Abstract
PURPOSE: We present a rapid computational method for quantifying interfraction motion of the esophagus in patients undergoing stereotactic body radiation therapy on a magnetic resonance (MR) guided radiation therapy system. METHODS AND MATERIALS: Patients who underwent stereotactic body radiation therapy had simulation computed tomography (CT) and on-treatment MR scans performed. The esophagus was contoured on each scan. CT contours were transferred to MR volumes via rigid registration. Digital Imaging and Communications in Medicine files containing contour points were exported to MATLAB. In-plane CT and MR contour points were spline interpolated, yielding boundaries with centroid positions, C(CT) and C(MR). MR contour points lying outside of the CT contour were extracted. For each such point, B(MR)(j), a segment from C(CT) intersecting B(MR)(j), was produced; its intersection with the CT contour, B(CT)(i), was calculated. The length of the segment S(ij), between B(CT)(i) and B(MR)(j), was found. The orientation θ was calculated from S(ij) vector components:θ = arctan[(S(ij))(y) / (S(ij))(x)]A set of segments {S(ij)} was produced for each slice and binned by quadrant with 0° < θ ≤ 90°, 90° < θ ≤ 180°, 180° < θ ≤ 270°, and 270° < θ ≤ 360° for the left anterior, right anterior, right posterior, and left posterior quadrants, respectively. Slices were binned into upper, middle, and lower esophageal (LE) segments. RESULTS: Seven patients, each having 3 MR scans, were evaluated, yielding 1629 axial slices and 84,716 measurements. The LE segment exhibited the greatest magnitude of motion. The mean LE measurements in the left anterior, left posterior, right anterior, and right posterior were 5.2 ± 0.07 mm, 6.0 ± 0.09 mm, 4.8 ± 0.08 mm, and 5.1 ± 0.08 mm, respectively. There was considerable interpatient variability. CONCLUSIONS: The LE segment exhibited the greatest magnitude of mobility compared with the middle and upper esophageal segments. A novel computational method enables personalized, nonuniform esophageal margins to be tailored to individual patients.