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Probability Dose Distribution for the Nuclear Halo in IMPT

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R Brosch

R Brosch1*, P Rez2 , M Bues3 , (1) Arizona State University, Tempe, AZ, (2) Arizona State University, Tempe, AZ, (3) Mayo Clinic Arizona, Phoenix, AZ


SU-I-GPD-T-133 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: In proton therapy, inelastic nuclear scattering creates a low-dose “halo” around the primary beam. Conventional analytic dose engines do not treat this halo directly, instead relying on Gaussian fits to Monte Carlo calculations that require custom tuning of the field size effect. We present an analytic method to calculate the low dose halo in the dose engine directly, without resorting to Monte Carlo calculations. The calculation time is independent of the number of primary protons.

Methods: We modeled the inelastic nuclear cross sections as single scattering only. The target was a homogenous water phantom irradiated with a 150 MeV monoenergetic primary proton beam and no beam divergence. The nuclear doubly differential cross sections for a given angle and energy were integrated over discrete angle and energy steps sizes defined in a lookup table. Ionization tracks were calculated for each scattering angle. The result is a probability dose map which was then normalized by incident particle number and bin size to determine the dose in each voxel. We compared the dose to a dose calculated using the FLUKA Monte Carlo code for 10⁷ proton histories with coulomb scattering, transport of neutrons and gammas disabled.

Results: Our integrated nuclear dose calculations from 2mm to 100mm are within 5.0% of the integrated FLUKA results at a depth of 5cm. Kernel parameters may be extracted from our calculations by fitting an exponential linear-quadratic function.

Conclusion: Lateral dose due to secondary protons may be calculated by building up a probability weighted dose map from the inelastic nuclear cross sections and then multiplying by the number of primary protons. The result is within 5% of FLUKA Monte Carlo calculations. This method may be incorporated into an analytic dose calculation engine.

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