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Quality assurance of three-dimensional processes in radiotherapy using polymer-gel and magnetic resonance

Research and development of a suitable three-dimensional
dosimeter to improve quality control procedures in radiation
therapy and suitable methods for intercomparison of
three-dimensional treatment planning systems.

Radiotherapy is at the present time an important treatment modality for cancer patients. The main aim of radiotherapy is to apply a sufficiently high dose to the tumour volume to cause the required biological sequelae of tumourous cells while sparing surrounding healthy tissues. The success of the treatment is strongly dependent on the accuracy of the applied dose. Current recommendations specify that for treatment to be successful, the deviation of the applied dose from the prescribed dose should not exceed +-5 % and in some cases even +- 3 %. Many special radiation therapy procedures (stereotactic treatment, conformal therapy, brachytherapy, etc.) require dose distribution in three-dimensional (3D) space. Present radiation dosimetry techniques only allow measurements in two-dimensions (2D) and therefore reconstructions from 2D measurements are necessary to obtain 3D distributions. A new method for 3D dose distribution measurement has been recently described, but it is not yet commercially available. Therefore extensive research and development in this field is desirable to develop a system which will be reliable, accurate and easily applicable to many clinical applications and which will bring about an improvement in the radiation therapy treatment planning and application. The main aim of this project is to develop a suitable radiation dosimeter based on polymer-gel and magnetic resonance imaging (MRI) which will make it possible to measure 3D radiation dose distributions. The system is based on the radiation-induced polymerization of monomers, dispersed in an aqueous gel. Water proton nuclear magnetic resonance (NMR) relaxation rates in the gel are strongly affected by local changes in the polymer molecular structure and dynamics, and thus the distribution of radiation dose may be visualised and quantified with high resolution using MRI. In this method the radiation detector itself forms the tissue-equivalent phantom and the measurement site is determined entirely by the measuring system. This means that the NMR gel dosimeter is totally non-invasive and no corrections of perturbation (after introducing a measuring probe into the phantom) are needed. Alternative techniques, such as thermoluminiscent and chemical dosimetry, involve transfer of the detector from the phantom to a measuring device and it is impossible to study the complete pattern of dosage except by sampling on a rather coarse grid of points. The photographic method has a property that the complete 2D pattern of the dose is preserved and can therefore be sampled repeatedly and interactively, concentrating on regions of special interest. However, the introduction of a photographic film represents a considerable perturbation of the tissue-equivalent phantom and gives rise to severe problems for quantitative photographic dosimetry. On the other hand, in the case of NMR gel dosimetry, the dosimetric system can be easily filled into any irregular volume simulating anatomic structure and after irradiation and MRI evaluation provides the observer with complete 3D dose distribution. The project should be developed in the following steps: 1) Development and improvement of a technology for preparation of new gel dosimeters. An influence of different chemical compounds on improvement of sensitivity, stability, reliability of dosimeter parameters will be investigated. Study of the dependence of the dosimeter response on the physical conditions during preparation, irradiation as well as during the dosimeter MRI evaluation. 2) Development and investigation of suitable methods for gel dosimeters evaluation with the help of MRI. Development of a suitable procedure for recalculation of MRI relaxation times into an absorbed doses. 3) Evaluation of dosimetric properties of gel dosimeters including their stability, reproducibility, fading of signal, influence of preparation procedures, dosimeter containers, non-homogeneities, etc. The long term stability required for postal intercomparisons of different radiation therapy sources will also be studied. Development of calibration procedures for 3D dosimetry including detail study of radiation quality dependence, dose rate dependence, dose range linearity, etc. 4) Development of computer graphic methods for isodose representation from MRI images. Methods for MRI image transfer to commercially available evaluation programmes for absorbed dose distribution display will be investigated in order to simplify evaluation of gel dosimeters. 5) Comparison of the dosimeter response with other dosimetric systems such as ionising chamber, film, thermoluminescent and semiconducter dosimeters. 6) Application of NMR gel dosimeter in the measurement of complete 3D dose distributions produced during some clinical radiotherapy modalities such as: * dynamic wedge, * multi-leaf collimator, * multi-arc small-field irradiation used in LINAC stereotactic radiosurgery, * multi-source isocentric irradiation used in Leksell gamma knife stereotactic radiosurgery, * afterloading brachytherapy, etc. 7) Development of quality assurance procedures for 3D treatment planning checks. Implementation of NMR gel dosimetry into a quality assurance programme. Testing the possibility of the use of gel dosimetery in national and international postal quality assurance programmes. It is expected that the development of 3D gel dosimetry will improve radiation therapy treatments, namely when special techniques are employed, it will provide a possibility for international comparison of 3D treatment planning systems, etc. Probably new applications of 3D gel dosimetry will be developed when the system is fully functional and commercially available.
Acronym: 
DIAGNOSIS
Project ID: 
1 758
Start date: 
01-10-1997
Project Duration: 
36months
Project costs: 
150 000.00€
Technological Area: 
Market Area: 

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