Protocols for the dosimetry of high-energy photon and electron beams: a comparison of the IAEA TRS-398 and previous international Codes of Practice

Abstract

A new international Code of Practice for radiotherapy dosimetry co-sponsored by several international organizations has been published by the IAEA, TRS-398. It is based on standards of absorbed dose to water, whereas previous protocols (TRS-381 and TRS-277) were based on air kerma standards. To estimate the changes in beam calibration caused by the introduction of TRS-398, a detailed experimental comparison of the dose determination in reference conditions in high-energy photon and electron beams has been made using the different IAEA protocols. A summary of the formulation and reference conditions in the various Codes of Practice, as well as of their basic data, is presented first. Accurate measurements have been made in 25 photon and electron beams from 10 clinical accelerators using 12 different cylindrical and plane-parallel chambers, and dose ratios under different conditions of TRS-398 to the other protocols determined. A strict step-by-step checklist was followed by the two participating clinical institutions to ascertain that the resulting calculations agreed within tenths of a per cent. The maximum differences found between TRS-398 and the previous Codes of Practice TRS-277 (2nd edn) and TRS-381 are of the order of 1.5-2.0%. TRS-398 yields absorbed doses larger than the previous protocols, around 1.0% for photons (TRS-277) and for electrons (TRS-381 and TRS-277) when plane-parallel chambers are cross-calibrated. For the Markus chamber, results show a very large variation, although a fortuitous cancellation of the old stopping powers with the ND,w/NK ratios makes the overall discrepancy between TRS-398 and TRS-277 in this case smaller than for well-guarded plane-parallel chambers. Chambers of the Roos-type with a 60Co ND,w calibration yield the maximum discrepancy in absorbed dose, which varies between 1.0% and 1.5% for TRS-381 and between 1.5% and 2.0% for TRS-277. Photon beam calibrations using directly measured or calculated TPR20,10 from a percentage dose data at SSD = 100 cm were found to be indistinguishable. Considering that approximately 0.8% of the differences between TRS-398 and the NK-based protocols are caused by the change to the new type of standards, the remaining difference in absolute dose is due either to a close similarity in basic data or to a fortuitous cancellation of the discrepancies in data and type of chamber calibration. It is emphasized that the NK-ND,air and ND,w formalisms have very similar uncertainty when the same criteria are used for both procedures. Arguments are provided in support of the recommendation for a change in reference dosimetry based on standards of absorbed dose to water.

Shift in absorbed dose for megavoltage photons when changing to TRS-398 in Australia

“…Previous work has examined the differences between TRS-398 and TRS-277: In Appendix I of TRS-398 the difference in the absorbed dose determined by these two protocols is found to be about 1% based on 60 Co calibrations traceable to the Bureau International des Poids et Mesures in Paris. These results were also published in a longer discussion in 2002 5 . In Belgium, a comparison of TRS-398 with different air kerma based protocols found a difference of 0.1 -0.6% 6 .…”

“…For megavoltage photons, the shift can be checked by measuring p dis and using the clinic's chamber's N K and N D,w calibration coefficients in equation (5). Representative values for these quantities indicate that the absorbed dose will appear to increase by between 0.1 and 1.1%, depending on chamber type and beam quality.…”

Quality assurance in proton beam therapy using a plastic scintillator and a commercially available digital camera

“…Many dosimeters have been reported for use in clinical proton dosimetry such as ionization chambers and ionization chamber arrays. However, 2D arrays of ionization chambers are not widely used although they provide fast measurements, because a high spatial resolution is required to measure the sharp fall in depth dose at the end of the protons' range, the protons spot shape, and the profile of a narrow proton beam 5, 6. Scintillators' properties appear to make them viable for use in proton quality assurance.…”

Comparison of linear accelerator photon outputs from the IAEA TRS-398 and TRS-277 codes of practice

“…Andreo et al 5 (see also the review article by Huq and Andreo 11 Another study comparing absorbed doses from air kerma codes of practice to those from absorbed dose to water codes of practice is the one by Palmans and coworkers 13 . In their study they used NE 2571, Wellhöfer FC65-G and PTW 30004 ionisation chambers in photon beams with 20 10 TPR ratios varying from 0.672 to 0.776.…”

“…The reason for this is not clear. However, because of the uncertainties involved it can be argued that the difference is not significant.Andreo et al5 do not give enough information to enable one to calculate c f accurately over a range of beam qualities. However, from their chamber an approximate value of 0.919 can be calculated for c f .…”