Use of megavoltage cine-images for studying intra-thoracic motion during radiotherapy for locally advanced lung cancer

Abstract Background and purpose Use of planning 4-dimensional CT (4DCT) scans often permits use of smaller target volumes for thoracic tumors but this assumes a reproducible pattern of motion during radiotherapy. We compared cranio-caudal (CC) motion on MV cine-images acquired during treatment with that seen on planning 4DCT. Methods and materials A pre-programmable respiratory motion phantom and a software tool for motion assessment were used to validate the use of MV cine-images for motion detection. MV cine-images acquired in 20 patients with node-positive lung cancer were analyzed using the same software. Intra-fraction CC motion on 6 MV cine-images from each patient was compared with CC motion on their planning 4DCT. Results Software-based motion measurement on MV cine-images from the phantom corresponded to actual motion. Mean CC motion of primary tumor, carina and hilus on 4DCT was 7.3 mm (range 2–13.8 mm), 6.8 mm (1.8–21.2) and 11.0 mm (4.2–15.1), respectively. Corresponding intra-fraction motion on MV cine was 4.1 mm (0.6–13.6 mm); 2.7 mm (0–10 mm) and 6.0 mm (1.8–14.4 mm), respectively. The tumor, hilus and carina could be tracked in 95%, 88% and 38% of the MV cine-images, respectively. Conclusions Intra-fraction motion can be reliably measured using MV-cine images from a phantom. Motion discrepancies identified on MV cine-images can identify patients in whom planning 4DCT scans are not representative.