Sherr, the study co-author, said sexual practices don't appear to be a likely culprit. Rectal cancer is not related to anal cancer, which is often spread through a virus when people have sex.
It's possible that rising rates of obesity could play a role because heavier weights are linked to the disease. However, he said, rates aren't up among people over 40 years of age.
What do the experts recommend? Sherr said doctors need to be aware of the possible risk that younger patients will develop the disease.
"When a patient under 40 comes in with rectal bleeding," he said, "they will almost reflexively say to the patient, 'You're too young for rectal cancer. It's hemorrhoids or something else.'"
Wishner cautioned that "we don't want people panicking as soon as they see a spot of blood on the toilet paper."
However, "when an older patient has a sign or a symptom, they're attuned to this. A 35-year-old blows it off. Cancer and things like that are not on the radar."
For more about colon and rectal cancer, visit the U.S. National Cancer Institute.
SOURCES: David L. Sherr, M.D., assistant professor, radiation oncology, Weill Cornell Medical College, New York City; Jerald Wishner, M.D., co-director, minimally invasive and robotic surgery, Northern Westchester Hospital, Mount Kisco, N.Y.; Aug. 23, 2010, Cancer, online
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