Fifty-seventh annual meeting of the American association of physicists in medicine
SU-D-304-05: Validation of Low-Dose-Tail Modeling for Proton Pencil Beam Spot Scanning Using a Quality Assurance Test Pattern
The purpose of this manuscript is to demonstrate the utility of a comprehensive test pattern in validating calculation models of the 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 and location, and for determining monitor units.
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). Planar measurements were compared with calculated dose distribution based on the weighted superposition of spot profiles previously measured using a pair-magnification method.
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 cutoff of the halo component is smaller at higher energies, and is not observable for the 225 MeV proton beam for UN #2.
In conclusion, the use of a comprehensive test pattern can facilitate the validation of the halo component of proton PBS spots at off axis locations. The cutoff of the halo component should be taken into consideration for large fields or PBS systems that intend to trim spot profiles using apertures.
This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17-W81XWH-07-2-0121 and W81XWH-09-2-0174.