ESTRO 2020 Abstract Book

S864 ESTRO 2020

patients spent on the couch was 8 min (range 6 min – 36 min).

PO-1590 Spirometer guided breathhold breast VMAT on Halcyon verified with portal images and surface tracking L. Delombaerde 1,2 , S. Petillion 1 , C. Weltens 1,2 , T. Depuydt 1,2 1 University Hospital Gasthuisberg, Department of Radiation Oncology, Leuven, Belgium ; 2 KU Leuven, Department of Oncology, Leuven, Belgium Purpose or Objective The fast imaging and radiation delivery capabilities of the Halcyon linac (Varian Medical Systems) are perfectly suited for breath-hold gated treatments. In this study we treated breast cancer patients, ages 70 and above, in deep inspiration breath-hold (DIBH) with the SDX spirometer system (Dyn’R). The stability and reproducibility of the spirometer-assisted breath-holds were assessed via portal images acquired at ‘tangential’ angles and by intra-bore surface monitoring using an in-house developed structured-light-based prototype. Material and Methods Four (preliminary data) left-sided breast cancer patients were treated using a triple-arc VMAT-SIB protocol in breath-hold with the SDX system (Dyn’R) for 21 fractions. Portal image frames were acquired at 15 Hz during DIBH VMAT. Three portal images are available per fraction (one per arc at 301° gantry angle) for verification of the anatomy during breath-hold. Concurrently the in-house developed surface scanning system monitored the movement of the patients’ chest surface in 6 degrees-of- freedom (DoF) during the entire treatment fraction, from completion of the setup until the end of radiation delivery. Offline, the portal image taken during the first arc was manually registered to the ribs and breast contour, delineated on the DRR generated from the planning CT. The superior-inferior (SI) and pseudo anterior-posterior (“AP”) systematic (Σ) and random (σ) errors were determined. The vertical chest displacement during breath-holds was extracted from the surface data. The intra fraction variability was calculated by subtracting the minimal from the maximal breath-hold per fraction. The inter fraction variability was calculated by subtracting the mean breath-hold per fraction from the mean breath-hold of the first fraction. The total number of breath-holds per fraction and the total time spent on couch was recorded. Results Portal image analysis (81 images) showed systematic and random errors for the SI direction of Σ = 0.9 mm and σ = 1.0 mm and for the “AP” direction of Σ = 0.7 mm and σ = 1.2 mm. The mean (± standard deviation) intra fraction vertical breath-hold variation was 1.4 (± 1.0) mm and the inter fraction variation was 1.0 (± 0.9) mm (56 complete fractions monitored). Median number of breath-holds per fraction was 4 (range 4 – 17) viz. one breath-hold for the CBCT and one breath-hold per arc. Median total time

Conclusion Using portal imaging and an in-house developed surface scanning system, we analyzed the stability of spirometer guided breath-holds during VMAT breast treatments. Both intra and inter fraction variation during spirometer guided breath-holds was found to be limited. Even in an older patient population, fast breath-hold treatments can be performed using the SDX system. In conclusion, breath- hold breast treatments with the SDX spirometer are feasible within the closed-bore gantry of Halcyon with good reproducibility. PO-1591 Dosimetric accuracy of beam gating, MLC tracking and couch tracking to manage prostate rotation R. De Roover 1,2 , R. Hansen 3 , W. Crijns 1,2 , C.G. Muurholm 4 , K. Poels 1,2 , S. Skouboe 4 , K. Haustermans 1,2 , P.R. Poulsen 4,5 , T. Depuydt 1,2 1 KU Leuven, Department of Oncology, Leuven, Belgium ; 2 University Hospitals Leuven, Department of Radiation Oncology, Leuven, Belgium ; 3 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark ; 4 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark ; 5 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark