Radioactivity measurements of ytterbium-169 brachytherapy sources

Ytterbium-169 is being considered as a new radiation source for brachytherapy applications. This radioisotope emits photons with energies ranging from 50 to 308 keV (average energy 93 keV) and decays with a half life of 32 days. For these reasons, it is believed to offer radiological and radiobiological advantages over some other isotopes currently in use. One impediment to widespread clinical use of this isotope is the determination of source strength in units of air kerma rate [cGy h-1]. The source strength can be measured directly with an ion chamber or calculated indirectly from the source radioactivity [Bq] with corrections for encapsulation. Our attempts to reconcile these two approaches have led to the development of a spectrometric technique for determining the radioactivity of ytterbium-169 brachytherapy seeds. A High Purity Germanium (HPGe) spectrometer is used to count the 307.7 keV photon emitted within a defined solid angle. The intrinsic photopeak efficiency of the detector was determined by Monte Carlo simulation followed by experimental verification with an activity-calibrated europium-152 source. Finally, the HPGe system has been used to calibrate a re-entrant ionisation chamber, allowing routine of Ytterbium source activity for clinical applications.