Session Item

High atomic number (Z) inorganic scintillators have properties suitable for ¹⁹²Ir BT in vivo dosimetry. Compared to organic scintillators, they have greater light output and negligible stem-effect. However, high Z leads to larger absorbed-dose energy dependence which requires thorough characterization. Up to now, experimental determination is prevalent though measurement accuracy can be compromised by steep dose gradients. Additionally, the measured signal may be convolved with other detector response artifacts that are not accounted for. Monte Carlo (MC) circumvents such problems and allows for broader investigation of factors affecting the absorbed-dose energy dependence of detectors under clinically relevant conditions. Therefore, aiming to improve in vivo dosimetry accuracy, three inorganic scintillators, which have been used in practice and have suitable luminescence properties, were characterized with MC in this study.  

The focus was on ZnSe (Z=32), and for comparison, CsI (Z=54) and Al₂O₃ (Z=11) were included. A general-purpose MC code PENELOPE was used to evaluate detector absorbed-dose energy response relative to water and its dependence on scatter conditions (full and patient-like), as well as patient anatomy (pelvic bones and prostate calcifications). 

ZnSe and CsI overresponded substantially compared to water in a patient-like phantom, but the normalized curves, which would correspond to the absorbed-dose energy correction, did not differ between the two media despite large difference in their atomic numbers (Fig. 1). Under full-scatter conditions, ZnSe response increased by 10% at 5 cm compared to limited scatter conditions, whereas the response of Al₂O₃ did not depend on phantom size. Pelvic bones did not affect ZnSe response in a mimicked prostate treatment. However, it decreased by 2% when an intermediate-size calcification was between the source and the detector. Finally, comparison with high-precision experimental data of ZnSe:O response showed good agreement with the MC results (Fig. 2). 

Fig. 1: a) MC-calculated ratios of the average absorbed dose to the detector cavity and water as a function of radial distance; b) Values normalized at 2 cm distance.

Fig. 2: MC-calculated ratios of the average absorbed dose to ZnSe detector and absorbed dose to water as a function of radial distance. All values were normalized to the ratio at 2 cm distance.

Experimental determination of absorbed-dose energy dependence of high-Z detectors should be performed under patient-like scattering conditions instead of adhering to TG-43 formalism. Additionally, it must be accounted for that the dependence is a function of radial distance and polar angle. While in these aspects inorganic scintillators are disadvantageous compared to the organic ones, we show that MC methods can aid and complement their characterization under clinically relevant conditions. Thus, allowing to introduce new detectors into the field that may benefit ¹⁹²Ir BT in vivo dosimetry.