ESTRO 2024 - Abstract Book

S332

Beachytherapy - Physics

ESTRO 2024

Thorsten Schneider, Angelika Kasper, Georg Winterbottom

PTB, 6.3, Braunschweig, Germany

Purpose/Objective:

The emitted radiation field and the dosimetric properties of LDR brachytherapy photon sources depend on the nuclide and the design of the seeds. BRAPHYQS and GEC-ESTRO encourage promotion of an efficient solution in Europe to monitor and assure seed design constancy, preferably like the CLA2004 - methodology controlled by AAPM. In Europe, Aspects of the Quality Assurance (QA) in the production of seeds, including the constancy of the sources, are not covered by the calibration of the seeds but by the process of CE-certification. Thereby, the company’s QA system must be outlined, and the effectiveness and permanent improvement must be demonstrated. Low energy photon spectrometry is a very sensitive tool to monitor the constancy of the radiation field of the seeds. However, establishing and maintaining a spectrometry unit requires considerable effort and such services are scarce within the brachytherapy community. The national metrology institute of Germany has decided to expand its calibration service for LDR seeds to include the determination of photon fluence spectra at specific polar and azimuthal angles around the source from 2025 on. The inclusion of spectrometry into regular calibration service requires highly automated spectra measurement and data evaluation.

Material/Methods:

A commercially available detector system from Canberra (HPGe) is used controlled by the Genie 2000™ software program.

To suppress stray radiation, the detector’s crystal is fully surrounded by a cylindrical steel cylinder. Collimators of 10 mm, 5 mm, 3 mm and 0.9 mm diameter are available. The detector’s efficiency was calibrated by means of radioactivity standards with an uncertainty of 3% (k=1) above 15 keV and 5% (k=1) below. The intensity of the discrete X-ray lines is analysed by the “GUPIX” X-ray spectra-analysing software package. The source holder is mounted to a computer-interfaced rotary stage, thus providing angular control while maintaining the proper orientation of the collimator axis to the source. In this way, spectra for a specific angle, as well as integral spectra, can be determined.

Results:

In the figure below the step of automated peak detection is exemplify with a spectrum of an I-125 Bebig S17 seed. The upper panel shows the whole spectrum. The lower panel is an enlargement of the region up to 20 keV.