The purposes of this study were to validate a discrete spot scanning proton beam nozzle using the Monte Carlo (MC) codeMCNPX and use the MC validated model to investigate the effects of a low-dose envelope, which surrounds the beam’s central axis, on measurements of integral depth dose (IDD) profiles.Methods:
An accurate model of the discrete spot scanning beam nozzle from The University of Texas M. D. Anderson Cancer Center (Houston, Texas) was developed on the basis of blueprints provided by the manufacturer of the nozzle. The authors performed simulations of single proton pencil beams of various energies using the standard multiple Coulomb scattering(MCS) algorithm within theMCNPX source code and a new MCS algorithm, which was implemented in the MCNPX source code. The MCmodels were validated by comparing calculated in-air and in-water lateral profiles and percentage depth dose profiles for single pencil beams with their corresponding measured values. The models were then further tested by comparing the calculated and measured three-dimensional (3-D) dose distributions. Finally, an IDD profile was calculated with different scoring radii to determine the limitations on the use of commercially available plane-parallel ionization chambers to measure IDD.Results:
The distance to agreement, defined as the distance between the nearest positions of two equivalent distributions with the same value of dose, between measured and simulated ranges was within 0.13 cm for both MCS algorithms. For low and intermediate pencil beam energies, the MC simulations using the standard MCS algorithm were in better agreement with measurements. Conversely, the new MCS algorithm produced better results for high-energy single pencil beams. The IDD profile calculated with cylindrical tallies with an area equivalent to the area of the largest commercially available ionization chamber showed up to 7.8% underestimation of the integral dose in certain depths of the IDD profile.Conclusions:
The authors conclude that a combination of MCS algorithms is required to accurately reproduce experimental data of single pencil beams and 3-D dose distributions for the scanning beam nozzle. In addition, the MC simulations showed that because of the low-dose envelope, ionization chambers with radii as large as 4.08 cm are insufficient to accurately measure IDD profiles for a 221.8 MeV pencil beam in the scanning beam nozzle.
The authors kindly thank Dr. Falk Poenisch and Dr. Martin Bues, M. D. Anderson Cancer Center for discussions; the staff of the information technology section of the Department of Radiation Physics, M. D. Anderson for technical support on issues related to the computer cluster; and Ann Sutton, Scientific Publications, M. D. Anderson for editing the manuscript. The authors also really appreciate the thorough review and recommendations provided by the referees and associate editor of this paper, which have helped the authors to significantly improve the presentation and quality of this work. This research was partially supported by Varian Medical Systems, Inc. Grant No. CS2005-00012856SP and National Institutes of Health Grant No. P01-CA21239.
II. METHODS AND MATERIALS
II.A. Experimental details
II.B.1. MC system and transport parameters
II.B.3. Particle source
II.B.4. 3-D dose distributions
II.B.6. IDD profiles
III.A. Pencil beam PDD profiles
III.B. Pencil beam in-air lateral profiles
III.C. Pencil beam in-water lateral profiles as a function of depth
III.D. In-water 3-D dose distributions
III.E. Pencil beam IDD profiles
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