A comparison of the AAPM "Protocol for the determination of absorbed dose from high-energy photon and electron beams" with currently used protocols.

Abstract

A comparison of the AAPM "Protocol for the determination of absorbed dose from high-energy photon and electron beams" (TG21) with currently used protocols for electron and photon dosimetry is presented. These protocols are the International Commission on Radiation Units and Measurements Report 21, "Radiation Dosimetry: Electrons with Initial Energies Between 1 and 50 MeV" (ICRU21), and the AAPM "Protocol for the Dosimetry of X- and Gamma Ray Beams with Maximum Energies Between 0.6 and 50 MeV" (SCRAD). Assuming a given radiation exposure and chamber parameters, doses to water at dmax for electron beams and at 5 g/cm2 for photon beams are calculated using the three protocols and then compared. The doses for photon beams calculated using the TG21 and SCRAD protocols are found to differ by 3% or less at energies below 10 MeV. The largest differences occur in photon doses at high energies where the dose calculated with the TG21 protocol is as much as 5.5% greater than that calculated with the SCRAD protocol for a typical thimble ionization chamber. For low electron beam energies, the doses calculated with the ICRU21 protocol are as much as 5% less than TG21 doses when using thimble chambers constructed of tissue-equivalent materials in a water phantom. If dosimetry measurements are performed in polystyrene, the dose calculated using TG21 may be greater than the ICRU21 dose, depending on chamber size and composition. An explanation for some of the differences between the protocols is presented emphasizing the dependence on chamber geometry, chamber composition, and phantom composition.