Clinical implementation of AAPM TG61 protocol for kilovoltage x-ray beam dosimetry

Historically, there have been a variety of dosimetry protocols used for kilovoltage x-ray therapy beams with a set of conversion factors and correction factors taken from different references. Corresponding to the continued installation and use of kilovoltage machines, the American Association of Physicists in Medicine (AAPM) presented a unified protocol developed by Task Group 61 (TG61). TG61 determines the absorbed dose to water with an ionization chamber calibrated in air in terms of air kerma (Nk). TG61 presents both an in-air method and an in-phantom method. In this work we only examine the TG61 in-air method. Our traditional dosimetry procedure, which is based upon NCRP Report 69 and on material found in standard medical physics texts, has been compared to the TG61. A variety of kilovoltage beam energies were examined with a set of various field sizes and source to surface distances. TG61 published updated data for the mass absorption coefficient ratios, backscatter factors, and the average energy per ion pair factor. The following conclusions have been reached: (1) Our traditional procedures and the TG61 protocol for in-air measurements are equivalent. (2) The conversion and correction factors used in TG61 are different by up to 4.5% compared to the old factors that we have used. (3) The application of the TG61 factors can result in up to 5% differences in the determination of the absorbed dose.