31. Machine and patient quality controls by portal imaging using independent software. (December 2016) - Fontaine, J.
Abstract : Introduction: Because of regulation and by good practice, machine and patient quality controls are implemented in radiotherapy. The objective of this study is to show the feasibility of some of these controls by portal imaging with an independent conversion algorithm of image grey level into absorbed dose in water. Material and methods: The irradiations were realized in 6 MV on an accelerator Varian Clinac iX equipped with a MLC 120 leaves and a detector in amorphous silicon (EPID aSi1000). The portal images were converted in dose with the EpiDream algorithm proposed by the company DREAM. After creating a calibration curve, it allows to convert the image into dose matrix without knowledge of irradiation conditions. For machine controls, the tests concerned the constancy measurement of dose rate, beam symmetry, dynamic filter transmission and Dose Leaf Separation (DLS). For the latter, plans simulating gaps on the leaf bench of 0.1, 0.2 and 0.5 mm were created to estimate the capacity of the system to reveal them. The results computed from portal images were compared to the measures usually realized in the department. For pre-treatment IMRT plan controls, doses distributions calculated with EpiDream from portal images were compared to those measured by EDR2 and calculated by TPS using a gamma index 3%–3 mm with a 10% threshold. In order to test the system sensibility, position leaf errors were also voluntarily introduced. Results: Quality controls realized with the EPID showed a good agreement with the measurements made periodically in the center. The study of the DLS showed a sensibility of the system close to ionization chamber for the detection of a gap on the leaf bench. The images converted in dose allowed to detect gaps lower than 0.2 mm. The study of IMRT plans showed, on 28 controlled beams, for the EPID-TPS comparison, an average gamma of 0.39 (±0.05) and an average percentage points passing the gamma criterion of 96% (± 3%). Concerning the EPID-EDR2 and EDR2-TPS comparisons, the percentages were respectively 90% (±4%) and 94% (±3%) and the average gamma indices are 0.59 (±0.06) and 0.47 (±0.06). EPID allowed to reveal all of the errors included in the treatment plans by highlighting visually the zones where errors were introduced and by degrading the value of the gamma index. Conclusion: The results obtained with EPID images converted in absorbed dose allowed to consider the use of the EPID for the realization of numerous quality controls in radiotherapy. So, the controls could be more quickly realized, in 2D and with a high spatial resolution. In this study, the possibility of realizing the control of the IMRT plans, the DLS, the transmission of the filter, dose rate and symmetry of the beams with the imager portal was demonstrated. A longitudinal follow-up is in progress to confirm these results.