Determining the absorbed dose to water in low-energy X-ray therapy

The fundamental dose quantity for low‑energy X‑ray therapy is the absorbed dose to water at the surface of a water phantom. The unit of this quantity is disseminated via a coplanar ionization chamber that is flush‑mounted inside a PMMA phantom. This ionization chamber is directly calibrated using a quantity referred to as the “absorbed dose to water at the surface of a water phantom”. This enables users to directly measure the absorbed dose to water in a clinical field. However, a number of correction factors must be applied. These data have now been published and included in the revised draft of TRS 398, the International Code of Practice for Dosimetry of the IAEA.

Low-energy X‑ray therapy is the official designation for the treatment of the skin surface with soft X‑rays. This type of radiation therapy is characterized by a low penetration depth of the X‑rays into the tissue, which preserves tissue located deeper under the skin. Devices generating soft X‑rays are operated at X‑ray tube voltages between 10 kV and 50 kV (at approx. 25 mA). However, some devices are operated at voltages of up to 100 kV. Low‑energy X‑ray therapy is used in the treatment of diseases such as basal cell carcinoma, metastatic melanoma and skin lymphoma. Depending on the application, the irradiation doses can range from a few grays (Gy) to 100 Gy. In some cases, these doses are administered in multiple applications.

The fundamental dose quantity for low‑energy X‑ray therapy is the absorbed dose to water at the surface of a water phantom. Unfortunately, there is no direct primary standard for this dose quantity. Instead, it is realized via air kerma (which is proportional to the absorbed dose in air) by multiplying it by calculated conversion factors. Air kerma is determined based on a so‑called free‑air ionization chamber by means of primary standard measuring facilities. The unit of this quantity is disseminated either via suitable ionization chambers that have been calibrated in air kerma in free air, or via a coplanar ionization chamber that has been flush‑mounted inside a PMMA phantom (see Figure 1). This ionization chamber is directly calibrated in the quantity absorbed dose to water at the surface of a water phantom. For technical reasons, it is not possible to use a water phantom in this case. In the first case, operators must convert the air kerma measured in the clinical field into the absorbed dose to water themselves. International Codes of Practice for Dosimetry [1] have been published for this purpose. In the second case, users can measure the absorbed dose to water directly in a clinical field; however, a number of correction factors must be applied to make up for the differences between the clinical field and the calibration field at a reference laboratory. This method of determining the dose is also described in published Codes of Practice [1,2].

The TRS 398 International Code of Practice for Dosimetry of the IAEA [1] was published in 2001 and is currently under revision. In this context, PTB is also taking part in revising the section on dosimetry for low‑energy X‑ray therapy. During the revision process, no full set of reliable correction factors was available for the first option of calibrations using the PMMA phantom. PTB has therefore determined this full set of geometric correction factors numerically and validated them by means of exhaustive measurements.

The geometric correction factor can be briefly explained as follows: The coplanar reference ionization chamber is flush‑mounted inside a PMMA phantom. This chamber is calibrated for the absorbed dose to water at the surface of a water phantom under reference conditions at the radiation quality Q₀ = TW 30 (see Table 1) at a distance of 30 cm and a field 3 cm in diameter.

Figure 1: The type PTW 23342 coplanar chamber, flush‑mounted on a PMMA phantom of 15 cm × 15 cm × 8 cm.

Table 1: The radiation qualities of PTB's TW series.

Furthermore, the corresponding calibration factors for radiation qualities TW 10 to TW 100 (see Table 1) are being determined under the same conditions. The result obtained is the energy dependence of the chamber's response under reference conditions. The photons scattered back by the water are a major source of the absorbed dose to water at the surface. The measuring chamber detects the photons scattered back by the PMMA phantom. By approximation, the chamber signal is thus proportional to the dose at the surface of a water phantom. The backscattering rate significantly depends on the field size of the X‑rays. The larger the field, the more back‑scattered photons are present at the reference point at the surface. The backscattering rate also depends on the energy of the X‑rays. In principle, the irradiation conditions at the clinical irradiation facility now deviate from those prevailing at calibration. Since the backscatter factor of PMMA is much higher than that of water, the chamber signal no longer exactly follows the variation of the absorbed dose to water at different field sizes. This effect is corrected by means of a geometric correction factor.

This factor was computed by means of Monte Carlo simulations [3] for the two most widespread coplanar chamber types, PTW 23342 and 23344. The correction factors for these two chambers were computed for field sizes from 2 cm to 20 cm with reference to the reference field size of 3 cm and the TW 30 reference radiation quality. The results are compiled in Tables 2 and 3. For the radiation quality with the highest energy (TW 100) and the largest field diameter (20 cm), the correction is up to 10 %.

The relative uncertainty of the computed values is 1 %. The simulations have been compared to measurements at field sizes of 3 cm, 5 cm, and 10 cm. Several chambers of each type were used. The result was composed of the mean value of the results of the specimens. The deviations between different specimens of the same type were smaller than the standard measurement uncertainty of 1.1 % (k=1). The results from the computations and the measurements agreed with each other within 1 %. The values compiled in Tables 2 and 3 have now been entered in the current draft of the revised version of the TRS 398 protocol. These values will thus be available to an international community of users.

Table 2: Geometric correction factors for type PTW 23342 chambers, standardized to the reference conditions Q₀ = TW 30 and f₀ = 3 cm.

Table 3: Geometric correction factors for type PTW 23344 chambers, standardized to the reference conditions Q₀ = TW 30 and f₀ = 3 cm.

References:

[1]    International Atomic Energy Agency 2001 Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water Technical Report Series 398 (Vienna: IAEA)

[2]    Clinical dosimetry – Part 4: X-ray therapy with X-ray tube voltages between 10 kV and 300 kV (2020-04)

[3]    Kawrakow I, Mainegra-Hing E, Rogers D W O, Tessier F and Walters B R B 2017 The EGSnrc code system: Monte Carlo simulation of electron and photon transport Technical Report PIRS-701 (Ottawa: National Research Council Canada) (https://nrc-cnrc.github.io/EGSnrc/doc/pirs701-egsnrc.pdf)

[4]    Büermann, Ludwig; Ketelhut, Steffen: Determination of chamber geometry correction factors for phantom-based absorbed dose determination in external therapeutic low energy kV x-ray beams, Medical Physics - Scientific Abstracts and Sessions: 47 (2020), 6, e255 - e880

Contact:

L. Büermann, Department 6.2, Working Group 6.25

S. Ketelhut, Department 6.3, Working Group 6.31