Investigation of Proton Beam Dose Distributions with 3D Dosimetry

Abstract

To evaluate proton beam dose distributions using a radiochromic polymer block dosimeter and compare measurements with calculations for a passively scattered proton beam and a spot scanning pencil proton beam. Two 7.5 cm high by 9.5 cm diameter cylindrical dosimeters were used. CT images were used to design two treatment plans with an Eclipse treatment planning system (TPS). The first plan used a single passively scattered field with a spread out Bragg peak (SOBP) of 4 cm. This plan delivered approximately 10 Gy(RBE) to the center of the SOBP. The second plan used a single pencil beam of energy 153.2 MeV to provide a range of 16 cm in water. Four regions of the dosimeter were irradiated delivering doses in the peak of approximately 6, 10, 13 and 19 Gy(RBE) by rotating the dosimeter between spot deliveries. The dosimeters were analyzed using an optical CT scanner. The measured optical densities were converted to dose via a calibration curve, exported to the CERR environment and fused to the treatment plan. Dose distributions were scaled to the SOBP plan. Dose profiles were taken along the axis of each spot and the SOBP and perpendicularly across the SOBP. The dose measured in the peak of single spot irradiations increased proportionally to the MU setting. The distal falloff was steeper than predicted by the TPS. The cross profiles for the SOBP plan matched within 2 mm with the TPS. The depth profile was noisy within the modulated area. Noise decreased as dose increased for both dosimeters. This formulation of dosimeter shows promise as a 3D dosimeter for proton therapy. The ease with which scanning beam spots can be measured suggests that the dosimeter will be a valuable tool in the ongoing process of commissioning and quality assurance of the scanning beam. Because the dosimeter records information in three dimensions, a single irradiation provides complete volumetric data for analysis. Depth dose and cross profile information can be obtained simultaneously. Gamma calculations are also being investigated for further analysis. The small amount of beam time necessary for the measurement is a significant benefit as beam time at clinically-active proton therapy centers is extremely limited. Noise reduction has been greatly improved with upgrades to the optical CT-scanning process. The dose response curve of the dosimeter for protons shows differences from that for photons; precise determination will facilitate further analysis and enable measurement of complex distributions. This will permit the direct comparison between treatment plans and measured data, in 3D.