Verification of dynamic radiotherapy: the potential for 3D dosimetry under respiratory-like motion using polymer gel.

Following the implementation of advanced treatment procedures in radiotherapy, there is a need for dynamic dose verification in 3D. Gel dosimetry could potentially be used for such measurements. However, recently published data show that certain types of gels have a dose rate and fractionation dependence. The aim of this study was to investigate the feasibility of using a polymer gel dosimeter for dose verification of dynamic radiotherapy. To investigate the influence of dose rate dependence during respiratory-like motion in and out of the beam, a respiration robot together with two types of gel systems (normoxic methacrylic acid gel (nMAG) and normoxic polyacrylamide gel (nPAG)) were used. Reference measurements were obtained using a linear diode array (LDA). Expected results, if there was no influence of the dose rate variation, were calculated by convolving the static irradiated gel data with the motion function controlling the robot. To investigate the fractionation dependence, the gels were irradiated using gated and ungated deliveries. Magnetic resonance imaging was used to evaluate the absorbed dose response of the gel. The measured gel data coincided well with the LDA data. Also, the calculated data agreed well with the measured dynamic gel data, i.e. no dose rate dependence due to motion was observed. The difference in the R2 response for the gels receiving ungated and gated, i.e. fractionated, deliveries was less than 1% for the nPAG and 4% for the nMAG, for absorbed doses up to 2 Gy. The maximum difference was 1.2% for the nPAG and 9% for the nMAG, which occurred at the highest given dose (4 Gy). The investigated gels were found to be feasible detectors for dose measurements under respiratory-like motion. For dose verification of dynamic RT involving gated delivery, e.g. breathing-adapted radiotherapy, relative absorbed dose evaluation should be used in order to minimize the effects of fractionated irradiation.