Stem signal suppression in fiber-coupled Al2O3:C dosimetry for 192Ir brachytherapy

The stem signal, composed of fluorescence and Čerenkov light, becomes a significant source of uncertainty in fiber-coupled afterloaded brachytherapy dosimetry when the source dwells near the fiber cable but far from the detector. A stem suppression technique originally developed for scintillators was adapted for on-line in-vivo dosimetry using fiber-coupled carbon doped aluminum oxide (Al2O3:C). The technique involved a two-channel optical filtration of the radioluminescence (RL) emitted from a pre-irradiated Al2O3:C crystal with enhanced sensitivity. The system responded linearly in the absorbed dose range 0.05–50 Gy, as needed under high dose rate (HDR) conditions. The dosimeter was irradiated in a water phantom using a 37 GBq 192Ir source at source-to-crystal distances ranging from 0.5 cm to 6.7 cm. For irradiation conditions that generated a stem component in the range 4%–15% in the unfiltered signal, and up to 5 ± 1% in a single-channel read-out optimized for Al2O3:C, the adapted stem suppression technique reduced the stem component to −1 ± 3%. The stem suppressed dosimeter response and the 192Ir source depth dose curve agreed within position uncertainties, and with a 5% maximum deviation, for distances up to 6 cm.