ESTRO 36 Abstract Book

S155 ESTRO 36 _______________________________________________________________________________________________

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 Nanotechnology- Center for Nanomedicine and Theranostics, Lyngby, Denmark 4 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark Purpose or Objective The purpose of this study was to estimate the intra- and inter-breath-hold tumour position uncertainty in voluntary deep-inspiration breath-hold (DIBH) radiotherapy for patients with locally advanced non-small cell lung cancer. Material and Methods Patients had liquid fiducial markers injected in mediastinal lymph nodes, and, if possible, in the primary tumours. Treatment was delivered during DIBH. Anterior and lateral fluoroscopic movies were acquired in free breathing (FB) and visually guided DIBH at three fractions (start, middle and end) during radiotherapy (33 fractions, 2 Gy per fraction) of nine patients with locally advanced non-small cell lung cancer. Fluoroscopies were acquired post treatment for two perpendicular gantry angles (Figure 1). Marker excursions in free breathing and DIBH, inter-breath-hold position uncertainty, systematic and random errors during DIBH in each of the three cardinal directions were investigated using an image based tracking algorithm, defining the marker template as one of the images from the middle of the first DIBH fluoroscopy. The mean marke r position during each DIBH, relative to a template frame for the first fluroscopy, was regarded as each fractions and markers uncertainty during the DIBH. A systematic error for the patient group was calculated as the standard deviation (SD) of all these mean marker positions. The standard deviation of the markers position within each DIBH was used to quantify the intra-breath- hold uncertainty (Figure 1). A root mean square (RMS) of the intra-DIBH SD was calculated to estimate random errors.

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 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.

Results A reduction of 2-6 mm in marker excursion in DIBH compared to FB was observed in the three cardinal directions (anterior-posterior (AP), left-right (LR) and cranio-caudal (CC)). Fourier transformation of the motion trajectories indicated that the lymph node motion during DIBH mainly originated from cardiac motion. The systematic errors during DIBH were 0.5 mm (AP), 0.5 mm (LR) and 0.8 mm (CC). The random errors during DIBH were 0.3 mm (AP), 0.3 mm (LR), and 0.4 mm (CC). The standard deviation of the inter-breath-hold shift was 0.8 mm (AP),

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