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Calorimetric determination of kQ factors for ionization chambers in photon radiation fields of small size


The ionization chambers used in dosimetry for radiation therapy are usually calibrated at 60Co radiation in the measurand absorbed dose to water. If used at other radiation qualities, e.g. at higher photon energies, the change of the response of the ionization chamber is taken into account by means of the energy-dependent kQ factor. Both - the calibration factor and the kQ factor - relate to a field size of 10 cm x 10 cm. Modern irradiation devices, e.g. medical linacs for IMRT, tomotherapy or cyberknife devices use, however, considerably smaller radiation fields. For dosimetry under these radiation conditions, it must be ensured that the original kQ factors of the ionization chambers used are still valid. With the aid of a water calorimeter, the energy-dependent kQ factors of ionization chambers for the photon radiation qualities 6 MVx and 10 MVx were determined for the first time for field sizes of up to 3 cm x 3 cm.

The water calorimeter used is operated at a water temperature of 4 °C and is largely identical in construction to the calorimeter used at PTB as a primary standard measuring device for the realization of the unit of absorbed dose to water at 60Co radiation. For the determination of the kQ factors of ionization chambers of the type NE2561, the water calorimeter was used at PTB’s medical linacs at photon radiation qualities of 6 MVx and 10 MVx. The field size of 3 cm x 3 cm was realized with the aid of a lead collimator which was installed in front of the radiation head of the accelerator.

Figure : Water calorimeter in front of the radiation head of PTB’s medical linac. The radiation entrance window of the calorimeter is in the red square.

The experimental determination of the kQ factors is carried out in two steps. First, the absorbed dose to water is determined at the point of measurement in a water phantom at a depth of 10 cm with the aid of the water calorimeter at a photon radiation quality Q. After that, an ionization chamber is placed into the water phantom at the same point of measurement - with the calorimeter open and at room temperature - and calibrated under identical irradiation conditions. The kQ factor is obtained as the ratio of the calibration factors at the radiation quality Q and at 60Co radiation.

For the determination of the absorbed dose to water, the water calorimeter is able to measure very precisely the temperature increases of approx. 0.5 mK which are caused by the radiation absorption. For the measurements in the 3 cm x 3 cm radiation field, heat conduction effects - which are, in particular, due to the dissolving of the temperature distribution correlating with the radiation field - are to be taken into account. The influence of these heat conduction effects on the determination of the absorbed dose to water is calculated with the Finite Element Method and corrected. For that purpose, the entire temperature profile in the water calorimeter must be simulated during and after each single irradiation and compared with the measured temperature profile. The resulting corrections amount to up to 4 % of the measured temperature increase.

The relative standard measurement uncertainty of the kQ factors, which have been determined by calorimetry for the NE2561 ionization chamber in the 3 cm x 3 cm field at the radiation qualities 6 MVx and 10 MVx, amounts to approx. 0.4 %. The comparison of the kQ factors with those in the 10 cm x 10 cm radiation field does not show, however, any dependence on the field size. At least up to field sizes of 3 cm x 3 cm, the kQ factors so far used can still be applied.