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


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Universitätsklinikum Frankfurt, Klinik für Strahlentherapie und Onkologie, Theodor-Stern-Kai 7, Frankfurt am Main, Germany. Electronic address: janett.koehn@kgu.de.
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J
Köhn
Universitätsklinikum Frankfurt, Klinik für Strahlentherapie und Onkologie, Theodor-Stern-Kai 7, Frankfurt am Main, Germany.
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J
Licher
Universitätsklinikum Frankfurt, Klinik für Strahlentherapie und Onkologie, Theodor-Stern-Kai 7, Frankfurt am Main, Germany.
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M
Mielke
Universitätsklinikum Frankfurt, Klinik für Strahlentherapie und Onkologie, Theodor-Stern-Kai 7, Frankfurt am Main, Germany.
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B
Loutfi-Krauss
Universitätsklinikum Frankfurt, Klinik für Strahlentherapie und Onkologie, Theodor-Stern-Kai 7, Frankfurt am Main, Germany.
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N
Blümer
Universitätsklinikum Frankfurt, Klinik für Strahlentherapie und Onkologie, Theodor-Stern-Kai 7, Frankfurt am Main, Germany.
B
B
Heine
Universitätsklinikum Frankfurt, Klinik für Strahlentherapie und Onkologie, Theodor-Stern-Kai 7, Frankfurt am Main, Germany.
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C
Rödel
Universitätsklinikum Frankfurt, Klinik für Strahlentherapie und Onkologie, Theodor-Stern-Kai 7, Frankfurt am Main, Germany.
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C
Scherf
Universitätsklinikum Frankfurt, Klinik für Strahlentherapie und Onkologie, Theodor-Stern-Kai 7, Frankfurt am Main, Germany.
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U
Ramm
D055615:Electrical Equipment and Supplies; D019047:Phantoms, Imaging; D011874:Radiometry / Q000295:instrumentation*; D012399:Rotation
Electronic Portal Imaging Device (EPID); Image movement; Verification