a Corresponding author: Samantha A.M. Lloyd, Department of Physics and Astronomy, University of Victoria, PO Box 1700, STN CSC, Victoria, BC, V8W 2Y2, Canada; phone: (250) 519 5500 ext 693731; fax: (250) 519 2024; email: slloyd@bccancer.bc.ca Comparison of measured Varian Clinac 21EX and TrueBeam accelerator electron feld characteristics Samantha A.M. Lloyd, 1a Sergei Zavgorodni, 1,2 Isabelle M. Gagne 1,2 Department of Physics and Astronomy, 1 University of Victoria, Victoria, BC, Canada; Department of Medical Physics, 2 British Columbia Cancer Agency – Vancouver Island Centre, Victoria, BC, Canada slloyd@bccancer.bc.ca Received 5 December, 2014; accepted 23 March, 2015 Dosimetric comparisons of radiation felds produced by Varian’s newest linear accelerator, the TrueBeam, with those produced by older Varian accelerators are of interest from both practical and research standpoints. While photon felds have been compared in the literature, similar comparisons of electron felds have not yet been reported. In this work, electron felds produced by the TrueBeam are compared with those produced by Varian’s Clinac 21EX accelerator. Diode measurements were taken of felds shaped with electron applicators and delivered at 100 cm SSD, as well as those shaped with photon MLCs without applicators and delivered at 70 cm SSD for feld sizes ranging from 5 × 5 to 25 × 25 cm 2 at energies between 6 and 20 MeV. Additionally, EBT2 and EBT3 radio-chromic flm measurements were taken of an MLC-shaped aperture with closed leaf pairs delivered at 70 cm SSD using 6 and 20 MeV electrons. The 6 MeV felds produced by the TrueBeam and Clinac 21EX were found to be almost indistinguishable. At higher energies, TrueBeam felds shaped by electron applicators were generally fatter and had less photon contamination compared to the Clinac 21EX. Differences in PDDs and profles fell within 3% and 3 mm for the majority of measurements. The most notable differences for open felds occurred in the profle shoulders for the largest applicator feld sizes. In these cases, the TrueBeam and Clinac 21EX data differed by as much as 8%. Our data indicate that an accurate electron beam model of the Clinac 21EX could be used as a starting point to simulate electron felds that are dosimetrically equivalent to those produced by the TrueBeam. Given that the Clinac 21EX shares head geometry with Varian’s iX, Trilogy, and Novalis TX accelerators, our fndings should also be applicable to these machines. PACS number: 87.56.bd Key words: TrueBeam, electron therapy, modulated electron radiation therapy I. INTRODUCTION Varian’s most recent generation of clinical linear accelerator, the TrueBeam (Varian Medical Systems, Palo Alto, CA), is a rebuild “from the ground up.” As a result of the reengineering process, there are differences in the design of the accelerator head compared to older Varian machines, and although the internal specifcations of the TrueBeam are proprietary, some of the changes are known qualitatively. In particular, the bending magnet, carrousel, and electron scattering foils have been redesigned, the primary collimator is thicker than in older units, and an antibackscatter foil has been added. (1-3) JOURNAL OF APPLIED CLINICAL MEDICAL PHYSICS, VOLUME 16, NUMBER 4, 2015 193 193