Doses aux tissus sains délivrées lors d'un traitement stéréotaxique pulmonaire au cyberknife : une application de deux méthodes de détermination dosimétrique

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Healthy tissue doses in a pulmonary stereotactic treatment using the Cyberknife: Application of two dosimetric methods

Introduction. Radiotherapy is a double-edged sword as this technique inevitably irradiates healthy tissues. This exposure may consequently trigger radio-induced complications. In recent years, assessing the risks induced by radiotherapy became a major concern as patients’ life expectancy improves and as modern techniques irradiate a more important tissue volume 1. Risks are directly correlated to the dose delivered to the volume of healthy organs. However, precisely determining those doses remains an important challenge 2. In that context, two approaches were investigated at IRSN to determine normal tissue doses: a Monte-Carlo (MC) model of the Cyberknife and an experimental tool using EBT3 films. In this study, we apply those tools to determine the doses delivered in a Cyberknife stereotactic treatment of the pulmonary region.Methods. The ATOM adult male phantom (CIRS) was scanned at the CFB (Caen). A target volume in the lung and several organs were delineated on this scan using the Multiplan TPS. A treatment plan was optimized and calculated using both RayTracing and the TPS MC algorithm. EBT3 films were prepared with a rigorous protocol and were placed between each phantom’s slice. Then the phantom was irradiated following the treatment plan. From the film measurements, the doses were reconstructed in 3D with a tool developed at IRSN 3 using MatLab. Moreover, the 55 treatment beams were simulated using a PENELOPE model of the Cyberknife validated for precise out-of-field dose determination 4.Results. RayTracing largely overestimates the healthy tissue doses whereas out-of-field doses are underestimated by MC TPS in comparison with the films. Discrepancies can be as high as 58% in the heart for MC TPS. Those discrepancies reach 33% and 256% for MC TPS and RayTracing respectively regarding the mean head and neck dose (measured at 9.1 cGy) and 32% and 623% respectively in the kidney (measured at 5.0 cGy). The pelvic mean dose (area not included in the CT scan) is estimated at 1.8 cGy with the films.Conclusions. This study enables to show the incorrect evaluation of the Multiplan TPS regarding out-of-field doses. The MC simulations are currently ongoing and will complete the results of this experimental study. Those dose evaluations could be used to estimate adverse effects risks induced by the treatments

Doses aux tissus sains délivrées lors d'un traitement stéréotaxique pulmonaire au cyberknife : une application de deux méthodes de détermination dosimétrique

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A new Monte Carlo model of a Cyberknife® system for the precise determination of out-of-field doses

International audienceIn a previous work, a PENELOPE Monte Carlo model of a Cyberknife system equipped with fixed collimator was developed and validated for in-field dose evaluation. The aim of this work is to extend it to evaluate peripheral doses and to determine the precision of the treatment planning system (TPS) Multiplan in evaluating the off-axis doses. The Cyberknife® head model was completed with surrounding components based on manufacturer drawings. The contribution of the different head parts on the out-of-field dose was studied. To model the attenuation and the modification of particle energy caused by components not modelled, the photon transport was modified in one of the added components. The model was iteratively adjusted to fit dose profiles measured with EBT3 films and an ionization chamber for several collimator sizes. Finally, dose profiles were calculated using the two Multiplan TPS algorithms and were compared to our simulations. The contributions to out-of-field dose were identified as scattered radiation from the phantom and head leakage and scatter originating at the secondary collimator level. Particle transport in the additional pieces was modified to model this radiation. The maximum differences between simulated and measured doses are of 20.4%. Regarding the detector responses away from axis, EBT3 films and the Farmer chamber give similar response (less than 20% difference). The TPS Monte Carlo algorithm underestimates the doses away from axis more importantly for the smaller field sizes (up to 98%). Besides, RayTracing simplifies peripheral dose to a constant value with no inclusion of particle transport. A Monte Carlo model of a Cyberknife system for the determination of out-of-field doses up to 14 cm off-axis was successfully developed and validated for different depths and field sizes in comparison with measurements. This study also confirms that TPS algorithms do not model peripheral dose properly. © 2019 Institute of Physics and Engineering in Medicine