Type of Document	Master's Thesis
Author	Knutson, Nels C.
Author's Email Address	nknuts1@lsu.edu
URN	etd-07062012-063441
Title	Evaluation of a Proton Pencil Beam Algorithm for Dose Calculations in Heterogeneous Media
Degree	Master of Science (M.S.)
Department	Physics & Astronomy
Advisory Committee	Advisor Name Title Fontenot, Jonas Committee Chair Deibel, Catherine Committee Member Hogstrom, Kenneth Committee Member Newhauser, Wayne Committee Member
Keywords	pencil beam algorithm proton
Date of Defense	2012-06-20
Availability	unrestricted
Abstract 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 profile of a Monte Carlo simulated proton beam in water and was scaled by a 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 lateral spread of the proton beam in heterogeneous media. The nuclear halo dose, which was caused by large angle and non-elastic scattering events, was modeled using two terms: a Gaussian distribution 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 at 4 energies and for 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% or 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% or 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 nuclear 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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