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Energy Dependence of the Response of Storage Foils

30.09.2010

For that purpose, it is often necessary to generate very complex dose distributions whose shape has to correspond as far as possible to that of the tumour and which is characterised by steep dose gradients (tumour-conform radiotherapy). This is achieved by superposing a large number of small, irregularly shaped radiation fields coming from different directions. 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 within the scope of quality assurance and patient protection. These verification measurements are, however, a problem due to the fact that the very small and irregular photon radiation fields used in modern types of radiotherapy differ significantly from those used in conventional radiotherapy. The established measurement provisions for the measurement of the absorbed dose to water such as, e.g., the German Standard DIN 6800-2 or the international dosimetry protocol IAEA TRS-398, therefore cannot simply be applied to small photon fields; in particular, many of the physical data 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. a radiation field of 10 cm × 10 cm).

To improve the verification of dose distributions in small and irregular radiation fields, storage foils, among others, are being investigated as to their suitability for dose measurement at PTB in Department 6.2 "Dosimetry for Radiation Therapy and Diagnostic Radiology" within the scope of the ERANET Plus Joint Research Project funded by the EU.

So far, storage foils have usually been used in diagnostic radiology because of their - compared to other digital detectors - relatively high robustness and their mobile usability.

Similar to film radiography, when using storage foils, dose D results in the following equation:

where SU (scanner unit) stands for the grey-scale values detected after the storage foils have been irradiated. The values of the parameters a and b depend on numerous influence quantities (e.g. on the energy of the photon radiation, on the time between the irradiation and the evaluation, the light incidence after the irradiation, the dose interval, etc.) and have to be determined correspondingly.

The principle of measuring the dose by means of a storage foil can be described (in a simplified way) as follows: X-ray storage phosphors composed of BaFBr:Eu2+ absorb the energy of the incident X-ray quanta directly by generating electrons and holes during the irradiation process. Some of these electrons and holes are stored in crystal defects in a metastable way. After the irradiation, these can be led to radiating recombination by means of appropriate, spatially resolved optical stimulation (red laser light). The intensity of the light released by recombination luminescence (also called photo-stimulated luminescence (PSL)) is detected with a high spatial accuracy by means of a photomultiplier within a special scanner, and the distribution of the dose on the storage foil can then be saved on the computer as a digitalised image. The measured light intensities are converted into grey-scale values by the computer so that, after reading out the storage foil, a grey-scale image is displayed on the monitor which is similar to that of an X-ray film. The electronic imaging and the subsequent digitalisation of the image have considerable advantages compared to classic film radiography - which has, over the past few years, led to the elimination or the replacement of classic film radiography in medicine.

Storage foils, as efficient digital X-ray detectors, have a large dynamic range enabling the imaging of a large dose interval. Furthermore, the modification of the hardware parameters and the control of the software open up the possibility of adapting the detection sensitivity of the entire system, according to the respective situation, to low or high doses. The images saved on the computer can be viewed on the monitor and can be simultaneously improved by means of image-processing algorithms and filters. Due to an optimisation of the storage phosphors and due to new lens systems for the detection of PSL and improvements of the scanning procedure, it is currently possible to achieve a spatial resolution (effective pixel size) of up to 40 µm.

Figure 1 shows clearly that storage foils (by Kodak) have a considerably higher response for low-energy photon radiation, which is directly connected to the absorption properties of the phosphor layer. In other words: if a storage foil is used in radiation fields which have different energy levels (or different photon spectra), then the change in the response caused thereby must be corrected by means of the two parameters a and b. The investigations described are, thus, aimed to determine these two parameters experimentally - which is basically the same as determining the energy dependence of the response of storage foils.

For this purpose, the parameters a and b were determined - first under reference conditions - in various photon radiation fields.

Figure : Dependence of the response of the energy of photon radiation for storage foils (Kodak 2000RT CR system). For low-energy X-ray radiation, a considerable increase in the response can be observed.

The investigations show that the parameters and can be determined with a relative standard measurement uncertainty of 0.1 % under reference conditions (i.e. in a 10 cm × 10 cm radiation field, 60Co). Furthermore, the investigation of different storage foils (of the same type) has shown that the specimen-specific variation of these parameters is small. It is therefore possible to indicate type-specific values of these parameters for different photon radiation qualities.

For dose measurement with a storage foil (by Kodak), the methods described in the "Guide to the Expression of Uncertainty in Measurement" (GUM) yield a total uncertainty of approx. 2 %. It must, however, be taken into account that the details of the analytical procedure play an essential role for the validity of the specified measurement uncertainty. As a result of the investigations, one can say that storage foils are a tool - similar to X-ray films - by means of which dose can be measured and dose distributions can be verified.