ESTRO 2020 Abstract book

S855 ESTRO 2020

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.

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