THE ADVANTAGES OF ABSORBED-DOSE CALIBRATION FACTORS

A formalism for clinical external beam dosimetry based on use of ion chamber absorbed-dose calibration factors is outlined in the context and notation of the AAPM TG-21 protocol. It is shown that basing clinical dosimetry on absorbed-dose calibration factors N(D) leads to considerable simplification and reduced uncertainty in dose measurement. In keeping with a protocol which is used in Germany, a quantity k(Q) is defined which relates an absorbed-dose calibration factor in a beam of quality Q0 to that in a beam of quality Q. For 38 cylindrical ion chambers, two sets of values are presented for N(D)/N(X) and N(gas)/N(D) and for k(Q) for photon beams with beam quality specified by the TPR10(20) ratio. One set is based on TG-21's protocol to allow the new formalism to be used while maintaining equivalence to the TG-21 protocol. To demonstrate the magnitude of the overall error in the TG-21 protocol, the other set uses corrected versions of the TG-21 equations and the more consistent physical data of the IAEA Code of Practice. Comparisons are made to procedures based on air-kerma or exposure calibration factors and it is shown that accuracy and simplicity are gained by avoiding the determination of N(gas) from N(X). It is also shown that the k(Q) approach simplifies the use of plastic phantoms in photon beams since k(Q) values change by less than 0.6% compared to those in water although an overall correction factor of 0.973 is needed to go from absorbed dose in water calibration factors to those in PMMA or polystyrene. Values of k(Q) calculated using the IAEA Code of Practice are presented but are shown to be anomalous because of the way the effective point of measurement changes for Co-60 beams. In photon beams the major difference between the IAEA Code of Practice and the corrected AAPM TG-21 protocol is shown to be the P(repl) correction factor. Calculated k(Q) curves and three parameter equations for them are presented for each wall material and are shown to represent accurately the k(Q) curve for all ion chambers in this study with a wall of that specified material and a thickness less than 0.25 g/cm2. Values of k(Q) can be measured using the primary standards for absorbed dose in photon beams.