Impact analysis for detector positioning and mechanical errors for reference dosimetry on FFF beam mode

The implementation of new technologies in radiotherapy has increased the accuracy level required in treatments. Absorbed dose calibration in water is an important indicator of the exactness on dose delivery, with an associated uncertainty on its measurements. FFF beam mode has been used in modern techniques, and due to singular aspects, require a better evaluation of deviations involved in the calibration process. Therefore, the work sought to evaluate such uncertainties by simulating mechanical setup and detector positioning errors, for different energy, dose rate and detector settings. The analysis was done in a relative way, based on the percentage deviation between the charge collection readings outside and within the reference conditions. The results were presented on three points of view, setting one of the configuration variables and evaluating the highest percentage deviation obtained within each parameter. At the end, a general analysis of the largest deviation obtained in each parameter was made for all energy, dose rate and detector configurations. The mechanical deviations reached values below 1.5%, within tolerable limits but highly influenced by the volume of the detector and the energy. Positioning errors of the detector were the most affected, with values above 2.0% and reaching its highest for vertical errors (3.6%). The beam profile, the chamber sensitivity, the influence of PDD curve settings and Pion influences has helped to explain such deviations. Thus, it became clear the importance of a rigid quality control in equipment operating under such mode, and the redoubled care that must be taken on experimental setup for reference dosimetry. (author)