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Barrier Shield Thickness
The barrier thickness requirements need to be calculated based on many parameters, such as patient workload, distances, machine energy, etc.
The amount of radiation protection material varies depending upon Low or High Energy units, workload, occupancy above and around the room and other factors. Ledite shielding attenuates both x-rays (photons) as well as neutrons. The primary ceiling areas require the greatest amount of attenuation. The most economical design would use an all Ledite ceiling shielding barrier.
Primary Barriers
Most linear accelerator rooms require a primary barrier which shields against direct beam radiation.
Secondary Barriers
The remainder of the walls are secondary barriers and typically require much less shielding than the primary.
As a design guide, the following thickness will be useful in preparing preliminary schematics for physics review. These are general representations only.
Modality and its effect on Primary Barrier Width
Conventional
Primary Barrier width usually includes a safety factor of 12" either side of the exiting beam width.
TBI (Total Body Irradiation)
Typically requires an additional 5' between isocenter and primary as compared to a conventional room. TBI also requires a wider primary to accommodate the increased beam width.
IMRT
IMRT typically has no effect on the width of the Primary Barrier.
IGRT
IGRT typically has no effect on the width of the Primary Barrier.
Stereotactic
The narrow beam divergence of Stereotactic treatment results in a proportionally narrow Primary Barrier width.
Beam Divergence
For design purposes, the widest potential beam angle should be used when calculating the width of the primary barrier.
Ceiling Shield Thickness
Ceiling thickness is directly affected by the machine energy which dictates the amount of shielding required. In turn, the weight of this shielding then dictates the size and height of the steel support beams.
Low Energy
Ceiling thickness is typically 36"providing 6 Tenth Value Layers of protection-generally sufficient protection for workloads of 50 patients per day, 5 days per week with exposure levels of less than 20 microsieverts per week (2 mrem/wk).
High Energy
Ceiling thickness is typically 42" - 48" providing approximately 6 Tenth Value Layers of protection depending on photon energy-generally sufficient protection for workloads of 50 patients per day, 5 days per week with exposure levels of less than 20 microsieverts per week (2 mrem/wk).
In areas that have height limitations, there are several options available to reduce the overall room height. Primary ceiling thickness can be reduced by substituting some of the Ledite with lead or steel placed below the Ledite block. Lead or steel can also be placed between the steel beam webs or laid on the top flanges of the steel beams.