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Realization of a comparison for eye tumour radiation therapy with a Ru-106 planar source


Every year, about 1000 persons contract a malignant eye tumour in Germany. About 70% of these patients are treated by means of brachytherapy with so-called beta eye applicators. These applicators are calottes made of stainless steel, silver or gold, which are adapted to the eye and which are covered, on their concave side, with the radioactive material Ru-106 and are fixed to the diseased eye for a duration of 1 to 10 days. For an effective and gentle treatment, an exact knowledge of the 3D dose distribution in the clinically relevant near field is necessary - on the one hand to apply the necessary dose required for the treatment in the tumour and, on the other hand, to minimize the dose in risk organs such as the optic nerve. For an optimal realization of eye tumour brachytherapy, an adequate dosimetry in the near field of the utilized applicators in units of absorbed dose to water is therefore required. To enable the users to trace back their dose measurements to a national standard, PTB has developed a primary standard measuring device on the basis of an extrapolation chamber for the realization of the unit of absorbed dose to water of beta brachytherapy sources. In further efforts to prove the comparability of this primary standard measuring device, an international comparison of the absorbed dose to water in the near field of various beta brachytherapy sources was carried out in 2006 and 2008. In 2008, the University Hospitals of Essen and Tübingen, which treat about 70% of patients in Germany with eye tumour beta brachytherapy, as well as the only manufacturer of eye applicators worldwide, the Berlin-based company BEBIG, asked PTB to carry out a verification of their respective basic dosimetry by means of a comparison. At the beginning of 2009, a measuring programme was set up with the participants. As reference field, the near field of a Ru-106 planar source was chosen in the water phantom, which is to be measured by the participants in units of the absorbed dose to water. For this purpose, the Ru-106 planar source was provided by BEBIG. It is a standard eye applicator which is not yet curved. The special problem of dosimetry in the near field of beta brachytherapy sources is the very steep dose distribution with gradients of up to a factor of 3 within one millimetre. Due to this fact, a local resolution of the detector probes used in the sub-millimetre range is required. The plastic scintillator detectors - which, for a large part, are an in-house development of PTB - come close to this requirement due to their relatively small detector volumes (cylinder with a diameter of 1 mm and a length of about 0.5 mm), which - in the past - allowed them to establish themselves in the dosimetry of therapeutic beta brachytherapy sources. At PTB, the measurements for the comparison were carried out by means of an absolutely calibrated plastic scintillator detector until the end of 2009. Now, the other participants will receive the Ru-106 source for the characterization of the dose distribution for three months each. In the fourth quarter of 2010, the source is to be measured finally once again at PTB. It is envisaged to evaluate and complete the comparison by the end of 2010.