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Incidences of colorectal cancer have been decreasing by about 1 percent a year since the mid 1980s, but incidences among people under 50 — the recommended screening age — has been increasing sharply, and these younger patients are more likely to present with advanced disease.

The study, published in JAMA Surgery, used a national database of 400,000 patients with colon or rectal cancer. Incidences decreased by about 1 percent a year over all but rose among people 20 to 34, with the largest increase — 1.8 percent a year — in disease that had already progressed to other organs.

Incidence rates today, per 100,000 people, are 3 for ages 20 to 34; 17 for ages 35 to 49; and 300 for people over 50. But by 2030, the researchers estimate, one in 10 colon cancers and one in four rectal cancers will be in people under 50, and rates among those over 50 will be 175 per 100,000.

The study draws no conclusions about whether screening should begin at a younger age. “There are always risks and unintended consequences of screening tests,” said the senior author, Dr. George J. Chang, an associate professor of surgery and health services research at the University of Texas.

For now, he said, “We have to pay attention to symptoms with which our patients present, and work them up by including colorectal cancer as a part of the differential diagnosis.”