Urgent warnings by government experts about the risks of routinely using powerful CT scans to screen patients for colon cancer were brushed aside by the Food and Drug Administration, according to agency documents and interviews with agency scientists.

After staying quiet for a year, the scientists say they plan to make their concerns public at a meeting of experts on Tuesday called by the F.D.A. to discuss how to protect patients from unnecessary radiation exposures. The two-day meeting is part of a growing reassessment of the risks of routine radiology. The average lifetime dose of diagnostic radiation has increased sevenfold since 1980, driven in part by the increasing popularity of CT scans. Such scans can deliver the radiation equivalent of 400 chest X-rays.

An estimated 70 million CT (for computed tomography) scans are performed in the United States every year, up from three million in the early 1980s, and as many as 14,000 people may die every year of radiation-induced cancers as a result, researchers estimate.

The use of CT scans to screen healthy patients for cancer is particularly controversial. In colon cancer screening, for instance, the American College of Radiology as well as the American Cancer Society have endorsed CT scans, in a procedure often called a virtual colonoscopy, while the American College of Gastroenterology recommends direct examinations in which doctors use a camera on a flexible tube.

For patients, navigating the debate can be difficult because doctors, patient advocacy groups and manufacturers often endorse positions that are in their economic self-interest. Radiologists, who often own and use CT machines, for instance, often endorse their use; while gastroenterologists, who often own and use camera scopes, often favor their own methods. Patient groups often get financing from drug and device makers, or physician-specialty groups.

The Food and Drug Administration, charged with sorting out such competing claims, has been just as torn on the issue. The internal dispute has grown so heated that a group of agency scientists who are concerned about the risks of CT scans say they will testify at the Tuesday meeting that F.D.A. managers ignored or suppressed their concerns, and that the resulting delay in making these concerns public may have led hundreds of patients to be endangered needlessly.

Scores of internal agency documents made available to The New York Times show that agency managers sought to approve an application by General Electric to allow the use of CT scans for colon cancer screenings over the repeated objections of agency scientists, who wanted the application rejected. It is still under review.