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

Abstract

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. 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. 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 7.3mm (range 2-13.8mm), 6.8mm (1.8-21.2) and 11.0mm (4.2-15.1), respectively. Corresponding intra-fraction motion on MV cine was 4.1mm (0.6-13.6mm); 2.7mm (0-10mm) and 6.0mm (1.8-14.4mm), respectively. The tumor, hilus and carina could be tracked in 95%, 88% and 38% of the MV cine-images, respectively. 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.