Technical Note: Validation of halo modeling for proton pencil beam spot scanning using a quality assurance test pattern

• Published in: Medical physics (Lancaster) Vol. 42; no. 9; p. 5138
• Main Authors: Lin, Liyong, Huang, Sheng, Kang, Minglei, Solberg, Timothy D, McDonough, James E, Ainsley, Christopher G
• Format: Journal Article
• Language: English
• Published: United States 01.09.2015
• Subjects: Models, Biological; Proton Therapy; Quality Assurance, Health Care; Radiation Dosage; Radiotherapy Planning, Computer-Assisted; Radiotherapy, Intensity-Modulated; Scintillation Counting
• ISSN: 2473-4209
• DOI: 10.1118/1.4928157

The purpose of this paper is to demonstrate the utility of a comprehensive test pattern in validating calculation models that include the halo component (low-dose tails) of proton pencil beam scanning (PBS) spots. Such a pattern has been used previously for quality assurance purposes to assess spot shape, position, and dose. In this study, a scintillation detector was used to measure the test pattern in air at isocenter for two proton beam energies (115 and 225 MeV) of two IBA universal nozzles (UN #1 and UN #2). Planar measurements were compared with calculated dose distributions based on the weighted superposition of location-independent (UN #1) or location-dependent (UN #2) spot profiles, previously measured using a pair-magnification method and between two nozzles. Including the halo component below 1% of the central dose is shown to improve the gamma-map comparison between calculation and measurement from 94.9% to 98.4% using 2 mm/2% criteria for the 115 MeV proton beam of UN #1. In contrast, including the halo component below 1% of the central dose does not improve the gamma agreement for the 115 MeV proton beam of UN #2, due to the cutoff of the halo component at off-axis locations. When location-dependent spot profiles are used for calculation instead of spot profiles at central axis, the gamma agreement is improved from 98.0% to 99.5% using 2 mm/2% criteria. The two nozzles clearly have different characteristics, as a direct comparison of measured data shows a passing rate of 89.7% for the 115 MeV proton beam. At 225 MeV, the corresponding gamma comparisons agree better between measurement and calculation, and between measurements in the two nozzles. In addition to confirming the primary component of individual PBS spot profiles, a comprehensive test pattern is useful for the validation of the halo component at off-axis locations, especially for low energy protons.