Characterization of microMOSFET detectors for in vivo dosimetry in high‐dose‐rate brachytherapy with ¹⁹²Ir

Abstract

Purpose: The objective of this study was to characterize the Best Medical Canada microMOSFET detectors for their application in in vivo dosimetry for high-dose-rate brachytherapy (HDRBT) with 192 Ir. We also developed a mathematical model to correct dependencies under the measurement conditions of these detectors. Methods: We analyzed the linearity, reproducibility, and interdetector variability and studied the microMOSFET response dependence on temperature, source-detector distance, and angular orientation of the receptor with respect to the source. The correction model was applied to 19 measurements corresponding to five simulated treatments in a custom phantom specifically designed for this purpose. Results: The detectors (high bias applied in all measurements) showed excellent linearity up to 160 Gy. The response dependence on source-detector distance varied by (8.65 AE 0.06)% (k = 1) for distances between 1 and 7 cm, and the variation with temperature was (2.24 AE 0.05)% (k = 1) between 294 and 310 K. The response difference due to angular dependence can reach (10.3 AE 1.3)% (k = 1). For the set of measurements analyzed, regarding angular dependences, the mean difference between administered and measured doses was À4.17% (standard deviation of 3.4%); after application of the proposed correction model, the mean difference was À0.1% (standard deviation of 2.2%). For the treatments analyzed, the average difference between calculations and measures was 4.7% when only the calibration coefficient was used, but it is reduced to 0.9% when the correction model is applied. Conclusion: Important response dependencies of microMOSFET detectors used for in vivo dosimetry in HDRBT treatments, especially the angular dependence, can be adequately characterized by a correction model that increases the accuracy of this system in clinical applications.

Monte Carlo characterization of high atomic number inorganic scintillators for in vivo dosimetry in 192Ir brachytherapy

“…Majority of experimental studies characterizing detectors for in vivo dosimetry use full scatter water phantoms to match absorbed dose to water calculations in TG‐43 based treatment planning systems 18–21 . However, results in Figure 4 show that the absorbed‐dose energy correction function for high‐ Z inorganic scintillators, such as ZnSe, changes depending on the phantom size.…”

“…For comparison, the signal of other medium-Z detectors, such as silicon-based diodes and MOSFETs investigated for HDR 192 Ir BT in vivo dosimetry, is up to 6% cm −1 . 3,21 However, it is interesting to note that although the mean atomic number Z of CsI is nearly twice as large as that of ZnSe, the change in the relative absorbeddose energy dependence as a function of r 0 is nearly the same at distances greater than 2 cm. As seen in Figure 3, the slope of the total mass energy-absorption coefficient as a function of photon energy is approximately the same for CsI and ZnSe down to 33-36 keV photon energies, which correspond to the K-edges of CsI.…”

“…We previously showed that for the examined case of ZnSe, the volume averaging effect is on the order of few percent,and detector encapsulation did not affect energy deposition regardless of the polar angle. For comparison, Ruiz-Arrebola et al 21 determined polar-and azimuthal-angle dependence of up to 10% for silicon-based MOSFET in vivo dosimeters in HDR 192 Ir irradiation.The dependence was due to the complicated MOSFET detector design itself and the source anisotropy effect on the absorbed-dose energy dependence of the detector is unknown. displays the absorbed-dose energy correction function for the same ZnSe:O scintillator-type detector in a 30 × 30 × 30 cm 3 polymer-based phantom determined experimentally by Kertzscher and Beddar.…”

Development of Experimental Brachytherapy Dosimetry Using Monte Carlo Simulations for Detector Characterization

“…A reference position for detector calibration is usually chosen at around 1-2 cm sourceto-detector distance as a compromise between uncertainties related to detector positioning and signal strength. Given a rigid setup and well-controlled measurement conditions, detector positioning uncertainty of 0.2-0.5 mm on the transversal place can be achieved, but due to steep dose gradients close to 192 Ir sources, it would still translate to the uncertainty of around 4% (k = 2) in the absorbed dose to water (Andersen et al 2009, Ruiz-Arrebola et al 2020. If there is a discrepancy between the experimentally determined and TPScalculated absorbed dose to water values, it is disguised in the detector positioning uncertainty.…”

Light-Dependent Resistors as Dosimetric Sensors in Radiotherapy

“…These devices take advantage of their small size, comparatively reduced cost and lower or no bias voltage requirements. The use of semiconductor electronic devices for in vivo dosimetry has a long tradition in the field of radiotherapy [7]. In most cases, devices specifically designed for dosimetry are usually manufactured using specialized procedures to provide them with high sensitivity to radiation.…”