Purpose: Source tracking and dose monitoring in HDR brachytherapy with a miniaturized fiber-integrated scintillator detector (MSD)
Methods: The MSD consists of a 0.23 mm large and 0.3 mm long detection cell (Gd2O2S:Tb) coupled to a 70-micron outer diameter optical fiber. The bare probe is tested in a water tank using a 8.2 Ci Ir-192 HDR source. The MSD is mounted onto a 2D motorized translation stage with micrometer accuracy. Detection signal is acquired at a rate of 0.08 s with a standard sCMOS camera coupled to a chromatic filter to cancel spurious Cerenkov signal. Dwell position and dwell time monitoring is then realized and compared to expected values from the treatment planning system for source-to-detector distances ranging from 6 to 60 mm. Measurements are realized for five different spacings between the probe axis and the source pathway (that are parallel from each other). The delivered dose and dose rate for each dwell position are deduced from the source-to-probe distance using the TG-43 dose calculation formalism.
Results: The MSD shows a linear response regarding the dose. The analysis of variance (ANOVA) was less than 2% for combined repeatability and reproducibly. The MSD was evaluated in terms of spatial and time source tracking for 966 dwell positions from 22 patient treatments. The standard deviation of the dwell position was always wihin 0.8 mm. Dwell times larger than 0.5 s were always detected (0.3 s dwell times were detected with a success pass rate of 86%). The mismatch between the measured and anticipated dose rates was always smaller than 5.0%. Our detector combines high spatial and temporal resolutions, high compactness, flexibility and ease of use.
Conclusion: MSD has the potential to become a valuable in-Vivo dosimetry system in HDR brachytherapy, measuring dwell position, dwell time and monitoring dose delivery.