Proton spectroscopic imaging of polyacrylamide gel dosimeters for absoluteradiation dosimetry

Proton spectroscopy has been evaluated as a method for quantifying radiatio n induced changes in polyacrylamide gel dosimeters. A calibration was first performed using PANG-type gel samples receiving uniform doses of 6 MV phot ons from 0 to 9 Gy in 1 Gy intervals. The peak integral of the acrylic prot ons belonging to acrylamide and methylenebisacrylamide normalized to the wa ter signal was plotted against absorbed dose. Response was approximately li near within the range 0-7 Gy. A large gel phantom irradiated with three, co planar 3 x 3 cm square fields to 5.74 Gy at isocentre was then imaged with an echo-filter technique to map the distribution of monomers directly. The image, normalized to the water signal, was converted into an absolute dose map. At the isocentre the measured dose was 5.69 Gy (SD = 0.09) which was i n good agreement with the planned dose. The measured dose distribution else where in the sample shows greater errors. A T-2 derived dose map demonstrat ed a better relative distribution but gave an overestimate of the dose at i socentre of 18%. The data indicate that MR measurements of monomer concentration can complem ent T-2-based measurements and can be used to verify absolute dose. Compare d with the more usual T-2 measurements For assessing gel polymerization, mo nomer concentration analysis is less sensitive to parameters such as gel pH and temperature, which can cause ambiguous relaxation time measurements an d erroneous absolute dose calculations.