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First measurements of the absorbed dose to water for interstitial brachytherapy

29.09.2008

As an organ-preservative minimally-invasive radiotherapeutic procedure, interstitial brachytherapy has constantly gained in importance for the treatment of the locally limited prostate carcinoma. The treatment is a "low-dose" brachytherapy in the case of which radioactive radiation sources (iodine 125 or palladium 103) are permanently introduced into the prostate tissue in the form of seeds via hollow needles.

Until now, this radiation therapy has been based on the dosimetric determination of the air kerma rate of these sources at a distance of 1 m from the central axis of the source. In radiation therapy it is, however, common practice to indicate the dose in the measurand "absorbed dose to water". In the report of Task Group 43 of the American Association of Physicists in Medicine (AAPM), a conversion method to the absorbed dose to water is described which is used in most European countries. When this method is applied, a total uncertainty of approx. 8 % (k = 1) must, however, be expected. Deviations between the applied dose and the calculated dose of 8 % are regarded as clinically significant. A direct determination of the dose in the measurand "absorbed dose to water" would reduce this uncertainty.

A large, parallel, air-filled plate chamber in a phantom was developed from a water-equivalent material (RW1) with the aim of serving, in future, as a primary standard for the direct determination of the absorbed dose to water. The measurement depth in the water phantom is realized by the selection of the thickness of the input plate.

In the past few years, a new measurement procedure has been developed to determine - for different measurement volumes - the charge Q generated by the radiation field in air with an extrapolation chamber situated in the phantom. With the aid of a conversion factor determined by means of Monte Carlo calculation, the water kerma in the boundary layer is determined from specific charges determined at different measurement volumes in the absence of a cavity.

The advantage of this procedure is that the MC calculations have to take only the photon transport into account. In the case of the current arrangement in a graphite phantom, the dose determined has an expanded uncertainty (k = 2) of approx. 2-3 %.

This year, the extrapolation chamber in the water-equivalent material was completed and used to perform first measurements. For the measurements, four hard-filtered X-radiation qualities with mean energies between 15 keV and 33 keV were used.

For each radiation quality, the absorbed dose to water determined for different plate distances is almost equal. Compared to studies performed earlier, however, on an extrapolation chamber of the same design, clearly higher standard deviations of the measurement values have been obtained; these deviations must be attributed to leakage currents during the measurement which are by approximately one order of magnitude higher than expected. In spite of this limitation, agreements in the range of percentages resulted in comparison to water energy dose values determined via free-air chamber measurements of the air kerma and with the aid of Monte Carlo calculations. This means that the procedure with the new chamber can be applied. The next step in this project is now aimed at reducing the leakage current.

Figure : The newly developed extrapolation chamber in front of X-ray equipment.