ESTRO 38 Abstract book

S535 ESTRO 38

shows the overlay of a 5 mm radial expansion of each fiducial contour based on the planning CT coordinates to compare with the current location of the fiducials. The treatment team pauses the beam (1-2 s) for assessment of the contour overlay, and determines whether to resume beam delivery if the fiducials are inside the overlay or re- image if outside it. Re-imaging starts with repeating the triggered image and assessment, and re-evaluating if a full CBCT localization is warranted. Retrospective analysis was performed on 129 fxs (30 pts) to quantify how frequently re-localization with CBCT was needed after commencing treatment, and how often the fiducials were not aligned in the initial BH attempt (false starts), but immediately repeating the triggered image with subsequent BH attempts (without shifting the patient) showed the fiducials within tolerance for treatment to proceed.

Results After initial localization, and treatment was either in progress or about to begin, 11.6% of fxs showed fiducials repeatedly outside of tolerance and required a new CBCT localization, requiring a mean vector (±SD) CBCT shift of 5.8 ± 2.7 mm. In 27.1% of fxs, a false start was recorded, but re-imaging with another triggered image allowed treatment to proceed without a repeat CBCT.

PO-0979 The Use of Triggered Imaging for Intrafraction Target Verification in Liver SBRT Breathhold M. Walb 1 , K. Jethwa 1 , S. Park 1 , C. Hallemeier 1 , D. Pafundi 1 1 Mayo Clinic, Radiation Oncology, Rochester, USA Purpose or Objective Target localization in liver SBRT is challenging due to organ motion and target visualization difficulties using volumetric CBCT. To mitigate these concerns, respiratory gating techniques, such as breathhold (BH) with implanted fiducials, are used. The Varian TrueBeam RPM system uses external markers as a surrogate for internal motion to measure respiration amplitude. This assumes that a patient BH during treatment correlates with internal motion observed at simulation. A larger PTV expansion may address this uncertainty but at the cost of less normal tissue sparing. To reduce uncertainty in target localization at treatment, we commissioned triggered imaging in the advanced imaging suite in TrueBeam 2.7. This allows for 2D kV images to be triggered at the beginning of each BH, before beam delivery, and verify that the internal fiducials are within an expanded contour overlay determined from simulation. A retrospective analysis was performed to determine the utility of triggered imaging for liver SBRT BH patients. Material and Methods Target localization is initially performed with a 2D-2D kV match followed by 3D or 6D CBCT match to the fiducials. Then, prior to each beam delivery period, a 2D kV image acquisition is triggered when the patient holds their breath within the gating window. The acquired image

Conclusion The results demonstrate a valuable role for intrafraction verification imaging for sites which rely on an external surrogate to gate the beam. Approximately 12% of patients required a second localization with a mean shift of 6 mm which can have a significant impact on a geographic miss. Plans include further analysis to guide liver margin expansions and assess the technology for additional SBRT sites, such as prostate.

Poster: Physics track: Adaptive radiotherapy and inter- fraction motion management

PO-0980 Dosimetric comparison of library of plans and online MRI-guided radiotherapy of cervical cancer J. Visser 1 , P. De Boer 1 , K.F. Crama 1 , Z. Van Kesteren 1 , C.R.N. Rasch 1 , L.J.A. Stalpers 1 , A. Bel 1 1 Amsterdam UMC- location AMC, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Online MRI-guided radiotherapy of cervical cancer has the potential to further reduce dose to organs at risk (OAR) as