AbstractID: 8183 Title: Correction of Setup Error in Six Degrees of Freedom by Portal Image Based
Modification of Treatment Geometry
Accurate setup is an important component of 3 dimensional conformal radiotherapy and intensity modulated radiotherapy. Portal
images and implanted gold marker seeds can be used to determine patient translation and rotation. The translational component can
be readily corrected using treatment couch motions; however, the correction of rotational errors is not easy to perform in a systematic
way. We have developed a method to correct both rotational and translational setup errors using adjustment of the planned gantry,
collimator, and table angles and of the couch position. Three gold marker seeds are placed in the patient and portal images, an AP and
two stereo views, are used to reconstruct the positions of the markers. To correct the rotational error, for each treatment port a nonlinear least square fitting technique is used to calculate the gantry, collimator, and table adjustment required to place the markers in the
correct treatment position in the beam’s-eye-view reference frame. The method was applied to an anthropomorphic head phantom.
Reference images of an AP and of a lateral field were obtained. The phantom was then skewed and the gantry, collimator, and table
angles required for correction of the AP and lateral beams were determined. Images were obtained using the corrected angles and
were compared with the reference images. The positions of the marker seeds and of bony landmarks agreed between the images to
better than 1 mm. This technique can be readily automated to provide fast, accurate correction of setup error in all 6 degrees of
freedom.