Purpose: To use Monte Carlo (MC) simulations for the characterization of several existing high atomic number (Z) inorganic scintillator dosimeters designed for in vivo dosimetry of ¹⁹²Ir brachytherapy treatments.
Methods: An MC code system PENELOPE-2014 was used to determine the absorbed-dose energy dependence of ZnSe:O (Z(eff)= 32) and CsI:Tl (Z(eff)= 54) relative to water (D(det)⁄D(wat)) and the influence of scatter conditions which change between detector calibration, experimental characterization, and in vivo measurements. Additionally, the change in photon spectrum due to the pelvic bones and its impact on detector response was evaluated to mimic treatments in the pelvic area. Detector design influence was also investigated.
Results: ZnSe:O and CsI:Tl have a large absorbed-dose energy dependence relative to water (up to four and ten times, respectively). Its change over the radial distance range relevant for in vivo dosimetry was comparable for the two materials despite a large difference in their Z(eff). The D(det)⁄D(wat) dependence on phantom size was negligible up to 2 cm distance but increased to 14 % at 5 cm distance from the source. Neither detector design nor pelvic bones had influence on the detector response. Volume averaging was few percent.
Conclusion: Most of the experimental studies investigating high-Z detectors use water phantoms that provide full scatter which differ from the in vivo measurement conditions. Our work shows that the change in ZnSe:O and CsI:Tl detector response is not negligible and shall be evaluated to avoid erroneous absorbed dose to water determination. Similar conclusions would apply for other high-Z detector systems. Additionally, although the D(det)⁄D(wat) dependence is large for high-Z inorganic scintillators, it can be well-characterized and agrees with the experimental data for ZnSe:O. Without an MC study, such conclusions could not be drawn since the experimentally determined detector response could be affected by its scintillation properties.