Standard electron cone ratio values for Varian, Siemens and Elekta (Philips) accelerators

The Radiological Physics Center (RPC), through its on-site dosimetry reviews at institutions participating in NCI cooperative clinical trials, has accumulated and measured electron cone ratio data for approximately 250 accelerators since 1987. For each make and model of accelerator, one can predict a cone ratio value within ±2% as a function of nominal incident energy (E_o), with the exception of the Clinac 1800. The Clinac 1800 data contain a mixture of cone designs and accelerator upgraded models that make them difficult to separate out. The Mevatron 67 series and KD data overlay each other for cone sizes greater than 10 × 10 cm² indicating very little difference between make and model of accelerator. The Elekta machines also indicate very little difference in cone ratios between different make/models of accelerator. The Elekta cone ratio values for sizes greater than 10 × 10 cm² decrease with energy, but increase with energy for the smaller cones. The Clinac 2100C and 2300C machines have similar cone ratio values for all cone sizes and energies, but the values for the Clinac 2500 are substantially different. The RPC measured data are in good agreement with the institution's data to within 2%. These cone ratio data can be used to predict the cone ratio for any electron energy for most modern accelerators as a redundant check when commissioning a new machine or performing annual QA checks.