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The role of humidity and other correction factors in the AAPM TG‐21 dosimetry protocol
Author(s) -
Rogers D. W. O.,
Ross C. K.
Publication year - 1988
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 - 10.1118/1.596292
Subject(s) - dosimetry , absorbed dose , calibration , ionization chamber , nuclear medicine , radiation , dose profile , materials science , radiochemistry , physics , chemistry , ion , nuclear physics , medicine , quantum mechanics , ionization
A detailed derivation is presented of the formulas required to determine N gas and D med in the AAPM TG‐21 dosimetry protocol. This protocol specifies how to determine the absorbed dose in an electron or photon beam when using exposure or absorbed dose calibrated ion chambers. It is shown that the expression given in TG‐21's recent letter of clarification is incorrect. Accounting for humidity correctly increases, by 0.4%, all absorbed dose determinations using an exposure calibrated ion chamber. Taking into account other correction factors in the equation for exposure could also have varying, but significant effects (possibly over 1%). These are the stem scatter correction, the axial nonuniformity correction and the electrode correction for electrodes made of different materials from the wall. Attention is drawn to differences in the definitions of the exposure and absorbed dose calibration factors N x and N D , respectively, as supplied by the NBS and the NRCC.