ESTRO 2020 Abstract book

S856 ESTRO 2020

without compromising target coverage, with a shorter irradiation time, compared to the SF technique.

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

PO-1499 Incidental dose to obturator nodes in prostate radiotherapy trials CHHiP, PROFIT and PIVOTALboost I. Gleeson 1 1 Addenbrooke's Hospital - Oncology Centre, Medical Physics, Cambridge, United Kingdom Purpose or Objective Elective nodal radiotherapy remains a controversial topic in high risk patients. Previous works suggest less conformal techniques result in higher incidental doses to pelvic nodes leading to increased failures among those receiving more conformal techniques. Incidental dose to the pelvic nodes differs between techniques with the obturator nodes been highlighted as a region of highest dose differences. This work investigates the incidental dose received by the obturator nodes with moderately hypofractionated prostate only radiotherapy techniques used in the trials CHHiP, CHHiP IGRT sub-study, PROFIT and PIVOTALboost arm A. Material and Methods 5 patients were planned by the same planner using 6 MV single arc VMAT following trial protocols CHHiP, CHHiP IGRT sub study, PROFIT and PIVOTALboost. Target volumes were grown as per their respective trial protocol as shown in the Table and one set of OAR were used for all plans which was outlined as per CHHiP. Plans were prescribed 60 Gy in 20 fractions. Satisfactory plans were produced in line with their protocols and obturator pelvic nodes were retrospectively outlined as per PIVOTALboost guidelines