ESTRO 2020 Abstract book

S915 ESTRO 2020

tumor motion gains in importance. Both beam gating and tumor tracking can mitigate the impact of intrafraction translation. However, their ability to manage rotation has not yet been thoroughly investigated. In this study, the dosimetric accuracy of beam gating, MLC tracking and couch tracking to mitigate the dosimetric impact of intrafraction prostate rotation was investigated by volumetric film measurements in a dynamic anthropomorphic pelvis phantom. Material and Methods Treatment plans for end-to-end tests were generated for the pelvis phantom (CIRS, VA) using anatomic structures of two men previously treated with prostate SBRT and MRI- directed focal tumor boosting. The prescribed dose was 35Gy to the prostate (CTV prostate ) with an iso-toxic focal boost (GTV boost ) up to 50Gy in 5 fractions. The CTV-to-PTV margin was 4mm. Plans were delivered on a TrueBeam linear accelerator (Varian, CA). Dose measurements were performed using a radiochromic filmstack (21 film planes with 0.17mm resolution and 2.5mm separation) inserted into the phantom. Three Calypso beacons were embedded in the filmstack for motion monitoring. The phantom applied internal pitch rotation (up to 25°) and coupled longitudinal/vertical translation. Three typical prostate motion tracks were applied for each patient case: high frequency anterior jumps (HF), a posterior drift with transient anterior jumps (Jump) and a rapid posterior drift (Drift). For each motion track, the dose was measured without compensation and with beam gating, MLC tracking and couch tracking. Gating used Auto Beam Hold to capture a 2D kV x-ray image every 3s and pause the beam of one of the beacons moved outside a 4mm gating boundary. A 6DoF couch correction was performed if the displacement was persistent in subsequent kV images. Tracking was performed using iTools Tracking, which adapted the MLC or couch to the Calypso-measured 3D translations. The dosimetric accuracy was quantified as the 3%(local)/2mm gamma agreement score (γAS) using a 10% low dose threshold and the TPS calculated dose as reference. The CTV prostate V7Gy, GTV boost V8Gy and urethra D0.1cc were used to estimate clinical impact. Results The mean γAS (Figure 1a) was 96.9% (static), 60.3% (no compensation), 72.4% (gating), 76.6%, (MLC tracking) and 76.5% (couch tracking). Gating and tracking improved the CTV prostate and GTV boost coverage and spared the urethra compared with no compensation (Figure 1). The three mitigation techniques performed similarly. Figure 2 visualizes sagittal and axial dose planes and DVH of Drift motion without compensation and with MLC tracking.

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 Purpose or Objective As modern prostate radiotherapy shifts toward targeted dose escalation, steeper dose gradients and fewer treatment fractions, mitigating the dosimetric impact of