Calculative technique to correct for the change in linear accelerator beam energy at off-axis points

The change in energy of linear accelerator x-ray beams from the central ray to off-axis points causes errors in the dose calculated by conventional techniques for large, irregularly shaped fields. A modification of conventional calculative methods to correct for the change in beam energy is presented. The results of measurements in irregular fields on a Clinac-4 are reported which verify the validity of the calculative method. A discussion of the clinical significance will point out errors of 3% to 4% in conventional dose calculations.