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But critics believe the Indiana University model’s results overstate the risks of using tungsten MLCs with proton therapy. “Unwanted neutron dose to the patient has been the subject of countless articles but is greatly overblown,” proton therapy physicist Bernard Gottschalk, PhD, of Harvard University’s Laboratory for Particle Physics and Cosmology told Oncology Nurse Advisor. “The lifetime risk of a fatal cancer from these neutrons is very poorly known but is thought to be less than 1%. In the vast majority of cases, depending on the shape and depth of the tumor, the main unwanted dose in proton therapy is not from neutrons. It is from protons. Residual radioactivity is even less of an issue.”
Real-world surveillance of neutron radiation doses to staff at the University of Pennsylvania’s proton therapy unit—which uses a tungsten-alloy MLC—do not support the Indiana University model’s predictions, according to Richard L. Maughan, PhD, Director & Clinical Chief of Medical Physics at the University of Pennsylvania’s Division of Medical Physics. “[Staff] wear monitoring film badges constantly and they’re sent away every 3 months,” Maughan said. “We’ve never seen a reading from any of the badges.” The badges have a detection threshold of 100 μSv (500 times lower than the occupational maximum permissible dose).
A study by Maughan’s team, published in the November 2011 issue of Medical Physics, found “nearly equivalent” neutron production in a practical tungsten alloy MLC and brass-block apertures used with proton therapy, even under the “overly pessimistic” worst-case scenarios described in the Indiana model involving a completely closed MLC.7 “The [Indiana] study is a Monte Carlo model, all calculations,” Maughan told Oncology Nurse Advisor. “It’s just modeling. We actually have four tungsten MLCs we use in proton therapy.” The University of Pennsylvania data are based on practical measurements.
Upstream areas close to the MLC should be inaccessible during neutron production, Maughan notes. “Neutron production is only occurring while the proton [unit] is on, so [staff] is not going to be in the room. It’s not going to affect staff because they’re never in the room when the beam’s on.”
When Maughan worked with neutron therapy, in contrast, staff did get film badge readings and had to be rotated through the room to avoid excessive radiation doses. “The collimator in a neutron facility becomes very activated,” he said. “But even with 2,000 times the neutrons, as seen in proton therapy, you can safely operate neutron facilities. For the production of radiation from residual activation, it really isn’t an issue whether you use tungsten or brass collimators with proton therapy…. The excess radiation to the patient from residual radiation is negligible compared to the dose from scattered protons and neutrons. Activation is an issue for staff. As the MLC is a fixed device, the staff never come in close contact with the activated material; brass apertures must be carried by the staff in close proximity to their bodies immediately after irradiation which may lead to higher exposures, [but] no proton therapist at any center, to my knowledge, has recorded any [badge] reading.”
Maughan readily admits tungsten yields more neutrons than brass, but the greater attenuation provided by the MLC leafs reduces the neutron dose to the patient. He further explains that from a mechanical point of view brass may not be a good material for high-precision MLCs. “The [MLC] gaps are small,” Maughan said. “Tungsten is very stable. Brass is comparatively not very stable. If you take a flat piece of material 5 mm thick or less, with brass, you can get a nice flat plate but once you machine it, there are stresses in brass and warping. With the intricacies you need in a collimator leaf, such tight tolerances of a few thousandths of an inch, the warping causes the plates to move together resulting in potential mechanical ‘jamming’ and failure.” ONA
Bryant Furlow is a medical writer based in Albuquerque, New Mexico.
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