ESTRO 36 Abstract Book

S1002 ESTRO 36 _______________________________________________________________________________________________

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 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

The plans were then recalculated implementing the shifts using the algorithm used for the clinical plans (Eclipse ™, Varian Medical Systems, Palo Alto, AAA algorithm, v 13.6). The mean and maximum doses for the lungs, kidneys, brain and the (body-lungs-5mm) structure were extracted and the difference between the planned and the recalculated doses determined.Results The mean doses change by a maximum of 0.6% (lungs), 0.6 (kidneys), 0.5% (brain) and 0.2% (body-lungs-5mm). The greatest difference between the maximum doses are 8.0% (lungs), 4.8% (kidneys), 2.6% (brain) and 12.0% (bodylungs- 5mm). The standard deviation of the difference between the calculated and recalculated doses are greater for the maximum doses than the mean doses (figure 2). Given that the minimum and maximum doses for SS TBI are typically in the range 90-110% of the prescribed dose, the differences in maximum dose should lead to care being taken when positioning patients for SS TBI.

Conclusion Patient positioning for a total of 63 fractions of SS TBI is such that the mean delivered doses differs from the planned by less than 0.6%. However, the maximum doses 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