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Comparison of high‐energy photon and electron dosimetry for various dosimetry protocols
Author(s) -
Araki Fujio,
Kubo H. Dale
Publication year - 2002
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1470208
Subject(s) - dosimetry , absorbed dose , kerma , calibration , nuclear medicine , primary standard , photon , ionization chamber , medical physics , materials science , physics , medicine , ion , optics , ionization , quantum mechanics
The American Association of Physicists in Medicine Task Group 51 (TG‐51) and the International Atomic Energy Agency (IAEA) published a new high‐energy photon and electron dosimetry protocol, in 1999 and 2000, respectively. These protocols are based on the use of an ion chamber having an absorbed‐dose to water calibration factor with a60 Co beam. These are different from the predecessors, the TG‐21 and IAEA TRS‐277 protocols, which require a60 Co exposure or air‐kerma calibration factor. The purpose of this work is to present the dose comparison between various dosimetry protocols and the AAPM TG‐51 protocol for clinical reference dosimetry of high‐energy photon and electron beams. The absorbed‐dose to water calculated according to the Japanese Association of Radiological Physics (JARP), International Atomic Energy Agency Technical Report Series No. 277 (IAEA TRS‐277) and No. 398 (IAEA TRS‐398) protocols is compared to that calculated using the TG‐51 protocol. For various Farmer‐type chambers in photon beams, TG‐51 is found to predict 0.6–2.1% higher dose than JARP. Similarly, TG‐51 is found to be higher by 0.7–1.7% than TRS‐277. For electron beams TG‐51 is higher than JARP by 1.5–3.8% and TRS‐277 by 0.2–1.9%. The reasons for these differences are presented in terms of the cavity‐gas calibration factor, N gas , and a dose conversion factor, F w , which converts the absorbed‐dose to air in the chamber to the absorbed‐dose to water. The ratio of cavity‐gas calibration factors based on absorbed‐dose to water calibration factors, N D , w60 Co , in TG‐51 and cavity‐gas calibration factors which are equivalent to absorbed‐dose to air chamber factors, N D , air , based on the IAEA TRS‐381 protocol is 1.008 on average. However, the estimated uncertainty of the ratio between the two cavity‐gas calibration factors is 0.9% (1 s.d.) and consequently, the observed difference of 0.8% is not significant. The absorbed‐dose to water and exposure or air‐kerma calibration factors are based on standards traceable to the National Institute of Standards and Technology (NIST). In contrast, the absorbed‐dose to water determined with TRS‐398 is in good agreement with TG‐51 within about 0.5% for photon and electron beams.