VALIDITY OF UNIVERSAL WEDGE EQUATION OVER THE RANGE OF CO-60 TO 25-MV PHOTON-BEAM ENERGIES

Purpose: Evaluating the validity of the universal wedge equation over the range of Cobalt-60 (60CO) to 25 MV photon beam energies. Methods and Materials: The universal wedge equation relates the beam weight of the wedged field to the effective wedge angle produced by combining the wedged field to the nonwedged field and is expressed as tan (theta) = B tan (theta(w)), where B is the normalized weight imposed on the wedged field, theta(w) is the maximum wedge angle of the wedge filter, and theta is the effective wedge angle. The isodose distributions from Co-60 to 25 MV photon beam energies were used. For each photon energy, the isodose distributions of wedged and nonwedged fields were combined in different proportions. The effective wedge angle was determined from each resultant isodose distribution. Results: The relationship between the weight of wedged field and the effective wedge angle was found to be nonlinear for wedge filters with maximum wedge angle greater than 30-degrees. The universal wedge equation predicts the effective wedge angle to within 2-degrees compared to the measured value.