Clinical Application of Wedge Factor

Choi, Dong Rak; Ahn, Yong Chan; Huh, Seung Jae

Abstract

PURPOSE: In general. The wedge factors which are used clinical practives are ignored of dependency on field sizes and depths. In this present, we investigated systematically the depth and field size dependency to determine the absorbed dose more accurately.
METHODS
The wedge factors for each wedge filter were measured at various depth (depth of Dmax, 5cm, 10cm, and 15cm) and field sizes (5 X 5cm, 10 X 10cm, 15 X 15cm, 20 X 20 cm) by using 4-,6-, and 10-MV X rays. By convention, wedge factors are determined by taking the ratio of the central axis ionization readings when the wedge filter is in place to those of the open field in same field size and measurement depth. In this present work, we determined the wedge factors for 4-, 6-, and 10-MV X rays from Clinac 600C and 2100C linear accelerators (manufactured by Varian Associates, Inc., Palo Alto, CA). To confirm that the wedge was centered., measurements were done with the two possible wedge position and various collimator orientations.
RESULTS
The standard deviations of measured values are within 0.3% and the depth dependence of wedge factor is greater for the lower energies. Especially, the variation of wedge factor is no less than 5% for 4- and 6- MV X rays with more than 45degree wedge filter. But there seems to be a small dependence on field size.
CONCLUSION
The results of this study show a dependence on the point of measurement. There also seems to be a small dependence on field size. And so, we should consider the depth and field size dependence in determining the wedge factors. If one wedge factor were to be used for each wedge filter, it seems that the measurement for a 10cm X 10cm field size at a depth of 10cm would be a reasonable choice.