SU-E-T-746: Electron Beam Dosimetric Characteristics for the Varian TrueBeam

Abstract

Purpose: The changes in the head design of the new Varian TrueBeam compared to the C‐series linear accelerators, required modification of the electron scattering foils and their placement in the beam line. As a result, the electron beam characteristics cannot be matched to existing Varian accelerator models (C‐series). Extensive measurements were needed for determining the beam characteristics for comparison and for commissioning purposes. We present a detailed analysis of the various dosimetric parameters measured for Truebeam compared to C‐series linacs. Methods: A Wellhoffer scanning system was used to measure PDDs and profiles at various depths and source to surface distances (SSD) for all applicators and all energies. Output measurements were performed for all electron applicators and different blocks with field sizes ranging from 2×2 to 15×15 cm depending on the applicator, at standard and extended SSDs, using a 0.03 cc ionization chamber (IBA Dosimetry CC04 chamber) in water, at the dmax depth for each energy. In‐air measurements of cross‐plane profiles were performed using a scanning system, to determine the effective SSD and electron dispersion (sigma‐theta‐x) which describes the angular scattering of electrons. Results: No significant changes were observed in the PDDs between the two accelerator systems. The effective SSD increased as a function of energy by 0.6 to 2.5 cm for TrueBeam (25×25 cm applicator), while the electron dispersion factor showed no change. The change in output factors was most significant for large applicators (20×20 and 25×25 cm), and low energies ranging from 1.5 to 6%. This difference increased systematically with increasing SSD. Conclusions: The changes in the electron beam parameters of the Varian TrueBeam compared to the C‐series linacs are significant enough to require unique modeling of electrons in our treatment planningsystem, as well as unique output factors for monitor unit calculations.