RELATIONSHIP OF WEDGE ISODOSE ANGLE TO PERCENT DEPTH-DOSE CURVES

Purpose: An expression that relates the wedge isodose angle to the gra dient of the percent depth-dose curves for photon beams has been formu lated and verified experimentally. Methods and Materials: The expressi on that relates a wedge angle to another via the gradient of their cor responding percent depth-dose curves was derived. Because the field si ze affects the percent depth-dose curves, measurement of wedged isodos e distributions were carried out for different field sizes to validate the expression. The 6 MV and 23 MV photon beams and a range of field sizes from 5 x 5 cm to 20 x 20 cm were used. For each field size, the wedge angle and percent depth-dose curve slope were measured at the de pth of 10 cm. Measured and calculated wedge angles were compared. Resu lts: The variation of the wedge angle caused by the change in field si ze is about 6 degrees. For field sizes larger than 10 x 10 cm, the var iation of wedge angle is less than 3 degrees. The calculated wedge ang les agree with the measured wedge angles to within 1 degree. The knowl edge of the extent of wedge angle variation due to the change in field size and the availability of an expression to determine an accurate w edge angle would be helpful in the use of the universal wedge equation .