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Evaluation of a Proton Pencil Beam Alogirithm for Dose Calculations in Heterogeneous Media

N Knutson

N Knutson1*, J Chapman1, J Fontenot1,2, W Newhauser1,2, K Hogstrom1,2, (1) Louisiana State University, Baton Rouge, LA,(2) Mary Bird Perkins Cancer Center, Baton Rouge, LA,

TH-F-211-2 Thursday 2:00:00 PM - 2:50:00 PM Room: 211

Purpose: To develop an improved nuclear halo dose model of a pencil beam algorithm (PBA) for dose calculation of proton beams in heterogeneous media.
Methods: The proton PBA consisted of a central axis term and an off axis term. The central axis term was determined from a central axis depth dose of a proton beam in water and was scaled by the mass stopping power ratio to account for other materials. The off axis term was determined from Fermi-Eyges scattering theory with material-dependent scattering powers to calculate the spread of the proton beam in heterogeneous media. The nuclear halo dose arising from large angle and non-elastic scatter was modeled using two terms: a Gaussian and a Cauchy-Lorentz distribution. Depth-dependent widths and amplitudes of each distribution were determined by fitting a simulated 1-mm x 1-mm pencil beam in water. The PBA was evaluated in approximately 30 test phantoms containing bone and/or air heterogeneities for 4 energies and 2 field sizes. Agreement between PBA and Monte Carlo simulations of the test conditions was quantified by computing the percentage of points within 2 percent dose difference or 1 mm distance to agreement.
Results: With the improved nuclear halo model, PBA calculations showed better than of 97% of dose points within 2%/1 mm of MC distributions for all geometries examined. For phantoms containing laterally infinite heterogeneities, agreement between PBA and MC distributions was 100% at 2%/1mm. For phantoms containing laterally finite heterogeneities, agreement was at least 97%. The points failing were due to the central axis approximation of the PBA in regions not influenced by the nuclear halo model.
Conclusions: The halo model developed in this work improves the agreement of the PBA with MC simulations in heterogeneous phantoms, particularly in low-dose regions that can be important for scanned-beam proton therapy.

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