CHICAGO (AP)— Flat growths on the colon wall are more common in Americans than previously thought and more likely to be cancerous than the more familiar knobby masses known as polyps, a new study finds.

New techniques can locate and remove the flat growths, but many doctors aren't aware of their cancer risk and may not know how to look for them.

The findings are likely to change the practice of colonoscopy, experts said, and may explain some colon cancers that arise between colonoscopies.

"I think it is very important. It's going to intensify the need for quality screening," said Dr. Stephen Hanauer, gastroenterology chief at the University of Chicago, who was not involved in the study. "You're not going to be able to do seven-minute colonoscopies."

The growths tend to be smaller when they are cancerous — the size of a nickel instead of a quarter — and are level with the colon wall or depressed like a pothole. They blend in with the surrounding tissue and are difficult to spot.

"They look like a pancake just lying on the floor," said the study's lead author, Dr. Roy Soetikno of the Veterans Affairs Palo Alto Health Care System in California.

Doctors have known about flat growths but haven't recognised their danger, experts said.

While knobby polyps were found in four times as many participants, more than half the colon cancers found — 15 of 28 — were in flat and depressed growths. Thirteen were in polyps.

Researchers found the flat growths were nearly 10 times more likely to be cancerous than the polyps. They believe the growths represent a separate colon cancer pathway, rather than being precursors to knobby polyps, Soetikno said. The study appeared in yesterday's Journal of the American Medical Association.

Since the 1980s, Japanese doctors have reported more flat colon growths than were seen in the United States, but Western scientists doubted their importance, said Dr. David Lieberman of the Oregon Health and Science University in Portland, who wrote an editorial in the journal.

"This paper will have a big impact on gastroenterology," Lieberman said. "It will heighten people's awareness that, yes, these are found in United States."

The findings came from colonoscopies of more than 1,800 mostly male veterans who were seen at one VA centre from July 2003 to June 2004. Doctors involved had been trained by Japanese specialists to use a dye during colonoscopies to spot flat growths. One-third of the patients were coming in for routine screening. The rest either had symptoms such as bleeding or pain, or were being screened because they had previous polyps or a family history of colon cancer.

Flat and depressed growths were detected in 170 patients, nearly 10 percent. Meanwhile, knobby polyps were found in 675 patients, about 37 percent.

Colorectal cancer is the nation's second leading cancer killer. The American Cancer Society estimates 49,960 people will die from it this year. Screening can save lives by finding growths before they turn cancerous.

Colonoscopies, considered the gold standard test, are recommended every 10 years, starting at age 50. A doctor snakes a long, thin tube equipped with a small video camera through the large intestine to view the lining.

Colorectal cancer is considered slow-growing so 10 years between screenings is usually enough to catch early growths.

Undetected flat growths could explain some mysterious "interval cancers" that show up between screenings in people who have regular colonoscopies, experts said.

"They get an exam and they're clean, and at the next exam they have cancer," Hanauer said.

The findings, along with prior research showing that doctors who take more time during a colonoscopy find more growths, underline the importance of high-quality screening, Hanauer said.

The high rate of cancer in the flat growths warrants further research, said Dr. Asad Umar of the National Cancer Institute.

"Techniques that can better identify and address these lesions are needed," Umar said.

The study was funded by the Palo Alto Institute for Research and Education, a nonprofit group associated with the Palo Alto VA.