SU-E-T-59: 3-D Dosimetric Assessment for a Proton Pencil Beam by Optical CT Scanning of Plastic Radiochromic Dosimeters

Purpose: Proton pencil beam scanning can provide precise and efficient treatment delivery for both conventional and intensity modulated proton therapy (IMPT) techniques. It is challenging to measure the 3-D dose distribution of a pencil beam (1 – 1.5 cm diameter) because of its small field size as well as the complexity of dose changes in various directions. This study is the first investigation in characterizing the dosimetry of a single proton pencil beam using PRESAGE radiochromic dosimeters and an optical CT scanner. Methods: In this study, cylindrical PRESAGE dosimeters and an optical CT scanner were used to implement dose distribution measurements for proton pencil beams at Massachusetts General Hospital. Two different dosimeter formulations, with different physical density and atomic number, were used for measurements for proton pencil beams, with energies between 93 MeV and 110 MeV. Optical density dose response was studied by irradiating the dosimeters to various known doses (up to 9 Gy). PDD distributions were studied for proton pencil beams, as well as unmodulated and modulated scattered beams.Results: PRESAGE has a linear dose response from 0 up to 8 Gy for all proton energies studied. The physical density of PRESAGE was used to scale the depth of PRESAGE measurement to depth of water. For pencil beams, normalized at surface, the dose at the Bragg peak is underestimated by 15–20% due to LET effects. The spread of a proton pencil beam as a function of depth can be observed. Measured PDDs were compared with that in water for both modulated (SOBP) and unmodulated scattered beams. Some LET effects are observed. Conclusion: The findings from this study suggest that it is feasible to use PRESAGE dosimeter for proton pencil beam dosimetry study. Future studies will be focused on the responses of various PRESAGE formulations in proton pencil beams.