Determination of chamber correction factors for X-ray therapy qualities

The primary measurement of the absorbed dose to water for X‑ray therapy radiation qualities using calorimeters is extremely time‑consuming. Alternatively, ionization chambers can be calibrated in the “air kerma” measurand, and then conversion factors can be used to calculate the calibration factor for the absorbed dose to water. In a joint research project with McGill University in Montreal (Canada), it has now been possible to measure this conversion factor for the first time for six ionization chamber types with relative standard measurement uncertainties smaller than 1 % and to calculate this factor with the aid of Monte Carlo simulations with relative uncertainties of 0.3 %. However, deviations of up to 3 % resulted from the comparison, the causes of which are still being investigated. 

Medium‑energy X‑rays with generating voltages between 80 kV and 300 kV are mainly used for radiation therapy of inflammatory and degenerative diseases of joints and soft tissue (medium‑energy X‑ray therapy). Reference dosimetry is performed with calibrated ionization chambers which are used to measure the absorbed dose to water at a water depth of 2 cm. At PTB, the absorbed dose to water is realized primarily by means of a water calorimeter. This type of realization is very demanding, since the radiation qualities have a high dose gradient in water and the intensity at the measurement location is relatively low. As the measurement procedure is extremely time‑consuming, two transfer chambers were calibrated directly in the water calorimeter at the time of the water‑calorimetric measurements and were used from then on for the traceability of routine calibrations [1].

A much less complex method is based on the calibration of a chamber in the “air kerma” measurand free in air and the subsequent conversion into the calibration factor with respect to the absorbed dose to water. The conversion factors required for this conversion are composed of the chamber‑independent, calculable ratio of the mass‑energy transfer coefficients of air to water averaged over the photon fluence spectra. These conversion factors also comprise a chamber‑specific correction factor which takes into account the different response of the chambers in air and in water and the different perturbation of the field by the chamber and the chamber stem.

The calculation of the average mass‑energy transfer ratios of air to water is possible with very small uncertainties of about 0.3 % [1]. However, correction factors of the chambers have so far been determined mainly from Monte Carlo simulations and are subject to relatively large uncertainties of up to 3 %. Furthermore, they are only available for a limited selection of chamber types, and some of them are already outdated. In recent years, significant progress has been made in terms of both measurement and computational techniques, making it possible to redetermine the chamber‑specific correction factors with significantly smaller uncertainties. In addition, significantly improved chamber types are now available on the market for which no such correction factors have previously been available. These were the main reasons for PTB to redetermine the chamber correction factors for the most frequently used chamber types in a joint project with McGill University in Montreal (Canada). Furthermore, these data are relevant for the international dosimetry protocol IAEA TRS 398 [2] that is used worldwide and is currently under revision.

At PTB, the six investigated chamber types were calibrated in the radiation fields of the TH series from 50 kV to 300 kV both in air kerma and in absorbed dose to water against the respective primary standards of PTB. The desired chamber correction factor is then obtained by simply dividing the calibration factors with respect to the absorbed dose to water and air kerma normalized to the mass energy transfer ratio of water to air. The estimated relative standard measurement uncertainties of this experimental determination of the correction factors are less than 1 %.

The project partner from McGill University calculated the respective correction factors with the user code “egs_chamber” of the EGSnrc Monte Carlo Code System [3] using photon fluence spectra measured at PTB with the aid of germanium detectors. For this purpose, realistic chamber models of the individual types were inserted into the simulation. Moreover, the dose in the chamber volume in water and air as well as the absorbed dose to water and the air kerma were calculated at the reference point. In doing so, it was possible to reduce the relative uncertainties of the correction factors to 0.3 %, which is a significant improvement compared to the previous uncertainties of up to 3 %.

Finally, the measured and the calculated correction factors were compared. The results for the PTW TM30013 chamber type can be seen in Figure 1 (from [4]). Unexpected deviations of up to 3 % were found. The deviations were similar for all six investigated chamber types and could be clearly assigned to the deviations of the calculated and the measured absolute ratio of the absorbed dose to the air kerma free in air. The causes of these deviations are the subject of further investigations.

Fig. 1: Measured (blue dots) and calculated (red dots) correction factors for the PTW TM30013 chamber as a function of the Cu half-value layers of the TH series radiation qualities.

Literature

(1)   KRAUSS, A., BÜERMANN, L., KRAMER, H.-M., SELBACH, H.J., Calorimetric determination of the absorbed dose to water for medium-energy x-rays with generating voltages from 70 to 280 kV, Physics in Medicine and Biology 57 (2012) 6245.

(2)   INTERNATIONAL ATOMIC ENERGY AGENCY, Absorbed Dose Determination in External Beam Radiotherapy, Technical Reports Series (TRS) No. 398, Vienna (2000).

(3)   KAWRAKOW, I., MAINEGRA-HING, E., ROGERS, D.W.O., TESSIER, F., WALTERS, B.R.B., The EGSnrc Code System: Monte Carlo simulation of electron and photon transport, Technical Report PIRS-701, National Research Council Canada (2017).

(4)   BANCHERI, J. et al., to be submitted to Physics in Medicine and Biology.

Your contact at PTB:

S. Ketelhut, Department 6.2, Working Group 6.25

L. Büermann, Department 6.2, Working Group 6.25