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

Abstract

An overview of common photon beam quality specifiers used in radiotherapy dosimetry introduces a reasoned discussion on the advantages and disadvantages of TPR20,10 and PDD(10)x. It is shown that some of the potential advantages of PDD(10)x are also present in other well known beam quality specifiers such as d80. However, all PDD-based beam quality indices, including PDD(10)x, are subject to electron contamination and their determination is affected by practical limitations. The proposed filtration of contaminant electrons 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 regard to the adequacy of using lead as an electron filter, but also in relation to its efficiency (if there were no contamination, restrictions for beam calibrations at dmax would be removed) and practical measurement. It is argued that (i) there is no unique beam quality specifier that works satisfactorily in all possible conditions, for the entire energy range of photon energies used in radiotherapy and all possible accelerators used in hospitals and in standards laboratories, and (ii) TPR20,10 remains to be the most appropriate specifier for clinical photon beams as it has less practical drawbacks than PDD-based quality indices. The final impact on clinical photon beam dosimetry resulting from the use of different photon beam quality specifiers, 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.

Dosimetry of small static fields used in external photon beam radiotherapy: Summary of TRS‐483, the IAEA–AAPM international Code of Practice for reference and relative dose determination

“…Last but not least, it must be noted that the slightly different relation between stopping‐power ratios and beam quality specifier between FFF and beams with flattening filter (WFF) should strictly be considered and taken into account in the relevant data. This has been included in the data tables of TRS‐483 for the reference dosimetry of both beam types.…”

Photosynthetic Regulation Under Salt Stress and Salt-Tolerance Mechanism of Sweet Sorghum

“…Plants adapt to osmotic stress mainly by reducing transpiration and accumulating osmotic adjustment substances (Chen M. et al, 2018). It has been reported that sorghum accumulated proline and soluble carbohydrates after NaCl treatment for 20 days (Heidari, 2009). The accumulation of these substances reduced the water potential of sorghum and maintained the waterabsorbing capacity under salt stress.…”

Polarity effect in commercial ionization chambers used in photon beams with small fields

“…Cylindrical ionization chambers are the instruments of choice for use in photon calibration and dosimetry [1]. For each ionization chamber, correction factors are defined as quantities that are not the subjects of measurement, but still affect the measurement.…”