Beam Characteristics of the First Clinical 360°-rotational Compact Scanning Pencil Beam Proton Treatment System and Comparisons against a Multi-Room System

Abstract

This paper is to present the dosimetric characteristics of the first clinical CBCT guided 360˚-rotational compact/single-room scanning pencil beam proton treatment system (SRPT) and comparisons against a recent beam data from a multi-room proton therapy system (MRPT) by the same vendor. The proton beam commissioning was conducted on the first clinical, newly-designed compact ProBeam 360˚ for its scanning pencil beams for protons ranging from 70MeV to 220MeV. The integrated depth dose curves (IDDs) were acquired in water with AP monogenic proton beams using a PTW Bragg peak chamber. The dose output factors, measured by a Marcus chamber at 15 mm transmission beam portion using single layer 10x10 cm2 proton plans, were then incorporated to the iDD's along with a factor of 1.1 for RBE offset. The beam spot sigma sizes were then derived from single Gaussian fitting for the in-air measured spot profiles. After beam modeling and validation for a treatment planning system, the proton dosimetric characteristics were compared against the recent data of MRPT. In contract to MRPT data, the Bragg peaks of SRPT have shown moderately extended proton beam range straggling in the peak region accompanied by a marginally elevated proximal transmission beam portion. This phenomenon displayed more noticeably in the mid-ranged or even high energies. Consequently, IDDs of SRPT showed about 0.41 to 0.94 mm broader Bragg peak widths (Rb80 - Ra80) in energies of 140 MeV or greater, peaked at 180 MeV. About 0.65-0.79 mm slightly extended distal falloffs (Rb20-80) were noticeable for energies between 140 MeV and 170 MeV, consistent with the findings of minimally raised energy sigma in AcurosPT Monte Carlo model. An independent check for the dose and energy accuracy of a spread-out Bragg peak plan on a simple phantom by the IROC showed 1.00 accuracy with 1.00 ±5% allowance. Additional comprehensive end-to-end independent check using a more realistic prostate phantom offered by IROC was reported a 96% accuracy and the passing score at a set passing criterion of 85%. The result over 108 patient specific individual field QAs with Octavius 1500 2D ionization chamber array showed 100% passing rate using a Gamma index of 3%/3mm or tighter from fields with or without range shifters. The high QA passing rates suggest a high confidence on the accuracy and quality of the commissioning for the newly designed SRPT. The commissioning results of the new-designed SRPT showed slightly widened proton Bragg peaks, elevated beam transition region and extended peak distal falloffs, particularly in mid-high energies, suggest minimally broader spectra for selected proton energies in comparing against MRPT data. This did not show a meaningful impact on the quality and accuracy of proton therapy program, indicated by high scores from independent dosimetry end-to-end checks and over 108 patient’s specific beam QA’s.