THE ROLE OF PHANTOM AND TREATMENT HEAD GENERATED BREMSSTRAHLUNG IN HIGH-ENERGY ELECTRON-BEAM DOSIMETRY

An analytical expression has been derived for the phantom generated br emsstrahlung photons in plane-parallel monoenergetic electron beams no rmally incident on material of any atomic number (Be, H2O, Al, Cu and U). The expression is suitable for the energy range from 1 to 50 MeV a nd it is solely based on known scattering power and radiative and coll ision stopping power data for the material at the incident electron en ergy. The depth dose distribution due to the bremsstrahlung generated by the electrons in the phantom is derived by convolving the bremsstra hlung energy fluence produced in the phantom with a simple analytical energy deposition kernel. The kernel accounts for both electrons and p hotons set in motion by the bremsstrahlung photons. The energy loss by the primary electrons, the build-up of the electron fluence and the g eneration, attenuation and absorption of bremsstrahlung photons are al l taken into account in the analytical formula. The longitudinal energ y deposition kernel is derived analytically and it is consistent with both the classical biexponential relation describing the photon depth dose distribution and the exponential attenuation of the primary photo ns. For comparison Monte Carlo calculated energy deposition distributi ons using ITS3 code were used. Good agreement was found between the re sults with the analytical expression and the Monte Carlo calculation. For tissue equivalent materials, the maximum total energy deposition d iffers by less than 0.2% from Monte Carlo calculated dose distribution s. The result can be used to estimate the depth dependence of phantom generated bremsstrahlung in different materials in therapeutic electro n beams and the bremsstrahlung production in different electron absorb ers such as scattering foils, transmission monitors and photon and ele ctron collimators. By subtracting the phantom generated bremsstrahlung from the total bremsstrahlung background the photon contamination gen erated in the treatment head can be determined to allow accurate dosim etry of heavily photon contaminated electron beams.