Perturbation correction factors for cylindrical ionization chambers in high-energy electron beams.

The aim of the study was to evaluate the perturbation correction factors at a reference depth for cylindrical ionization chambers in high-energy electron beams by means of the EGSnrc Monte Carlo user code cavity. The cylindrical chambers used in this study were the Farmer-type of PTW30010, PTW30011, PTW30012, and PTW30013 models. We calculated the wall correction factor, P (wall), the cavity or electron fluence correction factor, P (cav), the stem correction factor, P (stem), the central electrode correction factor, P (cel), and the overall perturbation correction factor, P (Q), for each chamber. The calculated P (cav) values were higher by from 2 to 1% than those recommended by the IAEA-TRS-398 code of practice, in an energy range of 6-18 MeV. The P (wall) values almost agreed with the analytical calculation performed with IAEA-TRS-398. The P (cel) values agreed with those of Ma and Nahum, performed with IAEA-TRS-398. The P (stem) values were approximately 0.995 on average and were independent of the electron beam energy. P (stem) needs to be considered in future dosimetry protocols. The P (Q) values were higher from 1 to 2% than those of IAEA-TRS-398 in an energy range of 6-18 MeV.