TRIDIMENSIONAL QUALITY CONTROL OF THE CRANIOSPINAL IMRT JUNCTION USING GEL DOSIMETER AND OPTICAL CT

Introduction: With the evolution of technologies for cancer treatment planning, the demand for quality control methodologies that provide three-dimensional information for dose distribution measurements also increased. In this context, gel dosimeters combined with an imaging technique are of great interest because they can perform three-dimensional dosimetry of the entire volume planned treatment. In this study, a Fricke – FX xylenol gel dosimeter (FXO) and an optical tomography using cone-beam geometry (optical CT) were used to evaluate the 3D dose distribution of two IMRT field junction used in craniospinal irradiation. Methods: Craniospinal irradiation was planned using two IMRT fields, one for the cranium and the other for the cervical region, the junction of these fields was evaluated in this study. A Unique LINAC (Varian Medical Systems) was used and a fraction dose of 2.1Gy was delivered to the phantom. The phantom was a cylinder of 12.5 cm diameter and 15 cm high, it was filled with the FXO gel dosimeter. Two other similar cylinders were irradiated in order to acquire a percent depth dose measurement with maximum doses of 3Gy and 1.5Gy to provide the data used in the association of the optical attenuation coefficient to the known absorbed dose, to calibrate the gel batch. The optical CT images were acquired with 360 ° phantom rotation in 0.5 ° steps. The images reconstruction were performed using the combination of FDK and SIRT-TV algorithms for noise removal. The reconstructed images were converted to dose and compared to the planning dose distribution using gamma analysis (3%/3mm) using in-house developed software. Results: By evaluating the gamma index images in the junction region only the edges presented values greater than 1, which failed in the evaluation. In the central region, inside the target volume, 96.5% of the points were approved in the gamma analyses. Considering all the volume, an average approval of 90.3% of the analyzed pixels was achieved. Conclusion: Gel dosimetry using FXO gel combined with CT-optics can be used for three-dimensional quality control in a complex procedure such as craniospinal irradiation.