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Revision of the IAEA TRS-398 international code of practice

23.12.2020

The IAEA TRS‑398 code of practice for dose measurement in external radiation therapy is currently being revised with the active participation of scientists from Department 6.2 of PTB. The main objectives of this revision are to take the ICRU’s latest recommendations on fundamental dosimetric constants into account, to include modern methods of radiation therapy (FFF, scanned beam), to provide harmonized, internationally accepted data, and to consider modern dosimetric methods and procedures.

One important treatment method for cancer is radiation therapy in which the tumorous cells are irradiated and destroyed by means of high-energy ionizing radiation. For this purpose, high-energy photon radiation from linacs is used in most cases. Other types of radiation such as high-energy electrons, protons, ions or X-rays may be used for special treatments. If this type of therapy is to be successful, the absorbed dose (in other words, the energy deposited in the tumor and in the surrounding tissue) has to be determined with great accuracy [1].

The absorbed dose is usually measured by means of ionization chambers. This is a complex process which is performed based on CoPs. CoPs state concrete instructions (measurement provisions) as well as the required data. Thus, in radiotherapy hospitals, the absorbed dose can be determined with a small measurement uncertainty, and this measurement is traceable to a primary standard for this measurand. It is, above all, larger countries that develop their own codes of practice (for example, the DIN 6800-2 standard in Germany; the AAPM TG-51 CoP in the USA and Canada), whereas many smaller countries use the IAEA TRS-398 [2] CoP that is issued by the International Atomic Energy Agency (IAEA). This international CoP has the special feature of including all of the above-mentioned radiation types (X-rays, photons, electrons, protons and ions), whereas many national CoPs often only deal with a selection of the most widely used radiation types (mostly photons and electrons).

The TRS-398 CoP was published in 2000. Since its publication, numerous new developments and findings have found their way into radiation therapy and dosimetry, which makes a revision of this code necessary. The main reasons for the revision are:

a)    Novel, modern types of ionization chambers have now become commercially available, and the data required must be specified if they are to be used. On the other hand, certain types of ionization chambers that have been in use for a long time can no longer be recommended for use as reference dosemeters. The list of dosemeter types suited to various measurement tasks must therefore be updated and adapted to the latest findings.

b)    New methods of radiation therapy that are mainly used for irradiation with high#8209;energy photons, protons and ions must also be considered. Among these methods are the use of flattening-filter-free (FFF) photon fields and the scanned beam technique for irradiation with protons or ions.

c)    The recommendations of the ICRU concerning new or modified values for fundamental constants of dosimetry must be taken into account [3].

d)    To simulate radiation transport, it is now possible to determine the measurands that are used in dosimetry by means of modern Monte Carlo methods. With these methods, a higher accuracy can be achieved than with the approximation procedures that were used before.

e)    New data and procedures that have been published in the relevant scientific literature must also be taken into account. This is particularly necessary for dosimetry in X-ray fields, for which it has, up to now, been possible to specify only very few data in IAEA TRS-398.

The IAEA is currently coordinating a project spanning several years for the revision of the TRS-398 CoP in order to take the above-mentioned aspects into consideration in dosimetry for radiation therapy. Scientists from PTB’s Department 6.2 are actively taking part in the work of the core working group that was set up for this purpose. This core working group has the task of coordinating the measurements and Monte Carlo numerical simulations carried out by other research groups to determine updated data, analyze the results of these investigations, reach a consensus regarding these data and methods and, last but not least, revise and update the content of the TRS-398 CoP. In this context, scientists from PTB are mainly in charge of revising the recommendations on dosimetry in X-ray fields and in high-energy photon radiation fields from accelerators. This revision will also include a large amount of data and findings regarding dosimetry in modern radiation therapy that have been obtained at PTB over the past few years (see, for example, [4]).

In dosimetry for X-ray fields, both the formalism and the data have changed considerably. This is in contrast to dosimetry for high-energy photon fields where determining a consistent set of correction factors for the influence of the beam quality (kQ) that is based on the ICRU’s recommendations is required. To this end, the values of the correction factor kQ determined by different research groups by means of Monte Carlo numerical simulations as well as by experiments were analyzed for 23 different types of ionization chambers. A total of 725 data points from numerical simulations and 179 data points from measurements were taken into account [5]. For each type of ionization chamber, the parameters a and b of a fit function of the formatmathematical formula

were determined from the data points, so that it will now be easily possible to calculate the correction factor kQ corresponding to each value of the beam quality index Q.

As an example, figure 1 shows the individual data points and the fit function determined from them for ionization chambers of the type NE 2571.

 diagram

Figure 1:   Correction factors for the beam quality kQ determined experimentally and by means of Monte Carlo simulations by different research groups for ionization chambers of type NE 2571. The data were fitted to a function of the format mentioned in the text (green curve). The area shaded in green represents the measurement uncertainty of the kQ values calculated by means of the fit equation.

The measurement uncertainty of the kQ values determined in this way is much smaller than the measurement uncertainty of the values stated in the previous version of the TRS-398 CoP. Moreover, due to the harmonized approach and by taking the current ICRU recommendations [3] into account, it is expected that the correction factors determined for different types of ionization chambers will be consistent with each other.

All in all, the revised version of the TRS-398 CoP will lead to improved international harmonization and the enhanced accuracy of reference dosimetry in radiation therapy and will thus contribute to increasing the amount of successful cures.

Literature

[1]  D. van der Merwe et al.: Accuracy requirements and uncertainties in radiotherapy: A report of the International Atomic Energy Agency. dx.doi.org/10.1080/0284186X.2016.1246801

[2]  P. Andreo et al.: Absorbed dose determination in external beam radiotherapy – An international code of practice for dosimetry based on standards of absorbed dose to water. IAEA Technical Reports Series No. 398. www-pub.iaea.org/MTCD/publications/PDF/TRS398_scr.pdf

[3]  ICRU: Key data for ionizing-radiation dosimetry: Measurement standards and applications. ICRU report 90. doi.org/10.1093/jicru/ndw043

[4]  A. Krauss & R.-P. Kapsch: Experimental determination of kQ factors for cylindrical ionization chambers in 10 cm × 10 cm and 3 cm × 3 cm photon beams from 4 MV to 25 MV. dx.doi.org/10.1088/0031-9155/59/15/4227

[5]  P. Andreo et al.: Determination of consensus kQ values for megavoltage photon beams for the update of IAEA TRS-398. doi.org/10.1088/1361-6560/ab807b

Contact:

Opens local program for sending emailR.-P. Kapsch, Department 6.2, Working Group 6.21