P209] Determination of the relative dose distribution around an intrabeam electronic brachytherapy source using Fricke gel dosimetry

Abstract

Purpose The INTRABEAM radiotherapy system is the most commonly used electronic brachytherapy (eBT) system in Europe for cancer treatment. To deliver the prescribed dose, medical physicists use the dose distribution data provided by the manufacturer (Zeiss). This work aims at verifying the correctness of these data for the specific case of breast cancer treatment by comparing the relative dose distribution predicted by Zeiss with that measured using gel dosimetry and that calculated using Monte Carlo simulation. Methods In this work, a Fricke gel dosimeter was used together with MRI readings. This method presents significant advantages as it enables relative dose distribution measurements in 3D, with millimetric spatial resolution. Firstly, the gel response was studied in terms of dose and energy dependence, using several dose levels in different X-ray reference beams. Meanwhile, these irradiation experiments were simulated using the Monte Carlo code Penelope for determining the relative absorbed dose distributions within the gel phantoms and the mean energies of the photon spectra at different depths. Once calibrated, the gel was poured into a dedicated phantom, surrounding the INTRABEAM source associated to a spherical 4 cm diameter applicator, irradiated under clinical conditions. Monte Carlo calculations were then used to convert the absorbed doses to gel into absorbed doses to water, and to obtain the relative absorbed dose to water distribution, required for the comparison. Results As no significant energy dependence was found within the uncertainty limits, the CCRI50b X-ray beam, whose energy spectrum is close to the one of the considered INTRABEAM source, was used to calibrate the gel response in absorbed dose. The polynomial relation obtained was then applied to the measurements with the INTRABEAM system to determine the relative absorbed dose distribution in gel, then converted into a relative dose distribution in water. The predicted, measured and calculated relative dose distributions are in excellent agreement, within the uncertainty limits. Conclusions The agreement obtained between the three methods validates both gel dosimetry as a powerful tool for measuring relative dose distributions delivered by low energy X-ray beams, and the relative dose distribution provided by Zeiss with the INTRABEAM system.