Colorectal Cancer in Young Adults

Routine screening for colorectal cancer (CRC) in adults > 50 years of age has led to overall reductions in CRC incidence and CRC-related mortality. Yet CRC incidence among young adults age < 50 continues to increase without a clear explanation. This review examines the changing epidemiology of CRC and emerging evidence regarding the influence of genetic and lifestyle factors on risk for colorectal neoplasia.

Young-onset CRC (yCRC), defined as CRC diagnosed in individuals younger than age 50, is a heterogeneous disease. Approximately, one in every five individuals affected with yCRC carries a pathogenic germline variant in genes associated with predisposition to cancer. However, most have no clinically identifiable risk factors. Analyses of birth cohorts estimate CRC risk among millennials to be 2-4 times higher than their grandparents', suggesting that changes in health behaviors and environmental factors are having an impact on CRC risk. Young individuals with CRC tend to be diagnosed at later stages and often present with metastatic disease. yCRC tumors arise predominantly in the distal colon and are more likely than older-onset tumors to exhibit microsatellite and chromosome stable (MACS) phenotypes. Although yCRC patients are more likely than their older counterparts to be treated with multimodality chemotherapy regimens, more aggressive treatments have not yielded measurable survival gains. Since one in ten new CRC diagnoses involve individuals age < 50, recent guidelines have proposed lowering the age for average risk CRC screening from 50 to 45; however, further studies are needed to evaluate testing strategies based on individuals' age and risk. Significant shifts in CRC epidemiology and diversity of tumor phenotypes support genetic and environmental factors as modifiers of cancer risk. Emerging data correlating tumor molecular features with outcomes justify further investigation into mechanisms of carcinogenesis to elucidate how specific factors (inherited and/or acquired) might stimulate young-onset colorectal neoplasia.