Relationship of wedge isodose angle to percent depth-dose curves

An expression that relates the wedge isodose angle to the gradient of the percent depth-dose curves for photon beams has been formulated and verified experimentally.

The expression that relates a wedge angle to another via the gradient of their corresponding percent depth-dose curves was derived. Because the field size affects the percent depth-dose curves, measurement of wedged isodose 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 depth of 10 cm. Measured and calculated wedge angles were compared.

The variation of the wedge angle caused by the change in field size is about 6 degrees. For field sizes larger than 10 x 10 cm, the variation of wedge angle is less than 3 degrees. The calculated wedge angles agree with the measured wedge angles to within 1 degree. The knowledge of the extent of wedge angle variation due to the change in field size and the availability of an expression to determine an accurate wedge angle would be helpful in the use of the universal wedge equation.