3D dose verification in HDR prostate monotherapy using polymer gels and MRI

Abstract

VIPAR polymer gels and 3D MRI techniques were evaluated for their ability to provide experimental verification of 3D dose distributions in a simulation of a 192Ir prostate monotherapy clinical application. A real clinical treatment plan was utilized, generated by post-irradiation, CT based calculations derived from Plato BPS and Swift treatment planning systems. The simulated treatment plan involved the use of 10 catheters and 39 source positions within a glass vessel of appropriate dimensions, homogeneously filled with the VIPAR gel. 3D high resolution MR scanning of the gel produced T2 relaxation time maps, from which 3D dose distributions were derived via an appropriate calibration procedure. Results were compared to corresponding dose distributions obtained from the Plato and Swift treatment planning systems. Quantitative comparison, on a point by point basis, was based on user adopted acceptance criteria of 5% dose-difference and 3 mm distance-to-agreement. Significant deviations between experimental and calculated dose distributions were found for doses lower than 50% due to the reduced dose resolution of the method in the low dose, low dose gradient region. Measurement errors were observed at 1.0-1.5 mm around each catheter due to MR imaging susceptibility artifacts. For most remaining points the acceptance criteria were fulfilled. Systematic offsets of the order of 1-2 mm, observed between measured and corresponding calculated isocontours at specific segments, are attributed to the 1 mm uncertainty in catheter reconstruction and 1 mm uncertainty in the alignment of the MR and CT imaging planes.

An evaluation of the TSE MR sequence for time efficient data acquisition in polymer gel dosimetry of applications involving high doses and steep dose gradients

“…Previous studies have established that a gel vial irradiated with a single 192 Ir HDR source dwell position comprises a time efficient and accurate source of calibration data spreading over a wide dose range. 6,7,25 In this respect, PABIG gel calibration data in this work were obtained from the CPMG and TSE axial scans of the brachytherapy gel. The cylindrical symmetry of the source with regard to the dose distribution, and consequently the R2 distribution, allowed for the determination of the central axial R2 map in the measured R2 matrices, i.e., the map including the position where the source center was located during the irradiation.…”

Dosimetric accuracy of a deterministic radiation transport based brachytherapy treatment planning system. Part II: Monte Carlo and experimental verification of a multiple source dwell position plan employing a shielded applicator

“…Aiming at the experimental verification of the findings from the comparison of TPS to MC dosimetry results, the polymer gel-MRI dosimetry method was employed to obtain experimental reference dose distributions for comparison with corresponding TPS calculations. The dosimetric properties of different polymer gel formulations and their relative advantages and disadvantages in the dosimetry of modern complex radiation treatment techniques as compared to con-ventional dosimeters are extensively reviewed in the literature, 14 where an increased number of studies reporting the successful use of polymer gels for the dosimetric verification of clinical radiotherapy applications [15][16][17] as well as for the dosimetric characterization of radiation units 18,19 and brachytherapy sources [20][21][22] can be found.…”

Mathematical Modeling of the Response of Polymer Gel Dosimeters to HDR and LDR Brachytherapy Radiation

“…Because bones and soft tissues have heterogeneous structures, it is difficult for medical physicists to predict the exact 3D dose distribution that will be delivered within and near the tumor 9, 30–32. As a result, they need 3D dosimetry tools to help them plan and achieve better brachytherapy treatments 33–36. Also PAG dosimeters may be useful in characterizing the basic radiation dose properties of new brachytherapy seeds,22, 32, 37–39 [e.g., to determine the seed anisotropy factor for nonspherical seeds,40 which is included in Equation 1 below].…”

“…Several research groups have performed experimental studies that address the issue of the reliability of polymer gel dosimeters for detecting 3D dose distributions near HDR brachytherapy sources 20, 22, 23, 33, 37. Other researchers have discussed the applicability and accuracy of polymer gel dosimetry for LDR brachytherapy applications 24, 41–44.…”