ESTRO 36 Abstract Book

S988 ESTRO 36 2017 _______________________________________________________________________________________________

Purpose or Objective The majority of patients affected by cancer are nowadays treated by radiotherapy, which consists in delivering a homogeneous dose with energetic particles. The main goal of this technique is to target and destroy tumoral cells without damaging the surrounding tissue. This treatment possesses a great adaptability to the broad variety of tumors. Therefore, a major effort was made on the last decades to improve technologies involved in the development and the optimization of this treatment. Our work consists on the development and validation of a new model designed to simulate the energy deposition of the particles used in radiotherapy (electrons, photons and protons), within human tissues. Material and Methods This model is based on a kinetic entropic closure of the linearized Boltzmann equation, which describes the transport of energetic particles in the matter. This equation takes a lot of computation time to be resolved due to the high number of variables. To simplify this, we replace fluences by angular moments, which allows us getting rid of the angular variables andimprove the calculation time. We obtain a set of angular moments equations, and we close thisset using the Boltzmann's principle of entropy maximization on the two first equations of the set. We show that this model has an accuracy comparable to references Monte-Carlo (MC)codes (Geant4, MCNPx, Penelope), and is less time- consuming than these ones. We found out this method applies to new approaches, as MRI-guided radiotherapy which consists in irradiating a patient under the in uence of a magnetic eld. We added the effect of the Lorentz force into our code, and compared it to a reference full MC code FLUKA. Results We obtain a good agreement between simulations from our model and FLUKA. We are able to highlight some effects that occur on the propagation of particles in the matter, which modify the dose distribution on the interface between materials of different densities in a presence of a magnetic field of a few Tesla. These effects have to be taken into account in order to prevent creation of hot spots or a spread of energy distribution in a human body, within computation times compatible with their use in the clinical environment. Conclusion Our model could be applied to future clinical cases and would allow a faster and more efficient way to plan a viable treatment for a patient. We plan to validate our results with an experimental campaign.This work takes place in the framework of POPRA (Programme Optique Physique et Radiothérapie en Aquitaine), which involves several laboratories around problematics on the topic of cancer treatment. EP-1832 Selecting head and neck cancer patients for proton therapy: the influence of dosimetric thresholds I.T. Kuijper 1 , M. Dahele 1 , A. Delaney 1 , B. Slotman 1 , W. Verbakel 1 1 VU University Medical Center, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Selecting head and neck cancer patients for proton therapy should be based on objective parameter(s) that indicate a reduced chance of toxicity, for example, on the dosimetric benefit for swallowing and salivary gland structures. We compared volumetric arc therapy photon (VMAT) and intensity modulated proton plans (IMPT) in order to estimate how many patients would be referred for proton therapy using different thresholds of reduction in organ at risk (OAR) dose. Material and Methods Non-robust IMPT plans were generated for 40 patients with locally advanced head and neck cancer (Varian Medical

are more sensitive to incorrect patient positioning, differing by up to 12% with the delivered dose being greater than the maximum. Correct patient positioning or SS TBI is pertinent. EP-1830 Simple method on bladder filling simulation to improve the soft-tissue evaluation on CBCT K.L. Jakobsen 1 , K. Andersen 1 , D. Elezaj 1 , D. Sjöstrøm 1 1 University Hospital Herlev, Department of Oncology, Herlev, Denmark Purpose or Objective The purpose of this study is to present a cost effective method on how to evaluate the robustness of the treatment plan on different bladder fillings during treatment planning. Furthermore the purpose is to evaluate how this method can be used to determine when a bladder is too small during treatment of the patient. Material and Methods Patients suffering from anal and rectum cancer were enrolled in the study. All patients were instructed to follow our bladder protocol where the patients are asked to empty their bladder 1 hour prior to scan/treatment and then drink 2 glasses of water. The bladder and the bowel were delineated on the CT image set according to QUANTEC guidelines. At the treatment planning stage different bladder fillings were simulated by cutting off ¼, ½ and ¾ of the bladder in the cranial-caudal direction (Figure 1). By using the different bladder volumes the corresponding bowel volumes were created. The robustness of the treatment plans was evaluated by identifying if the bowel constraint was fulfilled for the different simulated bladder fillings. If bowel constraint wasn’t fulfilled the treatment plan was re-optimized to improve the robustness. Before each treatment CBCT was acquired and the true bladder filling was compared to the simulated situations. For the situations where the bladder filling was identified to be too small so the bowel constraint was violated the patients were asked to drink more water. For some of the patients the true bladder was delineated on CBCT and the corresponding bowel was generated and compared to the simulated situation. Results For most of the rectum cancer patients the constraints was fulfilled for all simulated situations. Due to the higher prescription dose and also the location of the target the anal cancer patients didn’t match the constraints to the same extent. The study revealed that most of the treatment plans was robust to bladder filling changes but also identified situations were re-optimization could be done to create a more robust treatment plan (Figure 2). The RTTs found it feasible to compare the bladder on the CBCT with the simulations and was also able to identify when additional actions were needed. Conclusion This procedure has shown to be very cost effective as it doesn’t require additional imaging and it only takes 10-15 minutes to create the simulated structures. The latter can be optimized further in the future e.g. we consider to only simulating the smallest bladder (largest bowel) for the rectum cancer patients. This should be compared with our previous workflow with unreasonable demands on bladder filling and delineation of the bladder on CBCT with the rather subjective decision when the bladder was considered to be too small. Furthermore this workflow has made it able for the RTTs to get more involved in evaluating and react on differences in soft tissue. EP-1831 Entropic Boltzmann closure for MRI-guided radiotherapy J. Page 1 , J.L. Feugeas 1 , G. Birindelli 1 , J. Caron 1 , B. Dubroca 1 , T. Pichard 1 , V. Tikhonchuk 1 , P. Nicolaï 1 1 CELIA, Interaction- Fusion par Confinement Inertiel- Astrophysique, Talence, France