Setup Verification in Stereotactic Radiotherapy Using Digitally Reconstructed Radiograph (DRR)

Cho, Byung Chul; Oh, Do Hoon; Bae, Hoonsik

Abstract

PURPOSE
To develop a method for verifying a treatment setup in stereotactic radiotherapy by matching portal images to DRRs.
MATERIALS AND METHODS
Four pairs of orthogonal portal images of one patient immobilized by a thermoplastic mask 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), the displacement error between anatomical structure and fiducial markers (immobilization error), and the displacement error between fiducial markers and isocenters (localization error) were measured.
RESULTS
Localization errors were 1.5+/-0.3 mm (AP), 0.9+/-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(sqrt{(Delta AP)^2 +(Delta Lat)^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.
CONCLUSION
By comparing orthogonal portal images with DRRs, we find out that it is possible to verify treatment setup directly in stereotactic radiotherapy.