordered but never completed, or a positive FIT followed by no colonoscopy, has little protective value.