A general method to perform dose calculations along the axis of symmetrical and asymmetrical photon beams

Many manufacturers of modern linacs provide collimators with jaws that can be set asymmetrically. In the present work, the authors propose the use of a product of four independent output factors related to the four jaws to calculate the reference dose rate of asymmetrical fields. Each of these factors is normalized to the maximum opening of the collimator. A general algorithm for symmetrical and asymmetrical fields is proposed for dose calculation. Calculations have been checked by measurements at the reference depth in a 4-MV x-ray beam.