Comparison of electron beam characteristics from multiple accelerators

Abstract

Purpose To determine the relationships between electron beam depth dose characteristics, depth of maximum dose (d max), depth of 80% dose (d 80), and depth of 50% dose (d 50), and the nominal energy designation of electron beams from multiple linear accelerators for the purpose of electron beam treatment planning and quality assurance. Methods The Radiological Physics Center Staff, during its on-site dosimetry review visits to institutions participating in clinical trials, measured depth dose characteristics for more than 2000 electron beams. Measurements were performed on Varian, Siemens, and Elekta/Philips accelerators generating beams with nominal energy values ranging from 4–22 MeV. The depth dose data were determined at the nominal source-to-skin distance with the reference cone size in accordance with recommendation of the American Association of Physicists in Medicine Task Group 25 report. Results The important depth dose characteristics d max, d 80, and d 50 varied in a predictable fashion when plotted against the true beam quality indicator, R 50. However, d 80 and d 50 values overlapped considerably when plotted against the manufacturers' nominal electron energy values. For a specific nominal electron energy value, the values of d max, d 80, and d 50 varied by as little as 3 mm for low energy levels to nearly as much as 20 mm for high energies. Conclusions The manufacturer's nominal electron energy value does not adequately describe the depth dose characteristics of an electron beam for treatment planning purposes. Clinicians and physicists should determine and use only the specific depth dose data for their clinical beams and not the manufacturer's nominal value.