WASHINGTON — 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.