Title page for ETD etd-07062012-063441


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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