ESTRO 2020 Abstract Book

S806 ESTRO 2020

improved in all cases with cone. Up to 0.2 cc is saved in V10 and 8 Gy in average for Dmax, being the benefice higher as lower is the distance from the isocenter to the brainstem.

Conclusion The use of small cone improves significantly the brainstem sparing comparing with mMLC, allowing for an excellent gradient in TN treatment with linac. Based on this dosimetric analysis, the use of cones would be associated with a lower risk of toxicity. PO-1498 Dosimetric analysis of the use of FFF beams with SBRT for lung cancer M. Savanović 1,2 , M. Loi 2 , E. Rivin del Campo 3 , F. Huguet 3 , J. Foulquier 2 1 University of Paris-Saclay, Faculty of Medicine, Le Kremlin-Bicêtre, France ; 2 Tenon University Hospital, Radiation Oncology, Paris, France ; 3 Tenon University Hospital- Sorbonne University, Radiation Oncology, Paris, France Purpose or Objective The purpose of this study was to compare the dose received by organs at risk (OAR) in patients with lung cancer treated with stereotactic body radiation therapy (SBRT), using static fields (SF) or dynamic conformational arc (DCA) with flattened filter (FF) beams or flattening One-hundred patients with lung cancer were treated with SBRT between 2016 and 2018 by either SF (n=50) or DCA (n=50) techniques using FF beams (TrueBeam STx, 6 MV, IQ=0.67, 600 MU/min). A dose of 60 Gy was prescribed to the PTV at the 80% isodose line in 4 to 8 fractions, depending on the proximity of OARs. Treatments were planned with the TPS Pinnacle 9.10 with SF (7 to 9 beams) and DCA (2 to 4 partial arcs). These treatments were retrospectively replanned using 6XFFF beams (IQ=0.63, 1400 UM/min). The beam-on time and dosimetric gain on PTV coverage and OARs dose constraints (heart, spinal cord, spinal cord + 3 mm, esophagus, lung and ribs) were analyzed according to tumor location. The comparison of median values was performed using the non-parametric Wilcoxon test (significance level: p <0.05). Results The coverage of the PTV at the 80% isodose line was 98.9% vs 98.4% (DCA) and 98.8 vs 98.3% (SF) for the FF and FFF beams, respectively (Figure 1). The median D max received by the heart, spinal cord, spinal cord + 3 mm, esophagus, and ribs, as well as the median lung - PTV V20 and V16 were lower with FFF than with FF (p<0.05) (Table 1). The median beam-on-time was 144 s (FF) vs 89 s (FFF) using DCA with 2 partial arcs (p<0.001) and 346 s (FF) vs 238 s (FFF) using SF with 7 beams (p<0.001). Conclusion Using FFF beams for treatment of lung cancer with SBRT provides a dosimetric gain on all OARs, irrespective of technique (SF or DCA) and tumor location. Treatment using DCA with FFF provides a better protection of the OARs filter free (FFF) beams. Material and Methods

Conclusion The use of VMAT vs DA improves significantly the dose distribution in coverage and NT sparing. The modulation using VMAT-CS allows for internal dose escalation avoiding the constraint of tumor size. PO-1497 Small cone vs microMLC in SRS trigeminal neuralgia: Dosimetric improvement. M.J. Perez Calatayud 1 , N. Carrasco Vela 1 , A. Menéndez 2 , F.J. Celada-Álvarez 1 , A.J. Conde-Moreno 1 , M. Rodríguez Pla 1 , E. Martín García 1 , O.A. Prato Carreño 1 , J. Chimeno 1 , F. Lliso 1 , J.C. Ruiz 1 , J. Pérez-Calatayud 1 , A. Tormo 1 1 La Fe University and Polytechnic Hospital, Radiation Oncology Department, Valencia, Spain ; 2 La Fe University and Polytechnic Hospital, Neurosurgery Department, Valencia, Spain Purpose or Objective Trigeminal neuralgia (TN) SRS is a well-established radiotherapy treatment. Typically a high dose (70-90 Gy) is prescribed at the target center, being the fulfillment of brainstem dose-constrains the important challenge. For this purpose, the smallest available beam is used. The aim of this work is to study the advantage of cone vs mMLC with respect brainstem sparing. Material and Methods 10 patients have been retrospectively planned with 5x5 mm 2 defined with mMLC and a 4mm in diameter cone with FFF-X6 of the TrueBeam (Varian) linac. Treatment consists in approximately 13 non-coplanar homolateral arcs. SRS frame is used and patient set-up is performed by using CBCT-CT matching and 6D couch. Intrafraction is verified with CBCT. For equal prescribed dose, the DVH parameters of brainstem V10, Dmax and D10% are obtained. Results Table 1 presents the volume and dose differences for both techniques for the 10 studied cases. Also, the distance isocenter-to-brainstem is included. The brainstem dose is