ESTRO 2020 Abstract Book

S882 ESTRO 2020

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.

Conclusion Single breath-hold kV-CBCTs using a spirometer in lung cancer patients appears feasible and results in good intra- fraction reproducibility and improved CBCT image quality. Such an approach might prove beneficial when considering breath-hold delivery in this patient population or when implementing new adaptive delineation and planning strategies. PO-1618 Markerless Real-Time 3D kV Tracking of Lung Tumors During Free Breathing Stereotactic Radiotherapy K. De Bruin 1 , M. Dahele 1 , H. Mostafavi 2 , B. Slotman 1 , W.F.A.R. Verbakel 1 1 Amsterdam University Medical Centers, Radiation Oncology, Amsterdam, The Netherlands ; 2 Varian Medical Systems, Imaging application, Palo Alto- California, USA Purpose or Objective Real-time tracking during stereotactic lung radiotherapy (SBRT) could confirm the target remains inside the planning target volume (PTV). A gantry-mounted kilo- voltage (kV) imaging system can continuously acquire 2D kV images of the target during volumetric modulated arc therapy (VMAT). Markerless tracking of lung tumors in 2D kV images is considered challenging due to over-projection of internal structures and low contrast with surrounding pixels. Using a life-like moving thorax phantom, we investigated 3D markerless tumor tracking on kV fluoroscopy acquired during free-breathing VMAT lung SBRT. The same method was applied to clinical data. Material and Methods The 3D-printed/molded phantom contains 3 lung tumors (each ~4cm 3 ) in different locations, with different densities. It was moved in 3D in TrueBeam developer mode, using a simulated irregular breathing pattern. Planar kV images were acquired at 7frames/s during 11Gy/fraction 10MV FFF VMAT. 2D reference templates of the tumors+4mm were created for each gantry angle using the inspiration phase of a planning 4DCT. The acquired kV images were matched to templates using normalized cross correlation for determining 2D position projection on the kV panel. Respiratory phase-based triangulation of the 2D matched projection was used to determine the 3 rd dimension of target position. 3D target tracking performed on CBCT projections raw data is presented from 5 patients undergoing free-breathing lung SBRT with a 5mm PTV margin. 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

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 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