Basic investigations on the performance of a normoxic polymer gel with tetrakis‐hydroxy‐methyl‐phosphonium chloride as an oxygen scavenger: Reproducibility, accuracy, stability, and dose rate dependence

Abstract

Magnetic resonance (MR)‐based polymer gel dosimetry using normoxic polymer gels, represents a new dosimetric method specially suited for high‐resolution three‐dimensional dosimetric problems. The aim of this study was to investigate the dose response with regard to stability, accuracy, reproducibility, and the dose rate dependence. Tetrakis‐hydroxy‐methyl‐phosphonium chloride (THPC) is used as an oxygen scavenger, and methacrylic acid as a monomer. Accuracy, reproducibility, and dose resolution were determined for MR protocols at low spatial resolution (typical for clinical scanners), medium, and microimaging‐resolution protocols at three different dose levels. The dose‐response stability and preirradiation‐induced variations in , related to the time interval between preparation and irradiation of the polymer gel, were investigated. Also postirradiation stability of the polymer gel was considered. These experiments were performed using a beam in a water phantom. Moreover, we investigated the dose rate dependence in the low, medium, and saturation dose region of the normoxic polymer gel using a linear accelerator at photon energy of . MR scanning was performed on a whole body scanner (MEDSPEC , BRUKER BIOSPIN, Ettlingen, Germany) using several coils and different gradient systems adapted to the acquired spatial resolution investigated. For ‐parameter selective imaging and determination of the relaxation rate , a multiple spin echo sequence with 20 equidistant echoes was used. With regard to preirradiation induced variations increases significantly with the increasing time interval between the polymer gel preparation and irradiation. Only a slight increase in can be observed for varying the postirradiation‐time solely. The dose reproducibility at voxel volumes of about is better than 2%. The accuracy strongly depends on the calibration curve. THPC represents a very effective oxygen scavenger in methacrylic acid and gelatin. Polymer gels containing THPC offer high sensitivity to dose but their dose response also strongly depends on dose rate in the medium and high dose region.