Evaluation of the photon monitor backscatter in medical electron accelerators

Background: Modern linear accelerators permit the use of irregular fields d ue to their flexible collimator systems with separately movable jaws or mul tileaf collimators. When using such irregular fields in the clinical practi ce output factors have to be corrected for enhanced backscatter to the dose monitor as compared with the conventional block shieldings. Methods: A method is presented to detect the monitor backscatter contributi ons to the output factor for irregular field settings. Results: The monitor backscatter factors have been measured using a telesco pic device for 2 different treatment head geometries (Varian Clinac 2100C/D , General Electric Saturne 15) and for 3 photon radiation qualities (nomina l energies X6, X18, X12). A method is introduced to calculate the monitor b ackscatter for arbitrary irregular treatment fields from the experimental d ata for square or rectangular fields. Conclusions: Besides the corrections for changes in phantom scatter and cha nges in the aperture, corrections for monitor backscatter have to be taken into account in many clinical cases. They can contribute up to more than 10 % compared with the monitor values for free regular fields.