DOS 516 - Week 3 Discussion
Initial Post: Buying Time with Shielding
At SCCA Proton Therapy, our building is shielded to the point of absurdity. I could not get a definitive answer of exactly how thick the various shielded walls are, but I was told that the minimum wall thickness for a primary or secondary barrier is four feet of concrete, and some walls are much thicker. The numbers I heard as I asked around said the walls between treatment rooms are as much as 8 feet thick, and the outer walls are anywhere between 10 and 14 feet of concrete. One engineer I spoke to said that people are likely to get more radiation exposure outside the building from background sources than they would anywhere inside the building near the treatment rooms.
The three ways to minimize radiation exposure are to increase distance, decrease time, or increase shielding.1 In many cases, workflow can be planned to easily accommodate the time and distance goals, but some tasks do not offer this flexibility. The highest radiation source in our building is an assembly called a degrader, which sits next to the cyclotron that accelerates protons to 230 MeV. Since all of the protons coming out of the cyclotron have approximately the same 230 MeV energy, the purpose of the degrader is to rob the beam of some of its energy if a lower energy is desired for therapy. A wheel with varying thickness around the rim can spin around to place the appropriate thickness of material in the beam path to select the desired energy. Since the degrader is always in the beam line, it gets radiologically hot as the protons interact with the nuclei in the degrader material, creating radioactive species. Next in line from the degrader is a series of beam shaping devices including a collimator and focusing magnets that guide the degraded beam into the beam line for transport. The magnets use quite a lot of electricity, and they are water-cooled to keep them in an optimal operating temperature range. If there is a mechanical, electrical, or plumbing problem with the focusing magnets, an engineer will have to go into the beam line area to fix it. This means standing near the degrader.
Since the magnets are fixed in place, the engineer can not take advantage of distance by getting farther from the degrader. The engineer may also be unable to tune the time they spend working in the area, since the job is going to take as long as the job is going to take. One thing that they can control is shielding. If a worker has to work near the degrader, they can bring a heavy lead-lined blanket in with them and lay it over the degrader assembly. The lead in the blanket will absorb some of the radiation and provide a bit of shielding to the engineer. The engineer will still have to be careful to monitor the remaining dose rate, but the shielding can buy some time before unsafe levels are reached.
- Lenards N. Applying radiation safety principles: safe clinical practice. [SoftChalk]. La Crosse, WI: UW-L Medical Dosimetry Program; 2014.