The intra- and inter-batch accuracy and precision of MRI (polyacrylamide gelatin gel fabricated at atmospheric conditions) polymer gel dosimeters are assessed in full 3D. In the intra-batch study, eight spherical flasks were filled with the same polymer gel along with a set of test tubes that served as calibration phantoms. In the inter-batch study, the eight spherical flasks were filled with different batches of gel. For each spherical phantom, a separate set of calibration phantoms was used. The spherical phantoms were irradiated using a three-field coplanar beam configuration in a very reproducible manner. The calibration phantoms were irradiated to known doses to obtain a dose-R₂ calibration plot which was applied on the corresponding R₂ maps of all spherical phantoms on an individual basis. The intra-batch study showed high dosimetric precision (3.1%) notwithstanding poor accuracy (mean dose discrepancies up to 13.0%). In the inter-batch study, a similar dosimetric precision (4.3%) and accuracy (mean dose discrepancies up to 13.7%) were found. The poor dosimetric accuracy was attributed to a systematic fault that was related to the calibration method. Therefore, the dose maps were renormalized using an independent ion chamber dose measurement. It is illustrated that with this renormalization, excellent agreement between the gel measured and TPS calculated 3D dose maps is achievable: 97% and 99% of the pixels meet the 3%/3 mm criteria for the intra- and inter-batch experiments, respectively. However renormalization will result in significant dose deviations inside a realistically sized anthropomorphic phantom as will be shown in a concurrent paper.

Recently, novel radiochromic leucodye micelle hydrogel dosimeters were introduced in the literature. In these studies, gel measured electron depth dose profiles were compared with ion chamber depth dose data, from which it was concluded that leucocrystal violet-type dosimeters were independent of dose rate. Similar conclusions were drawn for leucomalachite green-type dosimeters, only after pre-irradiating the samples to a homogeneous radiation dose. However, in our extensive study of the radio-physical properties of leucocrystal violet- and leucomalachite green-type dosimeters, a significant dose rate dependence was found. For a dose rate variation between 50 and 400 cGy min(-1), a maximum difference of 75% was found in optical dose sensitivity for the leucomalachite green-type dosimeter. Furthermore, the measured optical dose sensitivity of the leucomalachite green-type dosimeter was four times lower than the value previously reported in the literature. For the leucocrystal violet-type dosimeter, a maximum difference in optical dose sensitivity of 55% was found between 50 and 400 cGy min(-1). A modified composition of the leucomalachite green-type dosimeter is proposed. This dosimeter is composed of gelatin, sodium dodecyl sulfate, chloroform, trichloroacetic acid and leucomalachite green. The optical dose sensitivity amounted to 4.375 × 10(-5) cm(-1) cGy(-1) (dose rate 400 cGy min(-1)). No energy dependence for photon energies between 6 and 18 MV was found. No temperature dependence during readout was found notwithstanding a temperature dependence during irradiation of 1.90 cGy °C(-1) increase on a total dose of 100 cGy. The novel gel dosimeter formulation exhibits an improved spatial stability (2.45 × 10(-7) cm(2) s(-1) (= 0.088 mm(2) h(-1))) and good water/soft tissue equivalence. Nevertheless, the novel formulation was also found to have a significant, albeit reduced, dose rate dependence, as a maximum difference of 33% was found in optical dose sensitivity when the dose rate varied between 50 and 400 cGy min(-1). By pre-irradiating the novel leucomalachite green-type dosimeter to 500 cGy, the apparent difference in dose response between 200 and 400 cGy min(-1) was eliminated, similar to earlier findings. However, a dose response difference of 38% between 50 and 200 cGy min(-1) was still measured. On the basis of these experimental results it is concluded that the leucodye micelle gel dosimeter is not yet optimal for dose verifications of high precision radiation therapy treatments. This study, however, indicates that the dose rate dependence has a potential for improvement. Future research is necessary to further minimize the dose rate dependence through extensive chemical analysis and optimization of the gel formulation. Some insights into the physicochemical mechanisms were obtained and are discussed in this paper.

In MRI (PAGAT) polymer gel dosimetry, there exists some controversy on the validity of 3D dose verifications of clinical treatments. The relative contribution of important sources of uncertainty in MR scanning to the overall accuracy and precision of 3D MRI polymer gel dosimetry is quantified in this study. The performance in terms of signal-to-noise and imaging artefacts was evaluated on three different MR scanners (two 1.5 T and a 3 T scanner). These include: (1) B₀-field inhomogeneity, (2) B₁-field inhomogeneity, (3) dielectric effects (losses and standing waves) and (4) temperature inhomogeneity during scanning. B₀-field inhomogeneities that amount to maximum 5 ppm result in dose deviations of up to 4.3% and deformations of up to 5 pixels. Compensation methods are proposed. B₁-field inhomogeneities were found to induce R₂ variations in large anthropomorphic phantoms both at 1.5 and 3 T. At 1.5 T these effects are mainly caused by the coil geometry resulting in dose deviations of up to 25%. After the correction of the R₂ maps using a heuristic flip angle-R₂ relation, these dose deviations are reduced to 2.4%. At 3 T, the dielectric properties of the gel phantoms are shown to strongly influence B₁-field homogeneity, hence R₂ homogeneity, especially of large anthropomorphic phantoms. The low electrical conductivity of polymer gel dosimeters induces standing wave patterns resulting in dose deviations up to 50%. Increasing the conductivity of the gel by adding NaCl reduces the dose deviation to 25% after which the post-processing is successful in reducing the remaining inhomogeneities caused by the coil geometry to within 2.4%. The measurements are supported by computational modelling of the B₁-field. Finally, temperature fluctuations of 1 °C frequently encountered in clinical MRI scanners result in dose deviations up to 15%. It is illustrated that with adequate temperature stabilization, the dose uncertainty is reduced to within 2.58%.

A quantitative comparison of two full three-dimensional (3D) gel dosimetry techniques was assessed in a clinical setting: radiochromic gel dosimetry with an in-house developed optical laser CT scanner and polymer gel dosimetry with magnetic resonance imaging (MRI). To benchmark both gel dosimeters, they were exposed to a 6 MV photon beam and the depth dose was compared against a diamond detector measurement that served as golden standard. Both gel dosimeters were found accurate within 4% accuracy. In the 3D dose matrix of the radiochromic gel, hotspot dose deviations up to 8% were observed which are attributed to the fabrication procedure. The polymer gel readout was shown to be sensitive to B0 field and B1 field non-uniformities as well as temperature variations during scanning. The performance of the two gel dosimeters was also evaluated for a brain tumour IMRT treatment. Both gel measured dose distributions were compared against treatment planning system predicted dose maps which were validated independently with ion chamber measurements and portal dosimetry. In the radiochromic gel measurement, two sources of deviations could be identified. Firstly, the dose in a cluster of voxels near the edge of the phantom deviated from the planned dose. Secondly, the presence of dose hotspots in the order of 10% related to inhomogeneities in the gel limit the clinical acceptance of this dosimetry technique. Based on the results of the micelle gel dosimeter prototype presented here, chemical optimization will be subject of future work. Polymer gel dosimetry is capable of measuring the absolute dose in the whole 3D volume within 5% accuracy. A temperature stabilization technique is incorporated to increase the accuracy during short measurements, however keeping the temperature stable during long measurement times in both calibration phantoms and the volumetric phantom is more challenging. The sensitivity of MRI readout to minimal temperature fluctuations is demonstrated which proves the need for adequate compensation strategies.

No abstract