ESTRO 38 Abstract book

S150 ESTRO 38

largely reduces required PTV margins not only by decreasing motion, but also by reducing deformations and avoiding error-prone marker localization in CBCT’s. Target delineation errors were not considered in this study.

tumour locations. We showed that MIDR based on planning data does not accurately resolve the delivered dose even in the case of regular motion. Our method may be used to validate MIDR for other motion models and treatment sites. OC-0297 Detailed PTV margin assessment for liver SBRT with CBCT-guidance or realtime monitoring and gating E. Worm 1 , R. Hansen 1 , M. Høyer 2 , J. Bertholet 3 , B. Weber 2 , A. Dolcet 4 , P.R. Poulsen 1 1 Aarhus University Hospital, Oncology, Aarhus C, Denmark; 2 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus C, Denmark; 3 Institute of Cancer Research, Division of Radiotherapy and Imaging, London, United Kingdom; 4 University Hospital Virgen de las Nieves, Radiophysics, Granada, Spain Purpose or Objective Implanted markers are often used to guide liver SBRT treatments, but position errors remain due to marker localization uncertainties, liver deformations, and intrafraction motion. Knowledge of the full error budget is lacking thus hindering realistic margin estimations. Here, we analyze in detail all error contributions to determine appropriate PTV margins in marker-based liver SBRT in two treatment scenarios: (A) CBCT guided free breathing or (B) respiratory gating with realtime motion monitoring. Material and Methods The van Herk formalism of systematic (Σ) and random (σ) errors was used to quantify the geometric errors from (1) CBCT match uncertainties (only Scenario A), (2) intrafraction motion , (3) interfraction deformations and marker migration, and (4) intrafraction deformations . Here, (1) was obtained by re-analyzing data of 29 liver SBRT patients where online manual CBCT marker match (Fig.1C) was compared with the marker positions accurately obtained from marker segmentation in individual CBCT projections [Bertholet , Acta Oncol,56(2017)]. (2) was reported in a recent study [Worm, IJROBP,101(2018)] of 15 patients receiving 3- fraction Calypso-guided liver SBRT (3 implanted electromagnetic markers, continuous monitoring, Fig.1A). Treatment delivery was respiratory gated around the end- exhale phase. Non-gated treatment was also simulated. (3+4) were based on individual marker motion in the Calypso-data (Fig.1A) using one marker as surrogate- tumor (Fig.1B) . (3) was derived from the mean difference between the marker- tumor vectors in the planning CT and the mean marker- tumor vectors during the first treatment field. Patients (n=1) with interfraction deformations >8mm were excluded since these receive re-planning in our daily clinical practice (based on marker-marker mismatch in CBCT). (4) was derived from the difference between the mean marker-tumor vectors during the first treatment field and the marker-tumor vectors during the rest of each treatment (Fig.1A+B). Results Table 1 shows the errors for scenarios A (CBCT guidance) and B (gating). For scenario A, errors were dominated by intrafraction motion and interfraction liver deformations. PTV margins of 4.6mm (LR), 9.6mm (CC), and 3.5mm (AP) were required for a 67% PTV dose prescription level (Nordic standard). Real-time monitoring and gating (scenario B) eliminated the need for the uncertain CBCT marker-match, reduced intrafraction motion, and slightly reduced the error of interfraction liver deformation. PTV margins of 2.9mm (LR), 3.9mm (CC) and 2.8mm (AP) were needed in (B). Conclusion Error analysis based on unprecedented detailed motion monitoring showed that intrafraction motion and interfraction deformations dominate the geometrical errors in liver SBRT. Gating based on realtime monitoring

OC-0298 MLC tracking for lung cancer SABR is clinically feasible: results of first-in-human clinical trial J. Booth 1 , V. Caillet 1 , A. Briggs 1 , N. Hardcastle 2 , D. Jayamanne 1 , K. Szymura 1 , O. Ricky 3 , T. Eade 1 , P. Keall 3 1 Royal North Shore Hospital, Northern Sydney Cancer Centre, St Leonards, Australia ; 2 PeterMacCallum Cancer Centre, Radiation Oncology, Melbourne, Australia ; 3 ACRF Image X Institute, University of Sydney, Sydney, Australia Purpose or Objective MLC tracking is an emerging technology to improve tumor targeting and reduce normal tissue irradiation during radiotherapy. The purpose of this work was to determine if MLC tracking for lung cancer SABR is clinically feasible, measure the target and normal tissue doses with comparison of these to SABR treatment. Material and Methods Seventeen patients with stage 1 lung cancer or lung metastases were recruited into the ethics-approved MLC