The proportion of rectal cancer cases diagnosed in people younger than 55 years doubled over the past 2 decades, according to a report published online Feb. 28 in the Journal of the National Cancer Institute.

In contrast, the proportion diagnosed in people older than 55 years has decreased over the last 4 decades, said Rebecca L. Siegel, MPH, strategic director of surveillance information services of surveillance and health services research at the American Cancer Society and her associates.

They examined time trends in colorectal cancer incidence using data from nine geographical areas in the Surveillance, Epidemiology, and End Results program regarding people aged 20 years and older who were diagnosed between 1974 and 2013. They used a statistical tool called age-period-cohort modeling to help differentiate factors that influence all age groups (period effects), such as changes in medical practice, from factors that vary by generation (cohort effects), which typically result from behavioral changes (J Natl Cancer Inst. 2017. doi: 10.1093/jnci/djw322).

The study population comprised 490,305 patients.

The incidence of rectal cancer increased by 3.2% per year during the study period among patients aged 20-29 years and in those aged 30-39 years. It didn’t begin rising until the 1990s in adults aged 40-49 years and 50-54 years, and then it rose by a smaller amount – 2.3% per year. In contrast, the incidence of rectal cancer generally declined throughout the 40-year study period among adults aged 55 and older.

Because of these opposing trends, there was a net increase in rectal cancer of 4% per year for people in their twenties together with a net decrease of 2% per year for those aged 75 years and older.

The decreasing rate of rectal cancer in older adults “may partly reflect detection and removal of precancerous lesions during clinical inspection of the rectum, which was common practice well before formal colorectal cancer screening. Inherent differences within the colorectum in the way environmental factors initiate and or promote carcinogenesis, as well as the influence of unknown risk factors, may also have contributed,” Ms. Siegel and her associates said.

The temporal pattern was somewhat different for colon cancer. The risk of colon cancer declined “for successive generations during the first half of the twentieth century but has escalated back to the level of those born circa 1890 for current birth cohorts.”

The rising incidence of both colon and rectal cancers among younger adults is “sobering,” given that such trends “often provide a bellwether of the future disease burden,” they noted.

“The strong birth cohort effects we observed signal relatively recent changes in exposures that influence risk,” including excess body weight, high intake of processed meat, low intake of dietary fiber, and low levels of physical activity. “New strategies to curb the obesity epidemic and shift Americans toward healthier eating and more active lifestyles” are needed, the researchers said.

In addition, both clinicians and the public must be educated about the rising probability of the disease in people younger than 55 years. Timely follow-up of symptoms, regardless of patient age, must be emphasized. Younger adults are nearly 60% more likely than are older adults to be diagnosed with advanced colorectal cancer, largely because they delay seeking medical care. The disease simply isn’t “on the radar” of young adults or their providers, the investigators added.


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