Systematic patient-dose errors for 4- and 10-MeV microwave linear accelerators associated with rectangular collimator...

Systematic patient-dose errors relating to standard equivalent-square approximations and movable collimator-exchange effect were evaluated for rectangular fields of photon beams from 4- and 10-MeV microwave electron accelerators (Varian Clinac 4 and Clinac 18). Errors approached 2.5% at nominal treatment depths for highly elongated rectangular fields. The collimator-exchange effect should be considered when (a) evaluating ionization-chamber stem effect, (b) specifying relative output factors for rectangular fields, and (c) evaluating equivalent-square-field algorithms.