Differences in electron beam dosimetry using two commercial ionization chambers and the TG‐21 protocol: Another reason to switch to TG‐51

Two of the most popular dosimetry systems used for calibration of megavoltage photon and electron beams in radiation therapy are (i) cylindrical Farmer-type chambers in liquid water and (ii) Holt Memorial parallel-plate chambers in clear polystyrene. Since implementation of the AAPM TG-21 calibration protocol, the Radiological Physics Center (which uses the Farmer in-water system) has compared machine calibrations on two occasions with those of Memorial Sloan-Kettering Cancer Center (which uses the Holt in-polystyrene system). Two years post publication of the TG-51 protocol, 70% of the clinics monitored by the RPC still use TG-21. Seventeen photon beams from cobalt-60 to 18 MV and 31 electron beams from 6 to 20 MeV were compared using the TG-21 protocol. These data represent the most comprehensive comparison of the two most popular systems in use. Based on the average percent difference, the two systems yielded the same absorbed dose to water at the reference point in phantom to within 1.5% for both modalities. No energy dependence was evident in the results; however, a systematic average percent difference between photons and electrons was seen, with the Farmer in-water system consistently predicting a dose 1.3% lower for electrons than the Holt in-polystyrene system. For photons both systems predicted the same dose to within 0.3% on average. When a physicist converts from TG-21 to TG-51, these data may be of assistance in explaining unexpected changes in output that are different from previously published values. Implementation of the TG-51 protocol should eliminate any of the observed differences in electron beam dosimetry between the two dosimetry systems because the Holt system cannot be used with TG-51.