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Absolute Dosimetry for Brachytherapy with Miniature X-ray Devices

04.01.2010

The intra-operative irradiation by means of a miniature X-ray device has increasingly established itself as an alternative to the methods used today in radiotherapy. In the case of this irradiation technique, the surgeon first removes the tumour and then places the tip of the X-ray source directly in the obtained cavity, in the so-called tumour bed, by means of an applicator. After the irradiation, the X-ray source can be removed and the operating field sealed.

This irradiation technique is already used today in the therapy of mammary carcinoma, but also in the intra-operative irradiation of brain tumours. The advantage of this procedure is that, due to an abrupt reduction of the dose, the highest irradiation dose is in the tissue located immediately around the applicator’s surface, whereby tissues which are situated further from the source are spared.

The miniature X-ray devices are operated with tube voltages of up to 50 kV. The thus generated, continuous bremsstrahlung spectrum contains some characteristic fluorescence lines which result from the materials used for the construction of the device.

Currently, no national metrology institute worldwide offers the realisation of the unit of absorbed dose to water, which is substantial to radiotherapy, in the near field of such a source and with the reference depth of 1 cm in water.

PTB has concluded a cooperation with a German manufacturer - Carl-Zeiss Surgical GmbH - in the field of absolute dosimetry for brachytherapy with miniature X-ray devices aimed at developing a procedure for the determination of the absorbed dose to water under the above-mentioned conditions.

Figure : Miniature X-ray device (left) with spherical applicator (right) for targeted positioning in the tumour bed.