Setup verification in stereotactic radiotherapy using digitally reconstructed radiograph (DRR)

To develop a method for verifying a treatment setup in stereotactic radiotherapy by matching portal images to DRRs. Four pairs of orthogonal portal images of one patient immobilized by a thermoplastic mast frame for fractionated stereotactic radiotherapy were compared with DRRs. Portal images are obtained in AP (anterior/posterior) and lateral directions with a target localizer box containing fiducial markers attached to a stereotactic frame. DRRs superimposed over a planned isocenter and fiducial markers are printed out on transparent films. And then, they were overlaid over orthogonal portal images by matching anatomical structures. From three different kind of objects (isocenter, fiducial markers, anatomical structure) on DRRs and portal images, the displacement error between anatomical structure and isocenters (overall setup error), and the displacement error between fiducial markers and isocenters (localization error)were measured. Localization errors were 1.5±0.3 mm (lateral), and immobilization errors were 1.9±0.5 mm (AP), 1.9±0.4 mm (lateral). In addition, overall setup errors were 1.6±0.9 mm (AP), 1.3±0.4 mm(lateral). From these orthogonal displacement errors, maximum 3D displacement errors(√(ΔAP)2+(ΔLat)2) were found to be 1.7±0.4 mm for localization, 2.6±0.6 mm for immobilization, and 2.3±0.7 mm for overall treatment setup. By comparing orthogonal portal images with DRRs, we find out that it is possible to verify treatment setup directly in stereotactic radiotherapy