Colorectal Cancer Screening

Abstract

Introduction Colorectal cancer (CRC) is well suited to screening. It is a common disease, affecting approximately 1 in 20 adults in the United States and Europe, ultimately proving fatal in almost 50%of cases. Symptoms are frequently nonspecific and are often common (eg, change in bowel habit and abdominal pain), leading many patients to ignore the condition until a relatively late (and hence incurable) stage. As for most cancers, prognosis is strongly related to disease stage at presentation, with early tumors (confined to the bowel wall) having nearly 95% 5-year survival compared with less than 50% if there is nodal involvement. Therefore, detection of early-stage cancer can reduce mortality by curing the patient of a disease that would likely be fatal if detected later. However, cure is not the only potential benefit of CRC screening; cancer can also be prevented. Most CRC is believed to develop from benign, but potentially premalignant, precursor lesions—adenomatous polyps. Although the natural history of colonic adenomas is not fully understood, a proportion of them undergo malignant transformation to carcinoma (the “adenoma-carcinoma sequence,” Fig. 1). In most cases, this transformation occurs slowly, averaging approximately 10 years. Hence, there is a window of opportunity during which adenomas can be removed, potentially preventing carcinoma from ever developing. The key target is the so-called advanced adenoma—one that is either large (Z10-mm maximal diameter) or shows significant dysplastic or villous components histologically, as these have the highest risk of malignant transformation. Accordingly, effective CRC screening programs can reduce both disease incidence and mortality, which may prove cost saving as well as clinically beneficial. Depending on the particular test used, screening programs combine the 2 approaches to varying degrees—prevention of cancer by removal of its precursor, or improved cure rates for established cancer via early detection.