The Midwest Proton Radiotherapy Clinic (MPRI) hopes to deliver beam to their first patient using the fixed beam line this summer. Before this can happen MPRI needs a state licensed medical physicist to certify that the facility meets all state regulations for radiation exposure. This paper models and predicts the radiation levels in the MPRI clinic due to standard operating procedures of the fixed beam line. Absolute maximum legal limits are established so a simple radiation survey will show that the clinic has meet minimum requirements.
Proton Radiotherapy combines the unique energy deposition properties of protons with conventional radiation oncology. Protons will deposit most of there energy within a short length without irradiating material beyond this point. This allows for precise targeting of tumors while minimizing the dose delivered to the surrounding healthy tissue.
After 30 years of service IUCF’s nuclear physicists found that many experiments would benefit being preformed using large accelerators at other national laboratories. This allowed IUCF to expand its research focus and facilities; the Midwest Proton Radiotherapy Institute is one of these expansions. MPRI brings the benefit of proton radiotherapy to the midwest as one of only three facilities in the nation. While the clinic is currently still under some construction, the first treatment room containing a fixed horizontal beam line is nearing completion. During this summer the final touches will be finalized so that treatment can begin early in the fall.
As a nuclear research facility, the state of Indiana left many of the safety concerns for IUCF to handle itself. Now as a medical clinic, there are host of regulations and guidelines that must be meet before the state will certify us to treat patients. This paper deals with the radiation produced by the clinic’s beam usage and ensuring that safe radiation limits are not exceeded.
In order to properly shield the facility it is necessary to identify the sources and types of radiation that might cause a problem. Even though protons are being accelerated, direct proton radiation is not a concern. The beam is highly controlled and with the exception of the patient no person will ever be exposed to it. What will be of concern is the secondary radiation produced when the beam hits various targets.
For energies above 10 MeV, neutrons are the dominant constituents of any secondary radiation produced by proton interactions. Fermi 1-35. The proton will collide with an individual nucleus producing a neutron in a (p,n) reaction. Any charged particles produced by these nuclear collisions will usually loose all their energy though ionization before they can escape the material.
With incident proton energies between 10 and 200 MeV there is a high angular dependence on the various properties of the produced neutrons. The total neutron fluence is greater at smaller emissions angle and the average energy of the neutrons is also higher. Actually predicting the secondary neutron radiation is a difficult matter that will be addressed later.
There are three simple methods in order to reduce a radiation dose in any area, time, distance, and shielding. While there are certain limits to the maximum rate of radiation, what is of more concern is the total radiation received over a longer period of time such as a week quarter or year. On of the easiest way to lower the accumulated dose is to simply decrease either the amount of time radiation is produced or how much time in spent in the radiation area. Since dose is a measure of energy deposited in mass, there is no dose accumulated if no one is present.
The distance form the source is also important when trying to minimize dose since the amount of radiation drops off proportional to the distance squared.
This relationship is caused by a set amount of radiation being spread out over a larger area. A lot of time and money can be saved in shielding by placing the sources of radiation as far away from high traffic areas as possible.
The third way to lower radiation to acceptable levels is to place a physical barrier between the source and area of concern. The goal is to get the radiation to attenuate inside the material leaving very low levels on the other side. When designing shielding for neutron radiation there are unique factors to take into account. While lead and other high Z materials are very good at stopping various different forms of radiation they are not the best choice for neutrons. Neutrons do not carry, so they don’t interact electromagnetically like most types of radiation. Instead they loose energy though scattering, both elastic and inelastic, with nuclei. In a scattering collision, the incident particle will transfer the most energy to nuclei of approximately the same size. This means that neutrons loose the most energy when colliding with hydrogen. When they hit larger atoms like Pb they will just bounce away retaining most of there energy. The analogy is a billiards ball hitting a bowling ball. For this reason hydrogenous materials usually provide the best shielding.
Using only organic hydrogenous materials is not a good idea however. This is because that the incident neutrons can produce their own secondary radiation inside the shielding, most often gamma rays. Elements that are good neutron attenuators are very poor at stopping ionizing radiation. To combat this problem it is common to see layers of shielding with lead and hydrogenous material.
While everyone would like to go out and get the newest and thinnest shielding available, cost is often a limiting factor. A thicker amount of cheap radiation shielding is often a viable solution in areas where space is that the most important factor. Weighing all these factors MPRI has decided to use concrete, as it’s primary shielding material. Concrete consists of a variety of high Z materials along with large amounts of water. It is neither the best shielding material for neutrons or gamma rays but it does a good job with all types of radiation, and it does it cheaply.
Identification of sources
Before any radiation level can be calculated the sources of radiation must first be identified. As stated earlier the prime sources of radiation are from when the proton beam hits some material and gives off secondary neutrons. So every location where the bean comes in contact with material must me taken into consideration.
The first target that the beam hits is the energy degrader. The protons coming out of the cyclotron will always have an average energy on 230 MeV. For patient treatment, however the protons will need a range of variable energies to reach all different tissue depths. This is achieved through the energy degrader. The degrader is made of beryllium and is placed in line with the beam. As the beam passes through it looses energy and exits the beryllium with the desired energy for treatment. Be gives off a lot of neutrons when bombarded with protons so this is important source of radiation.
Before a patient is irradiated the beam properties are checked to make sure that everything is in order. This occurs at the multi layer faraday cup or MLFC. A serious of thin copper sections the beam stops here for a brief period of time before every treatment. Copper is not much better then Beryllium when it comes to neutron production so this is our second source.
Finally while the beam is actually being delivered to the patient, human tissue will create neutron radiation just like anything else. For the patient this radiation is minimal when compared to their treatment, but for workers in other areas of the clinic this needs to be considered.
Other minor sources of radiation occur anywhere the beam is turned or manipulated in any way. Bending magnets, focusing and diagnostic equipment will all give off low levels of radiation but they should be insignificant compared to these three sources.
The sources will only produce radiation when the proton beam is located on them; this means that it is necessary to determine the standard operating procedures for the clinic. There are three separate beam usages in the clinic and each one will rest the beam on one or more sources, the most obvious being the actually treatment of patients. The beam will be delivered through the energy degrader and into the patient. Before each beam can be delivered the properties have to be checked at the MLFC. For this “Prime” configuration the beam will be the same as for treatment, with the exception that it will come to rest in the MLFC. The final situation is called Q&A and covers all the diagnostic and configuration procedures for regular upkeep of the beam. Through communication with MPRI’s personnel the average properties of these configurations have been determines and they are presented in table.
The method for calculating the radiation presented in the next section will depend on the total number of incident protons. To find these the currents and time of treatment were used as follows. The average patient receives 3 beams per treatment while the facility is designed to treat 20 patients per day. In a period of one week this means there are 300 beams delivered. Each beam lasts 1 minute while the prime last about 9.6 seconds. Q&A is approximately a four of the treatment. Using these numbers and the current in table (). The following was calculated.
Every area inside the clinic must be clearly defined and labeled for radiation purposes. The state of Indiana recognizes two distinct classifications for any area, open and restricted. An open area is accessible by the general population and admittance will not be securely controlled. Specifically people without any type of personal dosimetry device are of concern, they should be able to walk freely anywhere in the open areas for long periods of time and never have a chance of receiving a harmful dose. These areas must achieve radiations levels low enough so that they are not significantly about background. Classifying an area as “restricted” provides much more freedom by allowing higher levels of radiation. In these areas only authorized personal are allowed, they must be inaccessible to the public. Once in a specific area a person must be able to move freely and still not exceed the maximum allowable dose. This means that any area must be classified due to its worst possible radiation dose. Both open and restricted areas must meet all requirements for the entire space and worse case scenario. This means that every single section must meet standards, not just an average
Since MPRI is a medical clinic many non-employees will be in and out of the building. It is the desire of the clinic personnel that these patients and relatives be able to view the treatment rooms and feel comfortable in and open environment. This means that every area of the clinic should be classified as open for radiation purposes. This exception to this is when there is beam being delivered into the treatment rooms. During this time the area is completely sealed off and no one, employees or public is allowed in. Because of this fact we will not be concerned with the radiation in a treatment room while beam is being delivered to that room.
Radiation can cause a person damage if there is no one there to injure. This is the prime consideration behind the occupancy factors. Some areas of the clinic will simply not have people in them very often. The result of this is that the total radiation in can be higher in those areas since there is no one there and no dose is accumulated. To obtain occupancy factors the clinic operating procedures must be consulted. For example treatment room A contains a linac, used to deliver gamma radiation. The focus of this facility will not be gamma radiation treatment so this room will not be occupied nearly as often as the other treatment rooms used for delivering proton radiation. Six specific areas were chosen due to their proximity to the sources and/or high occupancy factors.
For the purposed of this paper the workload of the source will be defined as the amount of radiation produced for a single proton hitting one source. Each area with have a different workload for each source, this will include factors of distance and shielding.
The actual nuclear reactions that occur inside a source when bombarded by several nano Amperes of a high-energy proton beam are very complicated. To accurately predict the neutron dose at 1m from the source requires complicated Monte Carlo simulations, which are beyond the scope of this project. However these simulations have been run and simple mathematical models, which accurately imitate the results, are available.
Several of these models were looked taken into consideration and used to calculate the radiation. In the end that model that fit the best was one provided by Sullivan in A Guide to Radiation and Radioactivity Levels Near High Energy Particle Accelerators. This model is so successful because of the work of Vladimir Anferov in adapting it to the specifications at IUCF. The expression for neutron dose at 1 meter from the source per proton absorbed is given by equation bla.