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Experimental determination of beam quality correction factors

30.03.2009

Increasingly, modern methods of radiotherapy – such as, e.g., intensity-modulated radiation therapy (IMRT), CyberKnife radiosurgery or TomoTherapy – are applied in the treatment of tumour diseases. These methods are characterised by the fact that they make it possible to generate dose distributions which are extremely tumour-conform (i.e. the dose distribution generated inside the patient corresponds very precisely to the shape of the tumour), which minimises undesired side-effects of an irradiation. In order to protect healthy tissue, it is often necessary to generate very complex dose distributions with steep gradients. This is achieved by superimposing a large number of small, irregularly shaped radiation fields (with field sizes of down to 0.5 cm × 0.5 cm) from different directions inside the patient in such a way that the shape of the dose distribution corresponds to the shape of the tumour. For this purpose, complex calculations must be carried out beforehand to set up suitable radiation plans which must be verified by means of dose measurements in the course of quality assurance and patient protection.

These verification measurements are a problem due to the fact that the small and irregular photon radiation fields used in modern types of radiotherapy differ significantly from those used in conventional radiotherapy (e.g. different spectral fluence of the generated secondary electrons or disturbance of the secondary electron equilibrium). The established measurement protocols for the measurement of the absorbed dose to water, such as, e.g., the German Standard DIN 6800-2 [1] or the international dosimetry protocol IAEA TRS-398 [2], therefore cannot be applied to small photon fields without being adapted; in particular, much of the physical data (e.g. stopping power ratios, perturbation factors, etc.) tabulated in these documents are not valid for small radiation fields. Up to now, there has been no dosimetry protocol for the measurement of the absorbed dose to water in small and irregular radiation fields with a measurement uncertainty close to that of a dose measurement under reference conditions (i.e. in a 10 cm × 10 cm field).

In order to be able to carry out reliable measurements of the absorbed dose to water on a regular basis also for modern methods of radiotherapy, i.e. in small and irregular radiation fields, various aspects of dose measurement in small and irregular photon radiation fields will be investigated at PTB within the scope of the ERANET-Plus Joint Research Project T2.J07 "External Beam Cancer Therapy" [3] in cooperation with 8 other national metrology institutes.

One of the objectives of this project is to achieve the experimental determination of radiation quality correction factors which are necessary for the measurement of the absorbed dose to water.

All modern dosimetry protocols (e.g. [1] and [2]) are based on the use of ionisation dosimeters which were calibrated to indicate the absorbed dose to water in the 60Co radiation field. If such an ionisation dosimeter is used in a radiation field having a different energy (e.g. in a photon field generated by a medical linear accelerator), then the thereby induced modification of the response must be corrected by the radiation quality correction factor kQ. The absorbed dose to water DW is determined according to

DW = NMkQ                    (1)

where N is the calibration factor determined in the 60Co radiation field and M is the reading of the ionisation dosimeter which has already been corrected with respect to all other influence quantities (see [1][2]). The radiation quality correction factor kQ depends both on the radiation quality (i.e. on the energy) of the photon radiation and on the set-up and the dimensions of the ionisation chamber used. For dose measurements under reference conditions (i.e. in a 10 cm × 10 cm radiation field), values for the radiation quality correction factor are given in the dosimetry protocols. These values, however, have not been determined experimentally but are based on model calculations and Monte Carlo simulations and, with a relative standard measurement uncertainty of 1%, they represent the main contribution to the total uncertainty of dose measurement under reference conditions. For dose measurement in small radiation fields, presently, no reliable values of the correction factor kQ are known. With regard to a reduction of the measurement uncertainty - especially in small radiation fields - it is therefore necessary to experimentally determine radiation quality correction factors having a much lower measurement uncertainty.

Within the scope of the above-mentioned Joint Research Project "External Beam Cancer Therapy" [3], radiation quality correction factors have been determined in four different high-energy photon radiation fields - to begin with under reference conditions (i.e. in a 10 cm × 10 cm radiation field) - by Department 6.2 "Dosimetry for Radiation Therapy and Diagnostic Radiology". For this purpose, the absorbed dose to water DW was measured in the high-energy photon radiation fields by means of a water calorimeter [4]. Then, in the same radiation fields, the readings M of several ionisation chambers of different types were determined. These ionisation chambers had previously been calibrated in PTB's 60Co reference radiation field. From the knowledge of the calibration factor N, the reading M and the absorbed dose to water DW, the correction factor kQ was then determined according to Eq. (1) for each individual ionisation chamber.

As an example, Figure 1 shows the experimentally determined values of kQ for three different chambers of the type NE2561 in comparison to the values of the correction factor kQ given in the dosimetry protocol IAEA TRS-398 [2].

Figure : Experimentally determined values of the beam quality correction factor kQ for three different chambers of the type NE2561 in comparison to the values of this correction factor given in the dosimetry protocol IAEA TRS-398 (continuous green line). The dashed lines represent the standard measurement uncertainty of the kQ values given in IAEA TRS-398. For legibility reasons, the kQ values corresponding to the same respective radiation quality for the different chambers are slightly shifted in the representation.

The measurements show that it is possible to determine experimentally beam quality correction factors with a relative standard measurement uncertainty of 0.25%. This uncertainty is much lower than the uncertainty of the kQ values given in IAEA TRS-398 or DIN 6800-2 which is of 1%. Furthermore, it turns out that the exemplar-specific variation of the kQ values for different chambers of the same type is not larger than the measurement uncertainty of the values which have been determined experimentally – therefore, it makes sense to indicate the chamber-specific values of the correction factor kQ in dosimetry protocols (see also [5]).

These measurements of radiation quality correction factors are presently being pursued in small photon radiation fields. The objective is to determine kQ factors using a water calorimeter in photon fields of the size: 3 cm x 3 cm, with a relative standard measurement uncertainty of 0.5%.

Literature

  1. DIN Deutsches Institut für Normung e.V.: DIN 6800-2: Dosismessverfahren nach der Sondenmethode für Photonen- und Elektronenstrahlung - Teil 2: Dosimetrie hochenergetischer Photonen- und Elektronenstrahlung mit Ionisationskammern. Berlin, März 2008

  2. International Atomic Energy Agency: Absorbed Dose Determination in External Beam Radiotherapy, Technical Reports Series No. 398, Vienna, 2000,
    www-pub.iaea.org/MTCD/publications/PDF/TRS398_scr.pdf See www.euramet.org/index.php

  3. Achim Krauss, Ralf-Peter Kapsch:
    Calorimetric determination of kQ factors for NE 2561 and NE 2571 ionization chambers in 5 cm x 5 cm and 10 cm x 10 cm radiotherapy beams of 8 MV and 16 MV photons.
    Phys. Med. Biol. 52 (2007), 6243-6259

  4. Ralf-Peter Kapsch, Christian Pychlau:
    Exemplarstreuung von kQ-Werten.
    Proceedings der 39. Jahrestagung der deutschen Gesellschaft für Medizinische Physik e.V., ISBN 3-9809869-8-5, Berlin, 2008