P2.05-039 Assessment of Lung Tumor Motion Comparing 4DCT, 4DCBCT and Motion of Implanted Beacons during Imaging and Irradiation: Topic: RT Techniques

Abstract

Moving lung tumors exceeding the observed motion from planning 4D computed tomography (4DCT) can result in reduced dose coverage in stereotactic ablative body radiation therapy (SABR). 4D cone-beam CT (4DCBCT) facilitates verification of tumor trajectories before each treatment fraction. Using implanted Calypso beacons in the lung as ground truth, this work aims to assess how well 4DCT and 4DCBCT represent the actual motion range during imaging and irradiation. 4DCBCT was reconstructed for 1-2 fractions of 6 patients (three implanted Calypso beacons) receiving lung SABR from the projections acquired for treatment setup CBCT. Two reconstructions per projection set were created using the prior image constrained compressed sensing (PICCS) method based on the Calypso motion trajectories or an external respiratory signal (Philips Bellows). Calypso beacons were segmented for all 10 bins of the 4DCT and 4DCBCT sets and the centroid position calculated. Beacon centroid motion as seen on the 4DCT and 4DCBCT with respect to reference phase (end-exhale) was extracted and compared with the actual beacon centroid motion during CBCT acquisition and during irradiation. Both methods for 4DCBCT reconstruction failed to capture sudden motion peaks during scanning (see Fig. 1), but performed similar to the 4DCT. In general, 4DCT and 4DCBCT underestimated the actual beacon centroid motion. In the SI direction 22-27% of the actual motion exceeded the motion range from 4DCT and 4DCBCT imaging. In AP and LR direction up to 39-58% of the motion exceeded the observed motion range from 4D imaging. Both 4DCT and 4DCBCT failed to represent the full tumor motion range. For a safe treatment delivery this needs to be accounted for either by sufficient margins or more preferably real-time treatment adaptation directly tackling motion peaks and unpredictable motion.