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New calibration facility for brachytherapy

The possibilities available at PTB for the calibration of radiation sources for brachytherapy could be essentially improved with a new calibration facility which has now been put into operation. A collimated beam geometry with a significantly lower scattered radiation component has allowed both radiation protection and the uncertainty of the correction for scattered radiation to be significantly decreased. With the aid of an industrial robot, the secondary standards used for calibration can be precisely positioned, which leads to a further reduction in the measurement uncertainties.

Measuring set-up for the calibration of 192Ir brachytherapy sources in collimated radiation geome

In addition to conventional radiation therapy, brachytherapy is important for the treatment of tumours. Here, a radioactive radiation source is placed as close as possible to the tumour or directly in the tumour tissue via body openings or through hollow needles. Typically, iridium-192 sources approx. 5 mm in length and 1 mm in diameter are used which are brought into irradiation position in a catheter.

For calibration, the radiation source has up to now been positioned freely in the room, at a distance of at least 2 m from the walls, and the radiation measured with an ionisation chamber at a distance of 1.00 m. In the case of this measuring setup, the component of the scattered radiation from air and walls amounted to approx. 5 % of the total air kerma rate and had to be determined separately. As a result of the free positioning of the source in the measurement room and due to the tendency to use ever stronger sources for therapy (up to approx. 500 GBq), it was, however, difficult to comply with all radiation protection requirements. The new facility avoids these problems as the calibrations are now performed in a collimated radiation field. For this purpose, a lead box is installed in an especially established radiation protection area, in the centre of which the brachytherapy source to be calibrated is placed. With a commercial industrial robot, the secondary standard is positioned on the central axis of the radiation field defined by a diaphragm with an uncertainty of less than 0.1 mm. By collimation, the undesired scattered radiation component from the air – and thus the necessary correction – is considerably reduced (to below 1 %). Uncertainties in the positioning of the radiation source (approx. 0.3 mm) are largely eliminated through several measurements at different distances between radiation source and secondary standard. The clearly reduced uncertainties directly benefit the manufacturers of the radiation sources and thus ultimately the patient.

Contact at PTB:

Phone: +49-531-592-0