Abstract Book

ESTRO 37

S563

developed method can be transferred to model other commercial X-ray units. Detailed MC based investigations of the head scatter to improve imaging quality is current work in progress. PO-1008 Commissioning of IMRT/VMAT on the novel Varian Halcyon™ R. De Roover 1 , K. Poels 2 , W. Crijns 2 , A. Nulens 2 , B. Vanstraelen 2 , K. Haustermans 1,2 , T. Depuydt 1,2 1 Catholic University of Leuven, Department of Oncology, Leuven, Belgium 2 University Hospitals Leuven, Department of Radiation Oncology, Leuven, Belgium Purpose or Objective The novel Varian Halcyon™ O-ring linac design allows for delivery of IMRT and VMAT treatments at increased speeds by employing an encapsulated fast rotating gantry and performant dual-layer multi-leaf collimation system. The Halcyon™ is delivered with a pre-configured treatment planning system (TPS). As one of the first institutions that have implemented this novel linac system, we present an extensive dataset on the treatment delivery quality acquired with a variety of For the IMRT/VMAT commissioning of the pre-configured system, international codes-of-practice of AAPM MPPG5 and TG-119 and external IROC audits, were completed by end-to-end (E2E) tests on anthropomorphic phantoms and validation of patient-specific QA procedures. End-to-end measurements were performed for head&neck VMAT, cranial VMAT, rectum VMAT and lung IMRT. Target structures and organs-at-risk for the different treatment indications were mapped on a set of CIRS anthropomorphic phantoms. Treatment plans were generated according to Halcyon-specific class solutions. Low dose MV cone-beam CT image guidance prior to treatment delivery was integrated in treatment planning and used for phantom setup. IMRT/VMAT dose distributions were measured using multiple EBT3 film in different orientations and was supplemented with point ionization chamber (IC) measurements. Patient-specific QA of the first patient group for each treatment indication was performed using portal dosimetry, an ArCHECK diode array measurement and EBT3 film dosimetry and an A1SL IC inserted in the MultiCube phantom. Film dosimetry was compared with calculation in a relative manner. All gamma agreement scores (γAS) were determined using 3%(local)/3mm criterion and a 10% lower dose exclusion threshold. The treatment time was measured as the total beam-on time of the IMRT/VMAT delivery. Results The acquired E2E dataset included eight sites with at least two different EBT3 film planes for each. Patient- specific QA was performed for 16 patients varied across the different indications, but mainly head&neck. A selection of representative E2E results for head&neck VMAT, lung IMRT and rectum VMAT are visualized in Figure 1. The local γAS between the predicted dose by the TPS and the film measurements ranged between 90.6% and 99.8%. The relative difference of the point measurements ranged between 0.3% and 2.3% dosimetric techniques. Material and Methods

Table 1 shows an overview of the patient-specific QA results for the first patients. In 90% of patient plans the local γAS was above 97,7%. The IC point measurements agreed with calculations within 3% except for a single lung IMRT case (+4.1%).

The delivery time of VMAT plans (2-3 arcs) was on average 1 min. 33 sec. while for IMRT (9 fields) it took about 2 min. 37 sec. on average. Conclusion The novel Halcyon™ system showed fast treatment delivery of VMAT and IMRT with good dosimetric agreement between the measured and calculated dose using the pre-configured beam model in the TPS for a wide range of treatment sites. PO-1009 Plan comparison of non-coplanar VMAT, HyperArc and multi-criteria optimization for radiosurgery. P. Houston 1 , S. Currie 1 1 Beatson West of Scotland Cancer Centre, Radiotherapy, GLASGOW, United Kingdom Purpose or Objective To evaluate the plan quality of 4 modern stereotactic radiosurgery (SRS) treatment methods. The methods investigated are 4-arc non-coplanar VMAT(4-Pi); with and without multi-criteria optimisation (Trade-Offs) and HyperArc; with and without Trade-Offs. Material and Methods Ten patients, single fraction, multiple cranial metastases cases were planned using the Eclipse Treatment Planning System v15.5 [Varian Medical Systems, Palo Alto, CA,