3D dose verification in 192Ir HDR prostate monotherapy using polymer gels and MRI

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.