[Measurement of the distance of the rectal mucosa from a gynecologic afterloading applicator based on the drop in performance of a cobalt-60-source].

Purpose: Assessment of the distance from a gynecological afterloading applicator to the rectum mucosa.

Methods: The appropriate afterloading applicator for HDR-Ir-therapy is used for that purpose. A Co-60-source (370 MBq) with a guiding wire is positioned at the tip of the applicator and then withdrawn in steps of 1 cm. For each of these points the maximum dose at the rectum mucosa is measured with an ionisation chamber. From a reference curve it is then possible to determine the distance from the rectum mucosa to the applicator and to mark it in the calculated isodose plan. The actual daily dose to the rectum mucosa is planned not to exceed 3.0 Gy in the maximum.

Results: Up to now the measurements were performed in 91 patients. The minimal distances of the rectum mucosa from the applicator were found to range from 5 to 18 mm with an average value of 14 mm.

Conclusions: The comparison of the measurement with the readings of the rectum probe during each session showed, that the method is suitable to avoid an overdose at the rectum mucosa.