Rates of rectal cancer in people younger than 40, although low, have been rising steadily for the last 20 years for reasons that are mystifying scientists, researchers said Sunday.
Both colon cancer and rectal cancer are thought to share the same risk factors, but the incidence of colon cancer has remained steady during that period, while the incidence of rectal cancer has grown by an average of 3.8% per year, scientists reported online in the journal Cancer.
"We've scoured the literature for a cause and spoken to others in the field and we haven't identified anything that is able to explain this," said Dr. Joshua Meyer of the Fox Chase Cancer Center in Philadelphia, the lead author of the study. "It's a little bit puzzling."
The incidence of rectal cancer has also been growing in older people during the same time period, but that increase is generally attributed to changes in medical guidelines that called for more frequent screening, which would catch more cases of the cancer.
But people under 40 are generally not screened for colorectal cancer unless they have a family history of the disease, so screening does not account for the increase in that group, Meyer said.
Colorectal cancer is the second-most-common cause of cancer death in the United States; rectal cancer accounts for only a relatively small proportion of those deaths.
Meyer and his colleagues performed the study while he was at the Weill Cornell Medical Center in New York. The group noticed an increase in rectal cancer among their patients under 40 "and we wanted to see if it was real or a change in referral patterns," Meyer said.
The team studied data from the government's Surveillance Epidemiology and End Results cancer registry, which includes about 26% of U.S. cancer cases. They identified 7,661 colon and rectal cancer cases in patients under 40 from 1973 to 2005, then calculated the change in incidence over time.
Overall, the rates in this group were low, as might be expected: 1.11 cases of colon cancer per 200,000 people and 0.42 cases of rectal cancer. But colon cancer incidence remained flat over time, while rectal cancer grew from 1985 onward. The increase was the same for all races and both sexes.
The time between the first presentation of symptoms — typically rectal bleeding — and diagnosis averaged seven months, apparently because physicians assumed the bleeding was caused by hemorrhoids and didn't initially perform more tests.
Other symptoms of rectal cancer include a change in bowel habits, anemia, weight loss and diarrhea.
Risk factors for colorectal cancer include a family history of the disease, obesity, smoking, heavy alcohol use, a diet high in red meat and low in vegetables, and insufficient intake of vitamin D.
The team is not advocating routine screening for rectal cancer in patients under 40 because it would not be cost-effective, but does caution physicians to be more aware of the possibility that bleeding could be caused by cancer in younger people.
The researchers could find no evidence that the increase was linked to anal sex, Meyer added. Anal cancers linked to such sex are typically caused by the human papillomavirus, which is common in the gay community. But HPV generally causes adenocarcinomas, he said, and the rectal tumors are squamous cell carcinomas and unlikely to be related.