Abstract Book

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ESTRO 37

update the ECM for baseline drift between internal and external motion. No ECM updates occur during non- coplanar fields. The COSMIK method was validated in experiments and simulations using Calypso-recorded internal tumor motion with simultaneous camera- recorded external motion for 15 liver SBRT patients. The real-time estimated 3D positions were compared to the known tumor motion.

Conclusion We did not find any significant difference in OS between RP and EBRT-BT in the treatment of patients with high- risk prostate cancer. However, there was a survival advantage with the use of either of these modalities compared to treatment with EBRT alone. This suggests equivalence between RP and EBRT with brachytherapy boost, and inferiority of EBRT alone, in the treatment of high-risk prostate cancer. Ultimately, prospective trials will be needed for validation.

Award Lecture: Jens Overgaard Legacy Award

SP-0630 This is not a Target Volume… V. Gregoire UCL, Brussels Belgium

Abstract not received

Award Lecture: Academic award: Jack Fowler University of Wisconsin Award

OC-0631 First clinical demonstration of online real- time liver tumor motion monitoring on a standard linac J. Bertholet 1,2 , J. Toftegaard 1 , R. Hansen 1 , E. Worm 1 , H. Wan 3 , P. Parikh 3 , B. Weber 1,4 , M. Høyer 1,4 , P. Poulsen 1 1 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark 2 The Institute of Cancer Research and the Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom 3 Washington University School of Medicine, Department of Radiation Oncology, St-Louis, USA 4 Danish Center for Particle Therapy, Aarhus, Denmark Purpose or Objective Real-time treatment adaptation (gating, tracking) may mitigate the detrimental effects of motion during liver radiotherapy delivery, but it requires reliable target motion monitoring. We develop, validate and clinically demonstrate automatic tumor motion monitoring on a conventional linac by Combined Optical and Sparse Monoscopic Imaging with Kilovoltage x-rays (COSMIK). Material and Methods COSMIK combines auto-segmentation of implanted fiducial markers in CBCT projections and intra-treatment kV images with external motion monitoring. Fig. A illustrates the workflow: A pre-treatment CBCT is acquired with simultaneous recording of the motion EXT(t) of an external block on the abdomen. The internal 3D marker motion during the CBCT is estimated from the auto-segmented marker positions in the CBCT projections and used to optimize an External Correlation Model (ECM) of internal motion INT(t) as a function of external motion EXT(t). During treatment, the ECM estimates INT(t) from EXT(t) at 20Hz. Triggered kV images are acquired every 3s during coplanar fields, auto-segmented, and used to

For comparison, simulations were also performed with ECM-based monitoring with stereoscopic imaging updates and with continuous Kilovoltage Intrafraction Monitoring (KIM) at 10Hz without an external signal. The clinical translation of COSMIK included real-time tumor motion monitoring for 14 liver SBRT patients performed offline for 41 fractions and online for 2 fractions. The accuracy was assessed using the 3D marker positions estimated retrospectively from the intra- treatment kV images as ground truth. Results Fig. B shows an example of simulated COSMIK monitoring for a liver SBRT fraction. The mean 3D root-mean-square error of COSMIK in all simulations was 1.61mm (Table). Comparison with KIM (0.75mm) and ECM with stereoscopic imaging (1.49mm) shows that the accuracy was limited by the use of an ECM and not monoscopic imaging with a single kV imager. Clinical real-time monitoring with COSMIK was successful for all 43 fractions with similar accuracy in patient use as in the simulations (Table).