Gynecological applicator instrumented with GaN dosimetric probes for HDR brachytherapy

Abstract For independent in-vivo quality control (QC) in high-dose-rate brachytherapy (HDR-BT), a gynecological applicator instrumented with integration of GaN dosimetric probes is proposed. With association of a photodetection module and a PC, it allows real-time monitoring of dwell position, dwell time. Each employed GaN probe basically consists of a small-volume GaN crystal as radioluminescence (RL) transducer. Its response as a function of source-GaN distance is exploited for determining the treatment parameters of the afterloader. The characteristics of the GaN probe for this application were investigated by both measurements and Monte Carlo (MC) simulations. It was shown that the GaN probe's dosimetric response is dose-rate independent. By increasing the source-GaN distance, the energy spectrum at the GaN location changes with more significant attenuations on high-energy spectral lines and increased low-energy, wide-band contribution from scattered particles. Accordingly, the GaN overresponse (compared to referenced ionization chamber dosimetry) is more pronounced. However, there is no need to compensate this overresponse effect for this application as the GaN probe's dosimetric response is calibrated and used as reference. On the other hand, the GaN probe's response exhibits a temperature dependence, which is linear for the range from 18 °C to 40 °C with a slope of −0.4%/K. There is a shadow effect of GaN transducer on dose distribution with 1% decrease behind it. The instrumented applicator was tested in clinical conditions on both referenced treatment protocol and deliberately-introduced erroneous protocols. A modified gamma index (MGI) with MGI = 1 threshold was employed to indicate treatment delivery errors. All the introduced errors in dwell position and dwell time exceeding the acceptable criteria were detected.