Deformable image registration (DIR) is increasingly used in radiotherapy applications and provides the basis for a previously described model of patient-specific respiratory motion. We examine the accuracy of a DIR algorithm and motion model with respiration-correlated CT (RCCT) images of software phantom with known displacement fields; physical deformable abdominal phantom with implanted fiducials in liver; and small liver structures in patient images. The motion model is derived from a principal component analysis that relates volumetric deformations with the motion of the diaphragm or fiducials in the RCCT. Patient data analysis compares DIR with rigid registration as ground truth: the mean ± standard deviation in 3D discrepancy in liver structure centroid position is 2.0 ± 1.0 mm. DIR discrepancy in software phantom is 3.8 ± 2.0 mm in lung and 3.7 ± 1.8 mm in abdomen; discrepancies near chest wall are larger than indicated by image feature matching. Marker 3D discrepancy in physical phantom is 3.6 ± 2.8 mm. The results indicate that visible features in the images are important for guiding the DIR algorithm. Motion model accuracy is comparable to DIR, indicating that two principal components are sufficient to describe DIR-derived deformation in these data sets.

PURPOSE: To optimize the frame placement for patients receiving stereotactic radio surgery (SRS) using GammaKnife® Perfexion™, a simulation program was developed and its accuracy of collision clearance was compared with the planning system, Leksel Gamma Plan 8.0 (LGP). METHODS AND MATERIALS: A simulation program was designed in Matlab to do the following: import patient DICOM images with or without frame, automatically measure skull dimension from the images, simulate the frame, post, and pin placement, create isocenters, and check for collision for each isocenter. The program employs a graphical user interface that can be compiled to run on a personal computer. MR images from ten study patients were imported into the simulation program. Frame placement at the time of treatment was reproduced using the MR fiducial marker of the image in the simulation program. Post and pin length measured at the time of frame setup were used in the simulation. A total of 600 isocenters were selected for the comparison of clearance computed from the simulation program and from LGP. RESULTS: Average clearance at 600 isocenters tested was 6.7mm (standard deviation (SD) of 2.6 mm) from LGP and 6.9 mm (SD of 2.5 mm) from the simulation program. Mean differences between the computed clearance from the simulation program and LGP were −0.6, −0.3, 0.9 and 0.9 mm in anterior left (AL), anterior right (AR), posterior left (PL) and posterior right (PR) respectively. Standard deviations of the differences were 0.9, 1.0, 1.9, and 1.9 mm in the AL, AR, PL and PR directions respectively. CONCLUSION: A simulation program for virtual frame placement was developed to guide an optimal frame setup for patients receiving SRS using gamma knife Perfexion™. It is a reliable tool that can guide optimal frame setup to reduce the chance of collision.