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Impact of range shifter material on proton pencil beam spot
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To quantitatively investigate the effect of range shifter materials on
single-spot characteristics of a proton pencil
An analytic approximation for multiple Coulomb scattering
(“differential Moliere” formula) was adopted to calculate spot sizes of
proton spot scanning beams impinging on a range shifter. The
calculations cover a range of delivery parameters: six range shifter
materials (acrylonitrile butadiene styrene, Lexan, Lucite,
polyethylene, polystyrene, and wax) and water as reference material,
proton beam energies ranging from 75 to 200 MeV, range shifter
thicknesses of 4.5 and 7.0 g/cm2, and range shifter positions from 5 to
50 cm. The analytic method was validated by comparing calculation results with the
measurements reported in the literature.
Relative to a water-equivalent reference, the spot size distal to a wax or
polyethylene range shifter is 15% smaller, while the spot size distal to a range
shifter made of Lexan or Lucite is about 6% smaller. The relative spot size
variations are nearly independent of beam energy and range shifter thickness and
decrease with smaller air gaps.
Among the six material investigated, wax and polyethylene are desirable
range shifter materials when the spot size is kept small. Lexan and Lucite
are the desirable range shifter materials when the scattering power is
kept similar to water.
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