Monte!Carlo calculations of beam quality correction factors kQ for electron dosimetry with a parallel-plate Roos chamber

Current dosimetry protocols ( AAPM, IAEA, DIN) recommend the use of parallel-plate ionization chambers for the measurement of absorbed dose-to-water in clinical electron beams. For well-guarded plane-parallel chambers, it is assumed that the perturbation correction p(Q) is unity for all electron energies. In this study, we present detailed Monte Carlo simulations with the EGSnrc code for the widely used Roos parallel-plate chamber which is, besides other plane-parallel chamber types, recommended in all protocols. We have calculated the perturbation corrections p(cav) and p(wall) for a wide range of electron energies and for (60)Co. While our results confirm the recommended value of unity for the cavity perturbation pcav, the wall-correction factor pwall depends on electron energy and decreases with increasing electron energy. For the lowest electron energies in this study ( R(50) approximate to 2 cm), pwall deviates from unity by up to 1.5%. Using the perturbation factors for the different electron energies and those for the reference beam quality, (60)Co, we have calculated the beam quality correction factor k(Q). For electron energies E(0) > 9 MeV ( R(50) > 4 cm), the calculated values are in good agreement with the data published in the IAEA protocol. Deviations in the range of 0.5-0.8% are found for R(50) < 3 cm.