[Practical dosimetry and constancy check at introduction of intraoperative radiotherapy with Intrabeam (Zeiss)]

The check of dosimetry of the intraoperative radiotherapy system Intrabeam is predefined by the manufacture (Zeiss). The purpose of the study was to develop and implement a method to verify the internal dosimetry of Intrabeam (Zeiss). Additionally the long-term stability of Intrabeam was checked for dose and isotropy. For dose to water measurements an Unidos was combined with a soft jet chamber (TM 23342) which was calibrated in water absorbed dose and as a phantom the type 2962 (PTW Freiburg) was used. RW1 plates were inserted as build up material. The applicators were placed in a bag filled with water to consider the side-scattering. At the surface of the applicator there was a mean difference of 3 percent between the dose to water measurement and the internal dosimetry. The constancy of the dose rate showed a mean deviation of 0.3% at the reference point. The analysis of the dose distribution perpendicular to the applicator axis z (reference z-axis) resulted in a mean deviation of -2.7% (x-direction) and -7,1% (-x-direction) for the x-axis and, respectively -4.1% (y-direction) and -5.3% (-y-direction) for the y-axis. The proposed method is suitable to verify the absolute dose of Intrabeam. The dose values measured by this method were congruent to the dosimetry of the manufacture (Zeiss). From our point of view it is sufficient to verify the absolute dosimetry only at time of commissioning of the system or in the case of changing the applicator. For the daily routine the check of constancy specified by the manufacture is adequate, because the dose rate is checked on a daily basis. Additionally the test of constancy showed a high long-term stability in terms of dose rate and symmetry.