VALIDITY OF UNIVERSAL WEDGE EQUATION OVER THE RANGE OF CO-60 TO 25-MVPHOTON-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 a nd Materials: The universal wedge equation relates the beam weight of the wedged field to the effective wedge angle produced by combining th e 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 we dged field, theta(w) is the maximum wedge angle of the wedge filter, a nd 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 combine d in different proportions. The effective wedge angle was determined f rom each resultant isodose distribution. Results: The relationship bet ween the weight of wedged field and the effective wedge angle was foun d to be nonlinear for wedge filters with maximum wedge angle greater t han 30-degrees. The universal wedge equation predicts the effective we dge angle to within 2-degrees compared to the measured value.