ESTRO 36 Abstract Book

S983 ESTRO 36 2017 _______________________________________________________________________________________________

EP-1819 EORTC RTT Delineation Project: improving volume definition of OAR within the EORTC Lungtech trial M. Van Os 1 , M. Rossi 2 , C. Hurkmans 3 1 Erasmus MC Cancer Institute, Department of Radiation Oncology, Rotterdam, The Netherlands 2 Netherlands Cancer Institute, Radiation Oncology Department, Amsterdam, The Netherlands 3 Catharina Ziekenhuis EIndhoven, Radiation Oncology, Eindhoven, The Netherlands Purpose or Objective To accurately evaluate toxicity within a trial, it is important that the delineation of the Organs at risk (OAR) is performed consistently. The aim of the EORTC ROG/RTT delineation project is to reduce variability in OAR delineation to allow better comparison of toxicity in EORTC trials between institutes. Within the EORTC LungTech trial we evaluated whether provided delineation guidelines are explicit enough to avoid misinterpretation in volume definition of OAR and whether a hands-on delineation workshop adds to this. Material and Methods A questionnaire was developed and distributed amongst participating institutes requesting feedback after they had completed their benchmark delineation for the EORTC LungTech trial. Difficult areas could be identified. Subsequently two groups were defined: Group A to perform delineation of OAR according to protocol guidelines and with additional visual guidelines. Group B was invited to a delineation workshop where an expert teacher addressed the difficult areas provided from the benchmark cases. The consequent delineations were then evaluated per organ at risk between group A and B and also compared to the benchmark cases. The volume of agreement was calculated. Our hypothesis is that a delineation workshop prior to launching a trial improves consistent volume definition. Results Introducing visual guidelines as an extra aid to the written guidelines significantly reduced misinterpretation and greatly improves the consensus of delineation. Although not specifically tested between the groups the RTT’s performance in delineating the OARs was deemed as consistent as the clinicians. We aim to demonstrate the additional value of teaching by means of a delineation workshop, but this analysis is still ongoing. Conclusion The EORTC ROG advised to involve RTTs in future EORTC trials for volume definition of OAR. In future EORTC trials, a kick-off including a delineation workshop for participating institutes is planned to achieve more consistent delineation of OAR. EP-1820 RapidArc vs IMRT in adjuvant gastric cancer irradiation: any dosimetric advantage? K. Mashhour 1 , W. Hashem 1 , R. Fawzy 1 , H. Abdelghany 2 1 Kasr el ainy school of medicine-Cairo university, clinical oncology, cairo, Egypt 2 Kasr el ainy school of medicine-Cairo university, radiation physics, cairo, Egypt Purpose or Objective The outcome of intensity modulated radiation therapy (IMRT) plans depends on the choice of number of fields, beam orientations, optimization algorithms & planner experience. The RapidArc has the characteristics of both good plan quality as well as high delivery efficiency. The purpose of this work was to compare dosimetric endpoints Electronic Poster: RTT track: Treatment planning and dose calculation / QC and QA

between IMRT & RapidArc techniques for gastric carcinoma patients in the adjuvant setting. Material and Methods Twenty gastric cancer patients who had a radical gastrectomy with D2 dissection were eligible. The clinical target volume (CTV) included the gastric bed, anastomosis with 2-cm proximal/distal margins & regional LN areas. The planning target volume (PTV) consisted of CTV plus a 10 mm margin.All plans were created in the Eclipse treatment planning system (v8.6 Varian Medical System). For IMRT, a coplanar seven-field plan was performed. Regarding the RapidArc, plans were done using a double arc plan consisting of 2 co-planar arcs of 360° in clockwise & counter clockwise direction. Dose prescribed was 45 Gy to the PTV in 25 fractions using 6MV photons. The PTV dose coverage criteria were as follows: at least 95% of PTV received 45Gy; The conformity index (CI) & homogeneity index (HI) were calculated. The organs at risk (OAR) were the spinal cord, both kidneys and liver. Results Target coverage was similar for both techniques. The mean V95 was found to be 94.7% & 94.8% for the IMRT and RapidArc respectively (p = 0.32). The CI for IMRT and RapidArc were 0.93 ± 0.01 & 0.94 ± 0.01, respectively; while the HI was 1.15 ± 0.01 for IMRT & 1.14 ± 0.02 for RapidArc (both p > 0.04).All the plans met the required dose limitations. The maximum spinal cord dose for IMRT and RapidArc was 37.87 Gy vs 36.42 Gy (p = 0.34). For the right kidney, IMRT had significantly lower mean V20 (volume that receives 20 Gy) compared to RapidArc (23.2 vs. 30.3, p = 0.01). The mean V20 to the left kidney were 28.4 and 27.4 in the IMRT and RapidArc respectively (p = 0.01). The IMRT produced a similar liver mean V30 (volume that receives 30 Gy) (24.3 vs. 23.1, p = 0.52) to RapidArc. The treatment delivery time was 193.5 ± 25.0 s (range 157– 230 s) to IMRT and 66.0 ± 8.7 s (range 55–77 s) to RapidArc (p = 0.00). The total monitor units (MU) for IMRT and RapidArc were 343.0 ± 94.0 & 363.0 ± 44.0 (p = 0.07), respectively. Conclusion RapidArc obtained similar dosimetric outcomes to IMRT plans regarding target coverage & OAR sparing with an advantage of shorter delivery time & lower number of MU. EP-1821 Air gap between patient surface and immobili zation devices: dosimetric impact on H&N IMRT plans S. Moragues-Femenia 1 , M. Pozo-Massó 2 , J.F. Calvo- Ortega 2 , J. Casals-Farran 2 1 Moragues Femenia Sandra, HOSPITAL QUIRON BARCELONA- Radiotherapy, Barcelona, Spain 2 HOSPITAL QUIRON BARCELONA- Radiotherapy, HOSPITAL QUIRON BARCELONA- Radiotherapy, Barcelona, Spain Purpose or Objective In head and neck (H&N) treatments, there is an avoidable air gap between the typical mask-based immobilization device used and the patient surface ("air gap" from now on). Our aim is to evaluate the dosimetric effect of considering the "air gap" on the patient dose distribution on H&N IMRT plans. Material and Methods

A total of 5 H&N patients were selected. The "immobilization" device consisted of a thermoplastic mask covering the head, neck and shoulders, and attached to a board on the linac couch. Targets (PTVs) and organs-at-