ESTRO 2020 Abstract book

S934 ESTRO 2020

Results Target 1 of the phantom (upper lung, mean density - 130HU), 3D results available for 88% of kV projections. 3D RMS error of measured versus known position was 3.1mm. Target 2, (middle/medial lung, mean density -130HU) 3D results available for 90% of kV projections, 3D RMS error 1.6mm. Target 3, (inferior lung, mean density -478HU) 3D results available for 89% of kV projections, 3D RMS error 4.7mm. After rejecting outliers with a threshold of ±1mm of the maximum tracked amplitude, 3D results for target 1/3 were available for 83/76% of kV projections, and 3D RMS error 1.4/2.3mm. Phantom-derived software parameters were applied to 5 CBCT scans from 5 patients (tumor volume 1.4-18.3cm 3 , mean tumor HU -118-432). 3D tumor tracking results were available for, on average, 83±2.6% of projections. For patient 1-5, the estimated peak-peak motion (raw CBCT data/4DCT-scans) was 12/8mm, 2/3mm, 11/15mm, 13/12mm and 3/2mm respectively. Conclusion Phantom-based 3D markerless tumor tracking during free- breathing VMAT lung SBRT is feasible with template matching and phase-based triangulation of 2D kV images. The RMS error can be large, and is sensitive to (incorrectly located) outliers, but erroneous matches can often be visually recognized (Fig. 1+2). Improvements to the system and automatic rejection of matches with a high-probability of being incorrect are needed to make it possible to confidently track the majority of lung targets with high spatial-temporal resolution.

Research School, Tehran, Iran Islamic Republic of ; 3 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark ; 4 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark Purpose or Objective Radiotherapy of left-sided breast cancer may be performed with deep inspiration breath-hold (DIBH) gating to reduce the heart dose. Surface-guided radiotherapy (SGRT) can be used for the gating, but the accuracy is subject to inter- and intrafraction variations in the chest wall position relative to the patient surface. This study provides a detailed investigation of the SGRT error budget and investigates potential improvements by combining SBRT with image-guided radiotherapy (IGRT).

PO-1619 Detailed study of accuracy and potential improvements of surface-guided breast cancer radiotherapy S. Nankali 1,2 , R. Hansen 3 , E. Worm 3 , M. Skovhus Thomsen 3 , B. Offersen 1,3 , P. Rugaard Poulsen 1,4 1 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark ; 2 NSTRI, Radiation Application

Material and Methods Ten left-sided breast cancer patients received DIBH radiotherapy with tangential fields in 15-18 fractions. An