| For megavoltage electron beams, current dosimetry protocols assume well-guarded plane-parallel ionization chambers have electron perturbation correction factors of unity. Papers by other researchers have shown the contrary. The National Physical Laboratory (NPL), Teddington UK, has an electron calibration service that requires knowledge of perturbation factors in water over graphite, pref,w/pref,g, for their NACP-02 chamber. Using the Monte Carlo code EGSnrcMP, perturbation correction factors at zref in graphite for the NPL's Radiation Dynamics linac 4--19 MeV were calculated. These results combined with those in water calculated by Zakikhani (2006) showed that only the p ref,w/pref,g of the 4 MeV and 12 MeV beams, at 1.0080 +/- 0.27% and 1.0071 +/- 0.28%, were significantly different from unity.Perturbation correction factors in water and graphite for the Varian clinical linacs 4--18 MeV were also investigated. Important issues that arose requiring further study included the discrepancy in results caused by 1 keV vs. 10 keV transport cutoffs and whether the depth of the effective point of measurement should account for the phantom equivalent thickness of the chamber window. |
