A federal agency is investigating an incident at a Honolulu brain cancer treatment center in which a patient was mistakenly given a larger dose of radiation than prescribed.

Officials at the Gamma Knife Center of the Pacific, citing human error, reported that a patient undergoing radiosurgery was twice given about 3 percent more than the intended exposure.

“;We thought we had an absolute fail-proof system,”; said Dr. Maurice Nicholson, medical director of the center. “;It hasn't happened in 11 years, but it happened.”;

A resulting inspection by the U.S. Nuclear Regulatory Commission began yesterday and was expected to take a couple of days. A report is expected to be completed within two months, according to commission spokesman Victor Dricks.

On July 3 a brain cancer patient, described to be a woman in her 60s, underwent gamma knife radiosurgery to treat lesions. A precise beam of gamma rays is used to attack brain tumors while avoiding normal tissue. The woman was diagnosed with lung cancer that had metastasized to her brain.

In the first two doses, surgeons administered 3 percent more than the intended dose in each shot before they noticed the device that controls the diameter of the beam was the wrong size, emitting a slightly wider beam than needed. Nicholson said the device, called a collimator, was replaced, and the patient underwent five additional doses to other affected areas of her brain that day.

The patient was told of the error that occurred in the first two doses. The surgeons who administered the radiosurgery are independent doctors who use the center's equipment when necessary.

Nicholson said the increase in the first two doses was minuscule. The patient will not suffer from any adverse effects as the first two doses were conducted in noncritical areas of the brain, he said.

This is the first time such an incident occurred at the facility since it opened in 1998, said Nicholson. More than 1,200 patients have been treated at the center.

“;We are so adamant about meeting the criteria of treatment,”; Nicholson said. “;Patient safety is our No. 1 concern.”;

Medical centers are required to notify the commission of any such incidents as a condition of their license.

Stringent safeguards were put in place soon after the error to ensure a similar incident does not occur. While physicians triple-check equipment before any treatment is administered, physicians are now required to initial the checklists.