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Successful use of PTB’s water calorimeter in ion beams

21.04.2011

In the recent past, radiation therapy with ion beams (e.g. protons or 12C ions) has become of increasing importance. For this application, the ion beam is directed very precisely across the whole tumour volume by means of the so-called "raster scanning method". For carbon ions, this method has been developed at the Gesellschaft für Schwerionenforschung (GSI, Darmstadt) and is now used in therapy, among others, at the Heidelberg Ion-Beam Therapy Center (HIT). For dosimetry at these irradiation conditions, a suitable primary standard for determining the absorbed dose to water has so far not been available. In a first experiment, the transportable water calorimeter of PTB has, therefore, been used in a 300 MeV/u carbon beam at the GSI to demonstrate the suitability of the calorimetric measurement procedure.

Figure 1 :Photo of the transportable water calorimeter (on the right) with the associated cooling unit (on the left). The beam impinges horizontally on the beam entrance window of the calorimeter (in the red frame). The edge length of the - almost cubic - calorimeter amounts to approx. 60 cm.

For the measurements with the water calorimeter, a 12C pencil beam 9 mm in width was used. With the aid of the raster scanning method, a square field of about 50 mm × 50 mm in size was irradiated with 18 x 18 discrete points in the water phantom at a water depth of 5 cm. Due to the overlapping of the Gaussian intensity distributions of the single beam spots, an almost homogeneous dose distribution with a total dose of approx. 2.2 Gy is achieved. The duration of such an irradiation was approx. 80 s. During the beam time at the GSI, approx. 70 measurements could be performed with the water calorimeter. The results are shown in the figure below. The standard deviation of the distribution amounts to about 0.9 %. The mean value of the measured radiation-induced resistance change of the calorimetric detector was used to determine the absorbed dose to water DW.

For the determination of DW, the impacts of some influence quantities must be taken into account. These are, for example, heat conduction effects due to the dissolution of the original temperature distribution caused by the single beam spots, or possible impacts of the radiation-induced radiolysis of water. The heat conduction effects in the water calorimeter were simulated with the aid of finite-element calculations for the real time evolution of the irradiation measurements, i.e. the complete scanning process, including the "spill" structure of the ion beam, must be taken into account.

Figure 2 : Results of the measurement with the water calorimeter in the 300 MeV/u 12C ion beam. The figure shows the resistance change of the detector for each irradiation. The corresponding mean value was used to determine the absorbed dose to water DW.

Due to the operating mode of the accelerator, the ion beam is available in "spills" having a duration of approx. 2 s, with a pause of also approx. 2 s between successive "spills". On the basis of the finite-element calculations, the heat conduction effects in the water calorimeter can be corrected with a relative standard measurement uncertainty of about 0.4 %.

The results of the current investigation with the water calorimeter allow DW to be determined with a relative standard uncertainty of about 0.5 %. However, the assumption is made that a possible influence of the radiolysis on the result of the calorimetric determination of DW can also be neglected in the case of ion radiation. This assumption is supported by model calculations for the radiolysis of water, which predict that the so-called "heat defect" for the ultrapure water (in addition saturated with H2 gas) used in the detector of the calorimeter has the value "zero" also for radiation with a higher LET. It would be possible to verify the results of the model calculations for radiolysis in additional experiments using differently prepared detectors.

The measurements presented here were carried out at a water depth of 5 cm, i.e. in the plateau region of the depth dose curve of the 300 MeV/u beam. The therapeutically relevant region of the depth dose curve is, however, the so-called "Bragg peak" which - in this case - has a water depth of approx. 15 cm. Additional experiments with the water calorimeter will in future also be performed in the area of the spread-out Bragg peak (SOBP). It can be expected that the calorimetric determination of the absorbed dose to water DW within a standard uncertainty of less than 1 % will be possible. This could clearly reduce the measurement uncertainty of about 3 % which is usual today in reference dosimetry for radiation therapy with ion radiation.