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

Abstract

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.

Figure 1

Light detection portion of the plastic scintillation detector setup used in this study. The optical fiber was connected through a SMA connector and aligned to a red–green–blue photodiode (see inset). The blue and green outputs from the photodiode were connected via a triaxial cable to a double-input electrometer. The output of the electrometer was transmitted to a computer (not shown) via an RS-232 cable.

Figure 2

Positioning template (developed in-house) with the catheters, connected to transfer tubes, inserted for treatment delivery. The inserted plastic scintillation detector is covered with a black coating.

Figure 3

System of coordinates for the source and PSD positioning used in this study. The consecutive dwell positions were separated by 5 mm. The source (along with its cable) is represented in the second catheter from the left, while the PSD is shown in the middle catheter. The end of the PSD represents the scintillation component while the rest of it is the optical fiber. The stars represent the possible dwell positions along the catheters.

Figure 4

Acquired dose rate and integrated dose were measured as a function of time when the PSD was inserted (a) in the urethra and (b) at the rectum wall. The PSD was aligned to dwell position #9 for the urethra and position #7 for the rectum wall.

Figure 5

Dose measured in each catheter with the plastic scintillation detector in the urethra aligned to dwell position #9 and dose obtained from the planning system. Error bars for measured data were obtained from the standard deviation of three repeated measurements.

Figure 6

Ratio of the dose rate measured at different dwell positions over the treatment to the calculated dose rate from the planning system. The plastic scintillation detector was inserted in the urethra aligned to dwell position #9 and only positions with dwell times over 5 s were selected (total of 43 positions). Uncertainty on measurements was obtained using the standard deviation from ten consecutive data points (1 s span) for the same dwell position.

Figure 7

Dose rates measured (meas.) and planned (plan) for different plastic scintillation detector positions in the urethra mimicking positioning errors of 5 mm in either direction. Shown is the dose delivered in catheter #6.

Figure 8

Ratio of dose rate between the measured and planned values with and without chromatic removal of the stem effect. The plastic scintillation detector was inserted in the urethra aligned to dwell position #1, and the data shown here are for dose delivery in catheter #4. Uncertainty on measurements was calculated from the standard deviation from ten consecutive data points (1 s span) for the same dwell position.

Figure 9

Time elapsed for dose delivery in each of the 11 catheters for different plastic scintillation detector positions (pos.) in comparison with the expected time from the planning system. The error bars represent the standard deviation for three repeated deliveries.