The energy and angular distributions of electrons and contaminant photons from medical accelerator are the most important characteristics of a radiotherapy electron beam. A knowledge of clinical beams is essential for dosimetry, treatment planning, quality assurance, and design of an accelerator. In the radiation treatment of cancer the treatment plan can only be made correctly if characteristics of the beam are known. Beam quality has influence on the physical, chemical and radiobiological effects as well as on the response of various detectors. Due to the variations of an applicator's design the properties of scattered electrons are complicated[2][1] and these scattered electrons may have a large influence on the dose distributions in a patient. Experimentally it is difficult to obtain detailed information because of various limitations in the clinical environment and detectors[3]. In addition to this, it is virtually impossible using experimental methods to distinguish electrons which are scattered from the beam defining system.
The EGS4 Monte Carlo simulation code, BEAM, is a powerful and flexible tool to simulate realistic clinical radiation beams and to obtain a detailed knowledge of the characteristics of therapy beams from accelerators. A previous study[1] described the code and simulation in detail and demonstrated that BEAM can predict dose distributions very accurately. Here we will systematically analyze the simulated beams from a variety of electron accelerators including the NRCC research accelerator, the University of Wisconsin Varian Clinac 2100C, a Philips SL75-20, the Ottawa Regional Cancer Center Siemens KD2 and AECL Therac 20 and the Memorial Sloan-Kettering Cancer Center Scanditronix Racetrack MM50. These accelerators have very different designs and some of them produce scanned beams instead of scattered beams. The energy range of electron beams is from 5 to 50 MeV. Figures 1 to 8 are the representations of six of the accelerator's treatment head studied in this work as shown by EGS_windows[4]. The figures show the actual simulation of particles transported inside the treatment head.
For beams produced by various accelerators this report contains detailed information on the energy and angular distribution of: all electrons; scattered electrons from each beam defining component; and contaminant photons. Comparisons between calculated and measured dose distributions in water phantoms are presented along with calculated dose components contributed by contaminant photons and electrons scattered from beam defining components.
It must be emphasized that results apply to the specific accelerators studied and should not be taken as applying to all similar machines (e.g. the Univ. of Wisconsin Clinac 2100C had special foils to match beams with an earlier Clinac 1800).