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


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


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