ESTRO 2020 Abstract Book

S805 ESTRO 2020

for radiosurgery of single and multiple BM for mono- and multi-isocentric treatment. Material and Methods Ten patients treated with radiosurgery for BM with target volumes ranging from 0.1cm 3 to 5.2cm 3 were re-optimized with an automated treatment planning system (Elements V2.0, BrainLab AG, Munich, Germany) using dynamic conformal arc (DCA). Plans were optimized for a TrueBeam linac (Varian Medical System, Palo Alto, CA) with high definition multi-leaf collimator. Eight to ten plans per BM were generated with the PIL prescription equally distributed between 50% and 90%. All plans were normalized such that 99.5% ± 0.5% of target volume received the prescribed dose. The plan quality was evaluated with respect to the conformity index (CI), gradient index (GI), brain volume receiving 90% of the prescribed dose (V90%), V75%, V50%, V25% as a function of the PIL prescription and BM volume. Results The target mean dose had a linear relationship with PIL (Dmean = -0.289Gy x PIL + 46.3 Gy, R 2 = 0.97) and was independent of the target volume. The CI was lowest for PIL ranging from 86% to 75%, independent of the target volume size and the use of mono- or multi-isocentric treatment. The brain V25% - V90% and GI was the lowest for PIL ranging from 50% to 83% for PTV volumes < 0.7cm 3 . Target volumes ranging from 0.7cm 3 to 1.4cm 3 showed the lowest brain dose for PIL ranging between 50%-60% to 83%. For target volumes > 1.4cm 3 , the PIL resulting in the lowest V25%- V90% and GI ranged between 83% and 60%, while a PIL < 60% lead to increased brain doses and GI. The PIL resulting in the lowest V25%-V90% and GI for multiple BM vs. single BM was not different ( p = 0.2). Conclusion By optimizing the PIL prescription based on BM volume, the dosimetric quality of BM radiosurgery can be improved. For small BM volumes, decreasing the PIL prescription will result in a dose escalation without increasing the brain dose. For larger BM, an optimal range of PIL exists which results in optimal achievable CI and GI. The optimal PIL prescription was similar irrespective of the number of BM. PO-1496 Internal dose-escalation with FFF-VMAT and advantages in SRS metastasis treatments M.J. Perez Calatayud 1 , A. Menéndez 1 , M. Rodríguez Pla 1 , E. Martín García 1 , O.A. Prato Carreño 1 , J. Chimeno 1 , F.J. Celada-Álvarez 1 , A.J. Conde-Moreno 1 , J. Gimeno 1 , V. Carmona 1 , J. Pérez-Calatayud 1 , A. Tormo 1 1 La Fe University and Polytechnic Hospital, Radiation Oncology Department, Valencia, Spain Purpose or Objective Typically the treatment of metastasis in SRS is based on conformal dynamic arcs (DA) (4-6) and the prescribed dose is selected according the volume (16-21 Gy) because the normal tissue (NT) constrains. The aim of this work is to evaluate the potential benefice of VMAT in internal dose- escalation in these treatments. Material and Methods In 5 retrospectively DA-treated patients, SRS frame-based, with lesions around 4-6cc, two type plans have been created with RX 6MV FFF using VMAT of the TrueBeam HD Linac: A) preserving the homogeneity (VMAT-HOMO) and B) scaling the dose according the volume in concentric shells (VMAT-CS). The shells are built with sequential 1 mm inner margins scaling the dose according to: 4cc-18Gy, 2cc- 19.5Gy and 1cc-21Gy. Coverage, gradient and V12 NT parameters have been obtained. The arcs number have been maintained as DA. Results Table 1 presents the main DVH parameters for PTV (D90, D95, D10, Dmean), the Conformity Index and the V12 NT for the 5 cases with the 3 plans. In Table 2 the PTVs and

inner shells volumes-doses are also presented for the VMAT-CS modality. Normalizing to D90, better coverage and NT protection are obtained in all cases with VMAT. VMAT-CS allows dose escalation in the concentric shells keeping the conformity and NT sparing, as showed in Table 2.