SU-E-T-07: A Comparison of SSD and Field Size Dependence of Output for Megavoltage Electron Beams for Different Manufacturers




The purpose of this study was to compare the electron beam in-air output factor (H) for Varian and Siemens accelerators.


Measurements were performed among four accelerators: Varian CL2300ix, Varian TrueBeam, Siemens Oncor, and Siemens Primus. The nominal electron energy measured ranges from 6 to 21 MeV. Measurements were made in air using a diode detector for different square cone sizes (6 – 25cm) with circular field inserts with radius r between 1 cm and 8 cm and for different SSD (100 – 120 cm). H is defined asH(r,SSD)= X(r,SSD)/X(open,SSD), where X is the charge measured by the diode detector without buildup cap in air.


For the same nominal energy, cone size, radius of circular cutout, and SSD, H agreed within a maximum deviation of 2.3% and standard deviation of 1% among Varian accelerators but can be as much as 8.7% different between the Varian and the Siemens accelerators. H is a function of electron energy and cutout radius only when the field radius for H measured at different SSD is renormalized as r = r0*5/(SSD-95), where r0 is the physical field radius at the end of the cone. The radius dependence of H for five cone sizes (6×6, 10×10, 15×15, 20×20 and 25×25cm2) and three SSD(100, 110 and 120cm) for three energies was measured.


The in air output factor (H) measured for the same manufacturer can be treated as identical to within 1.1% for Varian machines and to within 1.6% for Siemens machines, but the difference in H measured for the different manufacturers is large enough (>2%) to require machine specific data set.