TG-21 versus TG-25: A comparison for electrons

Since 1984, the Radiological Physics Center (RPC) has used the American Association of Physicists in Medicine Task Group 21 (TG-21) protocol (absorbed dose determination) as the basis of its On-site Dosimetry Review visits to institutions participating in the National Cancer Institute's cooperative clinical trials. Subsequent to the TG-21 protocol, the Task Group 25 (TG-25) report on electron-beam dosimetry was published. The TG-25 report was not intended to supersede the TG-21 protocol, but to supplement it for depths other than d(max). However, both reports included measurement techniques and data regarding the calibration of electron beams. TG-25 was not intended for absolute calibrations made clear by the fact that it does not present all of the data required for plastic phantom calibrations, i.e., unrestricted stopping power ratios. As a result, some confusion has arisen at various institutions as to which protocol should be used for machine calibration. In this study, possible discrepancies that arise when using TG- 21, a version of TG-21 modified by the RPC, and TG-25 are compared. The differences in the results are calculated as a function of energy (6 and 20 MeV), chamber type (cylindrical or parallel plate), and the type of phantom material (water, polystyrene, or acrylic). The largest discrepancies noted were between TG-25 and the two TG-21 methods for low-energy electrons in either water or polystyrene. The mean difference for all conditions was 0.8% with a maximum value of 3.3% in polystyrene. The definition of the effective point of measurement; determination of the mean nominal incident energy (E₀), mean energy at depth (E(p.o)), and most probable energy at the surface (E(p.o)) for each protocol, and subsequent stopping power ratio, chamber replacement factor, and electron fluence correction factor are the major contributors to the calculated differences.