A phantom study of an in vivo dosimetry system using plastic scintillation detectors for real-time verification of ¹⁹²Ir HDR brachytherapy

Purpose: The goal of the present work was to evaluate the accuracy of a plastic scintillation detector (PSD) system to perform in-phantom dosimetry during ¹⁹²Ir high dose rate (HDR) brachytherapy treatments. Methods: A PSD system capable of stem effect removal was built. A red-green-blue photodiode connected to a dual-channel electrometer was used to detect the scintillation light emitted from a green scintillation component and transmitted along a plastic optical fiber. A clinically relevant prostate treatment plan was built using the HDR brachytherapy treatment planning system. An in-house fabricated template was used for accurate positioning of the catheters, and treatment delivery was performed in a water phantom. Eleven catheters were inserted and used for dose delivery from ¹⁹²Ir radioactive source, while two others were used to mimic dosimetry at the rectum wall and in the urethra using a PSD. The measured dose and dose rate data were compared to the expected values from the planning system. The importance of removing stem effects from in vivo dosimetry using a PSD during ¹⁹²Ir HDR brachytherapy treatments was assessed. Applications for dwell position error detection and temporal verification of the treatment delivery were also investigated. Results: In-phantom dosimetry measurements of the treatment plan led to a ratio to the expected dose of 1.003 0.004 with the PSD at different positions in the urethra and 1.043 0.003 with the PSD inserted in the rectum. Verification for the urethra of dose delivered within each catheter and at specific dwell positions led to average measured to expected ratios of 1.015 0.019 and 1.014 0.020, respectively. These values at the rectum wall were 1.059 0.045 within each catheter and 1.025 0.028 for specific dwell positions. The ability to detect positioning errors of the source depended of the tolerance on the difference to the expected value. A 5-mm displacement of the source was detected by the PSD system from 78 to 100 of the time depending on the acceptable range value. The implementation of a stem effect removal technique was shown to be necessary, particularly when calculating doses at specific dwell positions, and allowed decreasing the number of false-error detections-the detection of an error when it should not be the case-from 19 to 1 for a 5 threshold out of 43 measurements. The use of the PSD system to perform temporal verification of elapsed time by the source in each catheter-generally on the order of minutes-was shown to be in agreement within a couple of seconds with the treatment plan. Conclusions: We showed that the PSD system used in this study, which was capable of stem effect removal, can perform accurate dosimetry during ¹⁹²Ir HDR brachytherapy treatment in a water phantom. The system presented here shows some clear advantages over previously proposed dosimetry systems for HDR brachytherapy, and it has the potential for various online verifications of treatment delivery quality.