SU-E-T-103: Three-Dimensional Measurements of Dose and LET from a Proton Beam via Polymer Gel Dosimetry.

PURPOSE To obtain and assess the accuracy of 3D dose distributions and LET maps from a passively-scattered proton beam through the use of LET-dependent and LET-independent polymer gel formulations. METHODS Dose measurements were performed with BANG3-Pro2 (MGS Research, Inc, Madison, CT) polymer gel dosimeters. The dosimeters were mixed from kits and were poured into 14.5 cm high by 15 cm diameter acrylic cylinders. For LET measurements, a new BANG3-Pro variant intended for use as an 'LET- meter' was employed. Initial irradiations were performed using a 200 MeV passively scattered proton beam with a 4 cm spread out Bragg peak (SOBP), delivering a physical dose of 3 Gy to the center of the SOBP. Dosimeters were read out using the OCTOPUS-IQ (MGS Research, Inc, Madison, CT) optical CT scanner. The optical densities measured in the gel were compared against ion chamber data to assess the accuracy of the dosimeter. RESULTS Initial analysis indicates 80% of points measured along the central axis agreed with ion chamber data at the ±5%/±3mm level in the sensitive region of the gel. An artifact in the reconstructed image produced inaccurate readings beyond the depth of 50% dose on the distal edge of the SOBP, resulting in the majority of the agreement failures. CONCLUSIONS BANG3-Pro2 polymer gel dosimeters demonstrate promise as a 3D dosimeter for use in proton therapy. The dosimetric data obtained to date will be used as the baseline measurement against which the LET-sensitive BANG3-Pro formulation will be compared for the measurement of proton LET.