Emergency turnoff switches for radiation therapy equipment need to be tested regularly to maintain high quality standards. But a presentation made today at the American Association of Physicists in Medicine (AAPM) meeting suggests that some state regulations aren't necessarily based on safety facts.
Emergency off switches stop x-ray production and disable mechanical movements in case normal controls fail. They also terminate electricity if the equipment becomes an electrical or fire hazard. For this reason, they need to be tested at consistent intervals.
But a medical physicist from Alabama suggested to AAPM attendees that the rigid timing parameters mandated in his state to test the emergency turnoff switches of linear accelerators used for radiation therapy should be relaxed to reflect the reality of equipment failure rates, and also to provide leeway to staff performing the tests.
Ivan Brezovich, PhD, director of the division of radiation physics at the University of Alabama at Birmingham, and Richard Popple, MD, chairman of the department of radiation oncology, fully support the need to test emergency off switches in diagnostic imaging and radiation therapy equipment on a regular, frequent schedule. But they believe that Alabama state regulations requiring the testing to occur precisely every 90 days is not based on safety-related facts.
After receiving a safety violation citation for testing on the 94th day, Brezovich decided to calculate the risk of failure of an emergency off switch for a computer-controlled linear accelerator.
Assuming a mean time between failures of 100 machine years, the researchers determined that the emergency off system might not work for approximately 0.1 days during a 90-day interval following a successful test, and for 0.2 days during a 120-day interval. However, during the hypothetical observation period, no failure took place during 130 years of machine time.
"Safety tests should be performed regularly," Brezovich said, "but a safety regulation can actually be counterproductive when it creates a false feeling of security, and because it can divert resources from other areas on a specific day when they are needed."
Offering limited leeway to schedule a test over a several-day period would be more practical, pose no significant risk to patients, and could reduce staff stress, he suggested.
Besides, the rigid, specific-day testing rule doesn't guarantee safety, Brezovich pointed out: A service engineer had experienced a failure caused by plastic parts within an emergency off switch that had crumbled due to age and possibly exposure to radiation. Safety could be enhanced without undue cost by requiring that emergency off switches meet strict quality standards, but this has nothing to do with the exact date that the system should be tested, he noted.
By Cynthia E. Keen
AuntMinnie.com staff writer
July 22, 2010
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