TMR values in the build-up region for a 6MV linear accelerator

Abstract

Absorbed dose in the build-up region of high energy X- or gamma -ray beams varies with angle of incidence and scattering conditions; this region is not adequately treated by most presently used digital methods of dose computation. Contributions of dose from build-up regions are most important when dose distributions from beams of grazing angular incident and/or from a combination of fields are computed. In an effort to improve accuracy of clinical dosimetry the authors have developed a method of dose and TMR determination in the build-up region for any field size and angle and have applied it to the Varian 6 MV Linac. Dose distributions in an Alderson, rectangular or other appropriate phantom can be obtained by equidensitometry of individually calibrated radiographic film exposed in the phantom, using an electro-optical method of image density quantization. The inherently high spatial resolution of this method is verified by simultaneous exposure of Harshaw TLD rods with the film at selected points in the phantom. Dose distributions computed using these TMR values were compared with the experimental results.