RTY2003 – Radiation Therapy Physics – Shielding and Maze Design
Shielding is an extremely important part of radiation therapy, especially since even scatter or leakage radiation can exceed safety regulations. The objectives of shielding are to limit radiation exposure to staff, patients, visitors and the public. It is important to know that each linear accelerator bunker is unique and designed for that particular machine. A design of another bunker should never be used on a different machine. It is also important that we do not simply reference spreadsheets and tables from any online source, as regulations are different from country to country.
The three components of radiation are:
Primary radiation
Scatter radiation
Leakage radiation
Primary radiation contributes the highest dose of these three, and require the thickest barriers. For higher energy accelerators greater than 10MV, there may also be significant neutron contamination contributing to radiation dose.
Facilities that are designed as safe for leakage radiation are generally considered safe for scattered radiation.
There are several considerations for shielding design:
It is worth noting that for IMRT, the leakage workload is more than the workload of the primary and scatter radiation. This is because the field borders are often small compared to the gantry maximum size of 40cm x 40cm, and the leakage from the surrounding MLC is usually higher than the primary beam.
It is also important to consider the other, newer or more obscure treatment modalities. The Cyberknife, for example, consists of a robotic arm and no fixed isocenter, and only delivers 6MV photons. All barriers except the ceiling are primary barriers. For SRS, the use factors are different from 3D-CRT. This is because even though each treatment is of a higher dose, the setup time is significantly longer, and hence the treatments are less frequent.
Beam stoppers are devices that are attached to the C-arm of the gantry, opposite to the gantry head. They can either be retractable or fixed, and the purpose is to attenuate the primary beam by a factor of 103 so that the walls and ceiling that would otherwise be primary barriers, become secondary barriers. Beam stoppers are becoming more obscure today because the space on the C-arm gantry is better utilised with a portal image intensifier.
The primary barrier is one that is just sufficient to attenuate the primary beam / direct, useful beam to the required degree. The secondary barrier is the required barrier against stray radiation, which can be either leakage or scattered.
As such, the primary barrier is always thicker than the secondary barrier.
There are several types of maze designs:
The shielding thickness of the barrier depends on a myriad of factors, but most of them are built into the formulae and equations, so they should be relatively easy to derive. Most importantly, thickness depends on the radiation limits by the regulatory authority. It also depends on workload, use factor, occupancy, distance and materials used during shielding.
The information required for shielding calculations include the type of equipment, treatment techniques and hence workload, target dose and dose rate, though this is usually averaged out over all the patients being treated by the machine, the use factor and direction of primary beam, the distance to the area of interest, the occupancy of the area to be shielded, and the dose limit value in the area to be shielded.
It is important to consider if the room to be shielded is a controlled or uncontrolled area, whether it is accessible to working staff only, or to patients and general public. It is also important to note if the room is adjacent to low occupancy areas like the toilet, or the roof.
Controlled Areas are limited-access areas in which the occupational exposure of personnel to radiation or radioactive material is under the supervision of an individual in charge of radiation protection. Access, occupancy and working conditions are controlled for radiation protection purposes. Areas are usually in the immediate areas where radiation is used, such as treatment rooms and control booths, or other areas that require control of access, occupancy and working conditions for radiation protection purposes. The workers in these areas are individuals who are specifically trained in the use of ionising radiation and whose radiation exposure is usually individually monitored.
Uncontrolled Areas are all other areas in the hospital or clinic and the surrounding environment. Healthcare professionals or members of the public may frequent these areas. It is hence necessary to choose the appropriate occupancy factors such that we account for those who may be occupationally exposed, and those who are incidentally exposed.
The three important variables in beam shielding calculations are:
Workload (W): The amount of usage the machine undergoes, in suitable units
Use Factor (U): The percentage of the total time radiation is emitted in a particular direction (towards a particular barrier)
Occupancy Factor (T): The fraction of time that a person will be in the area to be shielded
Often, these three quantities will be multiplied together, and it is a good measure of how much shielding an area should be provided. It is hence easy to remember the terms as a “WUT”.
Workload is a measure of radiation output, and gives a sense of utilisation and usage of a particular machine. It is the projected absorbed dose delivered to the isocentre in a specified time.
For an x-ray machine below 500kVp, workload is expressed in milliampere minutes per week, which can be computed by multiplying the maximum mA with approximate minutes per week of beam on time.
For megavoltage machines, it is the number of patients treated per week with the dose delivered per patient at 1m. This is expressed in rad per week at 1m, which is synonymous to cGy/week.
It is important to remember that workload does not only include patient treatments, but also other potential uses of the treatment machine, like for quality assurance measurements, calibration of radiation beams, research, and irradiation of other items like animals and blood products.
In practice, it is always appropriate to add ~10% of clinical workload to take into account the work from other sources.
Use Factor describes the percentage of the total time which radiation is emitted in a particular direction. It is 0.5 for vertical beam orientations and 0.25 for other orientations.
Occupancy Factor is the fraction of time that a person would be occupying the space to be shielded. It is important to consider to occupancy for any given person instead of a particular individual.
When planning a new facility, assumptions and objectives must be clearly stated, verified and documented. Conservative assumptions should be used as under-shielding is significantly worse (and more damaging) than over-shielding. It may be worthwhile to plan ahead and take into account future possible expansions, considering the increase in workload or occupancy. Most design plans and considerations are expected to last for the next 20 years, including room for expansion.
Basic shielding calculations would entail calculating dose rate at a certain distance from the source due to primary, scattered and leakage radiation, and deriving the number of Tenth Value Layers (TVLs) needed to bring the radiation levels to the dose constraints for both occupational and public.
The Unshielded Dose Rate is computed as such:
The shielded dose rate can be calculated by multiplying this H by transmission, B. Ideally,
Hence the explicit equation for obtaining transmission is as shown:
We can relate the transmission with each term on the right hand side by rationalising how the transmission can afford to change as each variable changes.
B can afford to increase if the design goal is increased (P), or if the distance from source to barrier is increased (d). However, B has to decrease if the workload increases (W), or if the use factor increases (U), or if the occupancy factor increases (T).
The number of TVLs required to shield the unattenuated beam can be calculated directly from the transmission, because transmission is a measure of how much the primary beam should be reduced by, and the calculation is just to quantify in terms of TVLs. The formula is as follows:
The actual thickness of the barrier can hence be calculated as such, depending on the material chosen, the TVLs will change accordingly.
If you’re feeling ambitious enough, you can even combine both formulae together to form this:
While the calculation for primary barriers tends to be relatively accurate, calculations for secondary barriers tend to be conservative, and it is recommended to have a margin of factor 2 to 3 for shielded dose rate in order to account for variation in concrete density.
Width of primary barrier is another tricky concept. The key idea is that while the full uncollimated field of 40cm x 40cm is projected onto the barrier, the collimator angle can be changed to produce a shape with greater width.
It is hence necessary to take the widest width the primary beam could possibly produce, depicted in this diagram by the diameter of the circle. It is possible to calculate the width with a combination of the inverse square law and trigonometry:
Typically, 30cm of additional margin is added on both sides, making it a total of 60cm (0.6m in the formula).
d is the distance from source to barrier
l is the length of the full uncollimated field size
w is width of barrier
Usually most maximum field sizes are 40cm x 40cm, which makes our formula slightly easier:
When calculating for shielding, it is important to observe the maximum of all the properties like beam energy, dose rate and field size.
The identity of the chosen shielding material will affect the barrier thickness significantly. For low energy gamma and x-rays, lead is commonly chosen. For high energy (> 500keV) gamma and x-rays, concrete and high density concrete is commonly chosen.
Many other shielding materials exist:
Earth: density of 1.5g/cc, variable
Bricks: density of 1.65g/cc to 2.05g/cc
Borated (5%) Polyethylene (BPE): Shielding material used for neutrons in doors, on walls or around ducts. Used in conjunction with lead or steel in high energy rooms
For doors, polyethylene can be substituted for some of the BPE to save on cost
Composite materials like metal bits embedded in concrete
Assumptions
There are many assumptions when putting together shielding calculations.
This includes neglecting the attenuation of primary beam by patient. For a treatment with the patient on the couch, the patient usually attenuates the primary beam by 30% or more. However there is no patient during QA.
The calculations of the recommended barrier thickness often assume perpendicular incidence of radiation. This is where the dose is maximum. If the incidence is at an angle, the dose essentially has to be resolved to the perpendicular dose.
Leakage from equipment should be assumed to be at the maximum value recommended. This is opposed to scatter, which changes for different angles. The recommended values for occupancy factors of the uncontrolled areas are also conservatively high.
