On the beam quality specification of high-energy photons for radiotherapy dosimetry

An overview of common photon beam quality specifiers used in radiotherapy d osimetry introduces a reasoned discussion on the advantages and disadvantag es of TPR20,10 and PDD(10)(x). It is shown that some of the potential advan tages of PDD(10), are also present in other well known beam quality specifi ers such as d(80) However, all PDD-based beam quality indices, including PD D(10)(x), are subject to electron contamination and their determination is affected by practical limitations. The proposed filtration of contaminant e lectrons by Kosunen and Rogers [Med. Phys. 20, 1181-1188 (1993)] and by Li and Rogers [Med. Phys. 21, 791-798 (1994)] is questioned, not only with reg ard to the adequacy of using lead as an electron filter, but also in relati on to its efficiency (if there were no contamination, restrictions for beam calibrations at d(max) would be removed) and practical measurement. It is argued that (i) there is no unique beam quality specifier that works satisf actorily in all possible conditions, for the entire energy range of photon energies used in radiotherapy and all possible accelerators used in hospita ls and in standards laboratories, and (ii) TPR20,10 remains to be the most appropriate specifier fur clinical photon beams as it has less practical dr awbacks than PDD-based quality indices. The final impact on clinical photon beam dosimetry resulting from the use of different photon beam quality spe cifiers, is that they are not expected to yield a significant change (i.e., more than 0.5% and in most cases well within 0.2%) in the absorbed dose to water in reference conditions for most clinical beams. (C) 2000 A,American Association of Physicists in Medicine. [S0094-2405(00)00103-6].