Field size and depth dependence of wedge factor for internal wedge of dual energy linear accelerator.
OBJECTIVE
To investigate the field size and depth dependence of wedge factors for internal wedge in the isocentrical setup for 6 and 15 MV photon beams.


METHODS
Measurements were performed at d(max) 5, 10, and 15 cm depths for fields 5x5 cm(2) to 20x20 cm(2). The source-isocenter distance (SID) was 100 cm. Relative wedge factors (RWFs) were calculated to describe the dependence of the wedge factor on the field size and depth.


RESULTS
The dependence of wedge factors on field size and depth were less than 2% at 6 MV and 1% at 15 MV Xray beams for 15 degrees and 30 degrees wedges. When the field size increased from 5x5 to 20x20 cm(2), the field size dependence increased by 2.7% and 5.2% for 45 degrees and 60 degrees wedge, respectively, at 6 MV. The dependence on the field size was less than 2.5% for 45 degrees and 60 degrees wedges at 15 MV. The depth dependence of 60 degrees wedge was up to 6.5% for large field size at the depths at 6 MV. For 15 MV beam, RWFs showed only a small change with increasing depth.


CONCLUSIONS
The magnitude of error in using a single wedge factor was minimal for thin wedges. However, for 6 MV use of a single wedge factor produced errors up to 6% with 60 degrees wedge filter. Therefore, in the presence of 45 degrees and 60 degrees wedges in the beam, a field size and depth dependence wedge factor or a separate output factor (OF) curve for wedged beams should be used.
Kemikler
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Istanbul University, Oncology Institute, Department of Medical Radiophysics, Istanbul, Turkey.