Search

Recent use of very small photon beams (down to 4 mm) in stereotactic radiotherapy requires new detectors to accurately determine the delivered dose. Diamond detectors have been presented in the literature as an attractive candidate for this application, due to their small detection volume and the diamond atomic number (Z=6) which is close to water effective atomic number (Zeff∼7.42). However, diamond exhibits a density 3.51 times greater than that of water and recent studies using Monte Carlo simulations have demonstrated the drawback of a high-density detector on small beam output factors. The current study focuses on geometrical parameters of diamond detector, namely, the diamond dimensions and the electrode geometry, in order to solve the dosimetric issues still observed in small photon beams with diamond detectors. To give better insights to these open questions, we have used both computational method and experimental analysis. This study highlighted that reducing diamond dimensions is crucial for small beam output factor measurements and to limit the influence of its high density. Furthermore, electrodes covering the whole diamond surface were essential for a dose rate independence of the diamond detector. The optimal dosimeter derived from this work presented small diamond dimensions of approximately 1-1-0.15mm3, with diamond-like-carbon electrodes covering the whole diamond surface. A dose rate independence of this diamond detector (better than 0.5% over a wide range of dose rates available on a stereotactic dedicated facility) was obtained due to the electrode geometry. Concerning the output factor measurements, a good agreement (better than 1.1%) was observed between this carbon material detector and two types of passive dosimeters (LiF microcubes and EBT2 radiochromic films) for all beam sizes except the smallest field of 0.6-0.6 cm2 with a deviation of 2.6%. This new study showed the high performance of this diamond detector in small photon beams, in comparison with various commercially available passive and active dosimeters. [ABSTRACT FROM AUTHOR]

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]

Objectif La curiethérapie de haut débit de doses de surface avec applicateurs personnalisés semble être une alternative intéressante aux fils d’iridium ou à la contactthérapie. Nous présentons les résultats d’une série de patients ayant été traités avec cette technique à l’institut de cancérologie de Lorraine. Patients et méthode Les patients ont été d’août 2011 à février 2014 pour des lésions cutanées néoplasiques de la face et du cuir chevelu. Après délimitation de la zone à traiter les cathéters ont été inclus dans de la cire ou un bolus en silicone puis appliqués à l’aide d’une contention. Une dosimétrie avec optimisation graphique a ensuite été réalisée à partir d’une scanographie. Résultats Onze patients ont été traités, seuls trois patients avaient une lésion néoplasique en place. L’épaisseur moyenne de la zone à traiter était de 6,1 mm. La dose par séance était de 5 à 8 Gy à raison de deux séances par semaine. La dose moyenne était de 21,3 Gy. L’isodose moyenne de prescription était la 72 %. Il n’y a eu aucun cas de toxicité de grade III ou IV. Conclusion La curiethérapie interstitielle avec fils d’iridium nécessite l’implantation des cathéters dans la zone à traiter. La curiethérapie de haut débit de dose de surface avec applicateurs personnalisés permet d’éviter une effraction cutanée et offre des avantages indéniables en termes de dosimétrie comparativement à une radiothérapie externe. Cette technique est très intéressante pour le traitement des lésions cutanées superficielles fines où l’implantation de cathéters serait difficile, et convexes où le bolus peut être appliqué correctement. Dans le cadre des lésions cutanées superficielles fines et convexes la curiethérapie de haut débit de doses de surface utilisant des applicateurs personnalisés est une technique efficace et facile à mettre en œuvre. [ABSTRACT FROM AUTHOR]