Image movement of the Elekta EPID during gantry rotation: Effects on the verification of dose distributions

The use of Electronic Portal Imaging Devices (EPIDs) to acquire dosimetric information, especially for 3D-back-projection, has been increasingly extended. For a precise back-projection, the accurate knowledge of the movement characteristics of the EPID during gantry rotation is an essential requirement.

Measurements were conducted with different alignments of steel balls, which were mounted on the treatment table to avoid secondary effects such as the mechanical sag of gantry or jaws. The image movement of the EPID was determined by comparing the predicted projections of the phantoms with the EPID acquired image. Effects on dosimetric verifications were evaluated by γ-evaluation.

The measurement results showed that the shift of the EPID image is larger in Y direction than in X direction. A maximum rotation of 0.3° and nodding of 2.4° of the detector was calculated. Changes in SDD were found up to 10mm. The angles of nodding are overall higher at discrete gantry angles in comparison to images detected for continuous rotation. Using these results we were able to correct the EPID images used for verification measurements. γ-evaluation revealed a significantly improved agreement between planned and measured EPID signal values.

The measurement methods and algorithms introduced in this study are simple and comprehensive. Using these methods and algorithms we were able to quantify the major effects on geometrical and dosimetric characteristics. This allows the correction of EPID signal measurements for these effects related to the gantry angle, leading to an improved γ-evaluation for treatment plans.