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European Research Project in the Field of Dosimetry for Brachytherapy

04.01.2010

Approx. 100,000 cancer patients are treated by means of brachytherapy in Europe each year, whether as the only therapy form or in combination with chemotherapy or external radiotherapy. Brachytherapy is a form of treatment in which a radioactive irradiation source is placed close to or directly inside the tumour to irradiate it with as high a radiation dose as possible, whereby the surrounding, healthy tissue is spared. Thanks to the development of new radiation sources and irradiation methods - such as the remote-controlled positioning of the sources by means of afterloading systems - modern brachytherapy has, over the last decade, gained considerably in importance for the curative treatment of malignant diseases. It has therefore become increasingly necessary to improve basic dosimetry, which is indispensable for the planning of the therapy, and to make it traceable to primary standards.

At present, dosimetry for brachytherapy sources is based on the primary standards of air kerma at a distance of 1 m - vertically - to the source’s axis without the share of scatter radiation and air attenuation (reference air kerma rate - in vacuo - [1]). For the determination of the therapeutically relevant absorbed dose to water at a distance of 1 cm from the source in water, the value of the reference air kerma rate is converted by means of a dose conversion factor Λ is mainly based on calculations with Monte Carlo simulation methods and its uncertainty amounts to at least 5 % (k=1).

The European research project iMera+, Joint Research Project 06 "Increasing Cancer Treatment Efficacy Using 3D Brachytherapy" is therefore aimed at trying to considerably reduce this uncertainty. In total, 10 European national institutes are involved - to a different extent (PTB approx. 30 %) in this project which is supported by the European Union with € 1.2 m for three years. This project is aimed at developing, at several national institutes, primary standards for the realisation of the absorbed dose to water at a distance of 1 cm from the radiation source, and at disseminating the unit of absorbed dose to water directly to the clients all over Europe (hospitals which perform brachytherapy and manufacturers of brachytherapy sources) by means of transfer standards. Furthermore, it is planned to develop methods for the determination of the 3D distribution of the dose rate in the near field of the radiation source. The combination of the reference value of the absorbed dose to water at a distance of 1 cm with the 3D dose rate distribution makes it possible to re-arrange the systems of therapy planning and the protocols for clinical dosimetry in brachytherapy on the basis of the absorbed dose to water. This would ultimately lead to a considerable reduction of the uncertainty of the dose applied to the patient.

Of the 10 national metrology institutes (NMIs), five are developing new primary standards, both for high-dose-rate (HDR) sources, with nuclides such as 192Ir and 60Co, and for low-energy and low-dose-rate brachytherapy sources, so-called LDR sources with the nuclides 125I or 103Pd as they are used for the therapy of the prostate carcinoma. The remaining NMIs are involved (to different extents), amongst other things, in the determination of the 3D dose rate distribution, the development of transfer standards and the accompanying, comprehensive Monte Carlo calculations.

The progress of the project is regularly checked at bi-annual meetings of collaborators from all the institutes involved, and the further procedure is discussed. At the last meeting in Paris - which took place approximately in the middle of the project's running period - in October 2009, it turned out that, fortunately, up to now none of the project partners had any significant delay to report in the project planning.

The project is scheduled to end in the third quarter of 2011; it is planned to present the results to interested experts from universities, national metrology institutes and hospitals on the occasion of a symposium.

Literature:

  1. ICRU report 38. Dose and volume specification for reporting intracavitary therapy in gynecology. Bethesda, MD: International Commission on Radiation Units and Measurements; 1985. p. 1-20
  2. Nath R, Anderson L L, Luxton G, Weaver K A, Williamson J F and Meigooni A S 1995 Dosimetry of interstitial brachytherapy sources: recommendations of the AAPM Radiation Therapy Committee Task Group No. 43 Med. Phys. 22 209-36