Initial clinical observations of intra- and interfractional motion variation in MR-guided lung SBRT

Abstract

Objective:To evaluate variations in intra- and inter-fractional tumor motion, and the effect on ITV contour accuracy, using deformable image registration of real-time 2D-sagital cine-mode MRI acquired during lung SBRT treatments.Methods:Five lung tumor patients underwent free breathing SBRT treatments on the ViewRay system, with dose prescribed to a planning target volume (PTV; defined as a 3-6 mm expansion of the 4DCT-ITV). Sagittal slice cine-MR images (3.5x3.5mm2 pixels) were acquired through the center of the tumor at 4 frames per second throughout the treatments (3-4 fractions of 21-32 minutes). Tumor GTVs were contoured on the first frame of the MR cine and tracked for the first 20 minutes of each treatment using off-line optical-flow based deformable registration implemented on a GPU cluster. A ground truth ITV (MR-ITV20mins) was formed by taking the union of tracked GTV contours. Pseudo-ITVs were generated from unions of the GTV contours tracked over 10 second segments of image data (MR-ITV10s).Re...