ESTRO 2020 Abstract Book

S808 ESTRO 2020

breast were compared for VMAT and HYBRID planning techniques. The accuracy of dose delivery was estimated for both treatment methods. Material and Methods For this study 16 patients with left breast cancer were selected retrospectively. All of them had got hypofractionated radiotherapy (2.66Gy daily, 16 fractions). For each patient 3D,VMAT and HYBRID plans were calculated using Eclipse 13.7 TPS (Varian) for TrueBeam STxlinear accelerator. Pretreatment verification was performed using Octavius 4D phantom (729detector array). Respiratory Gating system (Varian) was used for voluntary breath hold.Dose evaluation was performed according to RTOG 1005, QUANTEC (ipsilateral lung:mean dose ≤ 13.5 Gy, V20 ≤ 30%, V15 ≤35%, V10 ≤40%, V5 ≤55%; heart: mean dose≤5 Gy,V10≤25Gy). Results The proportion of 3D and VMAT parts in HYBRID plan is an option which allows to find an optimum between decreasing mean dose and low dose for heart, lungs, breast and increasing V20 (volume of lung that receives radiation dose ≥20 Gy) for ipsilateral lung. For this study the proportion of 3D part had never exceeded 50% of all monitor unites per HYBRID plan.

Figure 1. Comparison of dose distribution for VMAT and HYBRID The HYBRID technique allowed us to reduce the mean dose for heart up to 45%, for contralateral lung up to 64% in comparison to VMAT plans. As for ipsilateral lung, the mentioned proportion optimum of 3D and VMAT was applied for all calculated plans and mean dose for HYBRID plans never exceeded mean dose in VMAT plans more than 6%. On the contrary, in some cases mean dose for ipsilateral lung in HYBRID plans was smaller, even up to 35%.

Figure 2. Two cases with the best edvantage for ipsilateral lung and heart All VMAT and HYBRID plans passed through the verification successfully. Conclusion The Eclipse13.7 TPS does not provide the possibility to use the skin flash tool for VMAT plans calculation. So, for breast cancer localizations it’s only possible to calculate either 3D or IMRT plans. The first one is not quite flexible in terms of conformity, the second is difficultin terms of fraction delivery time. Long treatment time is uncomfortable for patient which may lead to shifts of his positioning and inaccuracy of dose delivery. VMAT thus can greatly reduce treatment time. Respiratory Gating system allows to calculate VMAT plans for breast cancer without skin flash tool. But VMAT plans cause much greater coverage of normal tissue with low doses in comparison with 3D. Besides, VMAT optimization is not as flexible as IMRT. HYBRID in conjunction with voluntary breath hold can gives us a flexible treatment planning technique that allows to find an optimum between minimization of dose for critical organs and the complexity of the plan performance and dose delivery. PO-1501 Frameless workflow for radiosurgery of arteriovenous malformations - A new software prototype D. Schmidhalter 1 , D. Henzen 1 , E. Herrmann 1 , W. Volken 1 , P. Mackeprang 1 , E. Ermis 1 , H. Hemmatazad 1 , J. Honegger 2 , B. Haas 2 , M.K. Fix 1 , P. Manser 1 1 Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital- Bern University Hospital- and University of Bern, Bern, Switzerland ; 2 Varian