ESTRO 36 Abstract Book

S153 ESTRO 36 2017 _______________________________________________________________________________________________

irregularities. The limitation of CBCT for needing bony landmarks, surrogates, the need for large tissue density differences or the retrospective binning to assess motion data will be solved when using MRI. So MRI is at the very least a much better CBCT in the sense that it provides direct visualization of target and surrounding structures. CBCT guided proton therapy is lagging behind on the much needed image guidance offered by MRI and hybrid MRI radiotherapy systems will improve position verification. On-line MRI will also enable on-line re-planning strategies that are not, or only for some sites, feasible with CBCT as an input. This on-line re-planning fits seamlessly into the large research interest of the radiotherapy community to adapt the dose more to the actual anatomy and deliver more conformal dose distributions, currently being implemented via library of plans or off-line re-planning strategies. Moreover, integrated MRI allows imaging during radiation delivery. This way, assumptions on anatomical stability or motion as determined on pre-treatment data can be verified. Also, the intra-fraction volumetric imaging provides the input for dose reconstruction, so even if the pre-treatment assumptions are failing and the anatomy is moving/deforming unexpectedly, one can reconstruct exactly what the dose delivered is. This can be used for off-line re-optimization for remaining fractions. Additionally, as this dose reconstruction can be done in near real-time, one can also built adaptation triggers on it such as gating and ultimately intra-fraction re-planning strategies. The latter would be truly interventional radiosurgery where the dose distribution is continuously adapted to the mobile anatomy. Another advantage of integrated MRI radiotherapy systems is the capability to assess functional parameters such as perfusion or water diffusion, from the patient in treatment position. This can provide great insight in treatment response and temporal behavior during the course of radiotherapy. In summary , there is a clear desire from the image guided radiotherapy community to use more and better imaging prior and during radiation delivery. MRI guided photon therapy can fulfill this desire and will contribute to more precise radiation delivery and to a more hypo-fractionated approach. With that hybrid MRI radiotherapy systems will become the first choice for radiotherapy and CBCT guided proton therapy is mainly indicated in case the integral dose is treatment limiting, e.g. for pediatrics. SP-0299 Against the motion A. Lomax 1 Paul Scherrer Institute PSI, CPT, Villigen PSI, Switzerland

CBCT reconstruction. The kV images were continuously acquired at 7, 11, or 15 frames/s with a field size ranging from 10.5x9cm² to 26.6x20cm² (full field) during flattening filter free VMAT delivery. For reconstruction, a standard “spotlight” mode template was modified to suit our data, i.e. full 360° trajectory, full fan, no filters, and 100 kV. The FDK filtered back projection algorithm was used to reconstruct the CBCTs and the scans were matched to the planning CT in Offline Review (Varian Medical Systems, Palo Alto, CA). For validation purposes, the resulting match values were compared to the average spine offset values found using template matching + triangulation of the individual kV images. For lung SBRT, limited-arc CBCTs were reconstructed from fluoroscopic images acquired during irradiation of a lung lesion embedded in a 3D printed anthropomorphic thorax phantom and of one patient treated in breath-hold. In order to determine which arc length is required to obtain sufficient image quality for reliable CBCT-CT matching, multiple limited-arc CBCTs were reconstructed using arc lengths from 180° down to 20° in steps of 20°. Results 3D spine CBCT-CT registration revealed mean positional offsets of -0.1±0.8 mm (range: -1.5–2.2) for the lateral, - 0.1±0.4 mm (range: -1.3–0.7) for the longitudinal, and - 0.1±0.5 mm (range: -1.1–1.3 mm) for the vertical direction. Comparison of these match results to the average spine offsets found using template matching + triangulation showed mean differences of 0.1±0.1 mm for all directions (range: 0.0–0.5 mm). For limited-arc CBCTs of the lung phantom, the automatic CBCT-CT match results were ≤1mm in all directions for arc lengths of 60- 180°, but in order to perform 3D visual verification, an arc length of at least 80° was found to be desirable. 20° CBCT reconstruction still allowed for positional verification in 2 dimensions. The figure illustrates a limited-arc CBCT over 80° for a phantom and 100° for a patient.

Conclusion Using standard techniques, we have been able to obtain CBCT reconstructions of planar kV images acquired during VMAT irradiation. For treatments consisting of partial arcs, e.g. lung breath-hold treatments, limited-arc CBCTs can show the average tumor position during the actual treatment delivery. It is anticipated that this capability could be implemented clinically with few modifications to current treatment platforms. This could substantially improve positional verification during irradiation. OC-0301 Target position uncertainty during visually guided breathhold radiotherapy in locally advanced NSCLC J. Scherman Rydhög 1 , S. Riisgaard Mortensen 1 , M. Josipovic 1 , R. Irming Jølck 2 , T. Andresen 3 , P. Rugaard Poulsen 4 , G. Fredberg Persson 1 , P. Munck af Rosenschöld 1 1 Rigshospitalet, Department of Oncology- Section of Radiotherapy, Copenhagen, Denmark 2 DTU Nanotech and Nanovi Radiotherapy A/S, Department of Micro-and Nanotechnology- Center for Nanomedicine and Theranostics, Lyngby, Denmark 3 DTU Nanotech, Department of Micro-and

Abstract not received

Proffered Papers: Intra-fraction motion management

OC-0300 Proof of tumor position during SBRT delivery using (limited-arc) CBCT imaging C. Hazelaar 1 , M. Dahele 1 , B. Slotman 1 , W. Verbakel 1 1 VU University Medical Center, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective SBRT requires accurate patient positioning and robust positional verification during irradiation itself is desirable. We investigated if CBCT scans reconstructed from (collimated) fluoroscopic kV images acquired during irradiation, including over a limited arc length, can provide information on the average tumor position, for

spine and lung SBRT. Material and Methods

In total, 38 fluoroscopy datasets (1 dataset/arc) of 16 patients treated with spine SBRT were used for full-arc