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Introduction For small beams used in stereotactic radiotherapy, extra precautions need to be taken into account when determining dosimetric data used in the modelling of dose calculation algorithms [1]. In this work, we focused on the output factors determination of the Cyberknife M6 (Accuray Inc., Sunnyvale, USA). Measurements were performed with active and passive detectors and with fix collimation. We compared two new sets of corrective factors published by the IAEA (TRS 483) and by Francescon et al., recommended by Accuray [2] and we compared theses corrected results with radiochromic films. Methods OF were measured with three punctual PTW detectors: 60018 (SRS) and 60008 diodes, 60019 microdiamond and EBT3 Gafchromic films (Ashland Inc., USA) for all the fixed collimators in the reference conditions (SAD = 80 cm, depth = 1.5 cm), 200 MU for each beam. Five measurements were performed for each punctual detector and three for the films. Films were analyzed with the Epson V750 Pro (Epson, Japan) and the ImageJ software (NIH, USA). Results Application of these two sets of corrective factors enable the reduction of discrepancies (related to small beams measurements challenges) between detectors for collimators greater than or equal to 7.5 mm diameter (see Fig. 1). For the 5 mm fixed collimator, the maximal discrepancy between two detectors is 9.6% without corrections and is reduced to 4.4% and 8% with the Francescon's corrections and with the IAEA respectively. In average, corrected measurements are closer to films measurements with a maximal discrepancy varying from 3.8% to 2% and 2.1% for Francescon and for the IAEA respectively. Conclusions The two sets of corrective factors give similar OF results with a good agreement between detectors, allowing a better precision for OF modelling in the TPS. Those factors could be extended for the Iris collimator [3]. [ABSTRACT FROM AUTHOR]

Introduction We conducted a practice analysis as part of a quality assurance process related to the commissioning of a Cyberknife M6. In this context, several physicists have been specifically trained. The purpose of this study was to assess whether interoperator differences existed in the implementation of treatment plans and whether these differences could impact treatment. Methods We retrospectively analyzed treatments planned between September 2017 (first Cyberknife M6 treatments) and March 2018 using Precision 1.1 (Accuray) TPS. The data collection and formatting was automated under Excel macro from the export of the "Plan Overview" file generated by the TPS containing the specific technical and dosimetric data (DVH). The analysis focused on dosimetric indexes (PTV coverage, compliance index (reference isodose vol/ PTV flight), gradient index (isodose volt 50%/ reference isodose)] and efficiency indices of treatment delivery (number of beams, MU per session and estimated duration of the session). Numerical parameters were compared by variance analysis or by Kruskal–Wallis test according to the normality of the distribution (studied by a Kolmogorv-Smirnov test). The qualitative parameters were compared by a Chi-square or Fisher Exact test. The level of significance was set at 5%. Results 6 physicists (medical physicists only) with experience ranging from less than 1 year to 11 years completed 226 treatment plans. 40% of the treatment plans were intracranial. The proportion of intracranial/extra-cranial plans, the volume of the PTVs and the multiplicity of volumes were comparable between the 6 physicists. No significant differences between physicists were found on PTV coverage (p = 0.108) or Gradient Index (p = 0.219). Significant differences appear on the number of MUs (p = 0.008), the number of beams (p = 0.001) and the treatment times (p = 0.020). These 3 parameters are related to beam, time or UM reduction operations, during the last stage of plan optimization. No criterion on these 3 parameters or on the reductions was defined a priori within the team; they remain at the discretion of the physicist. Conclusions The statistical evaluation of the treatment plans carried out during the first six months following the initiation of Cyberknife M6 treatments at our institution made it possible to objectify the homogeneity of the dosimetric results obtained by the different physicists, despite different optimization practices. The analysis also showed that the recently integrated medical physicist obtained comparable results to other physicists on all the evaluated indicators. This validates her training by companionship. [ABSTRACT FROM AUTHOR]