ESTRO 36 Abstract Book

S444 ESTRO 36 2017 _______________________________________________________________________________________________

Results Both IMPT or target tailoring by excluding the proximal uterus resulted in significant reductions of V 15Gy , V 30Gy , V 45Gy and D mean for bladder and small bowel. Compared to conventional volumes, target tailoring by excluding the non-invaded uterus resulted in an average reduction of the primary ITV and PTV of 37% and 8%, respectively. IMPT would have reduced the estimated NTCP for small bowel toxicity (≥grade 2) from 25% to 18%, and would be additionally reduced to 9% when IMPT were combined with MRI-based target tailoring. Major NTCP reductions of >10% were predicted in four patients (36%) by IMPT, and in six patients (55%) when IMPT were combined with MRI-based target tailoring. Patients benefitted most (NTCP reduction >10%) from one of the investigated approaches if the V 45Gy for bowel cavity was >275 cm 3 during standard IGART alone; a similar reduction in NTCP from the combined approached would have been obtained in patients with a V 45Gy for bowel cavity >200 cm 3 . Conclusion In patients with cervical cancer, both 1) proton therapy and 2) target tailoring by excluding the radiologically uninvolved part of the uterine corpus led to a significant dose reduction to surrounding OARs, which separately would already yield a clinically important decrease in small bowel toxicity, which is cumulative if both approaches would be combined. Reference [1] de Boer P, Bleeker MCG, Spijkerboer AM, et al. Eur J Radiol Open. 2015;2:111–7. PO-0834 Automated planning to reduce integral dose in robotic radiosurgery for benign tumors L. Rossi 1 , A. Méndez Romero 1 , M. Milder 1 , E. De Klerck 1 , S. Breedveld 1 , B. Heijmen 1 1 Erasmus Medical Center, Radiation Oncology, Rotterdam, The Netherlands Purpose or Objective Highly conformal dose distributions and minimizing integral dose are essential in radiosurgery of benign vestibular schwannoma (VS) tumors to avoid long term side effects. This includes avoidance of secondary tumor induction in these long surviving patients. High delivery accuracy can be obtained with the robotic CyberKnife (CK, Accuray Inc, Sunnyvale, USA) due to real time image- guided tracking, allowing small PTV margins. However, optimal plan quality may be hampered by the current trial-and-error planning approach, as it strongly depends on the planner’s experience and available planning time. We have developed a system for fully automated CyberKnife treatment planning. In this study, we have used this system to automatically generate plans for vestibular schwannoma patients (AUTOplan) and we have compared them with plans that were manually generated in clinical routine (MANplan), both with the IRIS collimator. Material and Methods Both MANplans and AUTOplans were genereated with the Multiplan TPS (Accuray Inc). For AUTOplanning, a fully automatic pre-optimization was performed with our in-

Conclusion The use of the variable RBE-model results in substantially higher predicted doses to the CTV compared to the constant 1.1, due to the low α/β associated with breast cancer. Substantially higher RBE values are also predicted for the OARs. This decreases the potential benefit with protons, but could probably be neglected in cases where the physical doses are low. However, if e.g. the LAD is close to the target this could lead to substantially higher predicted doses. The variable RBE could therefore be of importance in certain cases when employing a NTCP model based comparison between proton and photon plans. PO-0833 Reducing small bowel dose for cervical cancer using IMPT and target tailoring in treatment planning P. De Boer 1 , A.J.A.J. Van de Schoot 1 , H. Westerveld 1 , M. Smit 1 , M.R. Buist 2 , A. Bel 1 , C.R.N. Rasch 1 , L.J.A. Stalpers 1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands 2 Academic Medical Center, Gynaecology and Obstetrics, Amsterdam, The Netherlands Purpose or Objective Current radiotherapy standards for cervical cancer patients lead to irradiation of large bowel volumes and bladder during external beam radiotherapy (EBRT). Highly conformal techniques such as IMRT, arc-rotation therapy and image guided adaptive radiotherapy (IGART) have resulted in considerable reduction in volume to organs at risk (OARs), but there remains room for further improvement. We previously showed that cervical invasion into the uterine corpus assessed by MRI correlates well to pathological invasion [1]. In the present study we wish to investigate the potential clinical benefit from target tailoring by excluding the tumor free proximal part of the uterus during IGART. Furthermore, we compare this benefit with the advantage of an improved dose conformity by intensity-modulated proton therapy (IMPT). Material and Methods Diagnostic MRIs and planning-CTs from eleven patients with locally advanced cervical cancer were used; all previously had photon radiotherapy and a substantial (>4 cm) tumor-free part of the proximal uterus as visualized by MRI. IGART and robustly optimized IMPT plans were generated for both conventional target volumes (including the entire uterus), and MRI-based target tailoring (excluding the non-invaded proximal part of the uterus), which yielded four treatment plans per patient. For each plan, V 15Gy , V 30Gy , V 45Gy and D mean for bladder, sigmoid, rectum and bowel bag were compared. The clinical benefit of either and both approaches were estimated by calculating the normal tissue complication probability (NTCP) for at least grade II acute small bowel toxicity.