Accuracy of an in vivo dosimetry‐based source tracking method for afterloading brachytherapy — A phantom study

Abstract

To report on the accuracy of an in vivo dosimetry (IVD)-based source tracking (ST) method for high dose rate (HDR) prostate brachytherapy (BT). Methods: The ST was performed on a needle-by-needle basis. A least square fit of the expected to the measured dose rate was performed using the active dwell positions in the given needle. Two fitting parameters were used to determine the position of each needle relative to the IVD detector: radial (away or toward the detector) and longitudinal (along the axis of the treatment needle). The accuracy of the ST was assessed in a phantom where the geometries of five HDR prostate BT treatments previously treated at our clinic were reproduced. For each of the five treatment geometries, one irradiation was performed with the detector placed in the middle of the implant. Furthermore, four additional irradiations were performed for one of the geometries where the detector was retracted caudally in four steps of 10-15 mm and up to 12 mm inferior of the most inferior active dwell position, which represented the prostate apex. The time resolved dose measurements were retrieved at a rate of 20 Hz using a detector based on an Al 2 O 3 :C radioluminescence crystal, which was placed inside a standard BT needle. Individual calibrations of the detector were performed prior to each of the nine irradiations.Results: Source tracking could be applied in all needles across all nine irradiations. For irradiations with the detector located in the middle region of the implant (a total of 89 needles), the mean AE standard deviation (SD, k = 1) accuracy was −0.01 mm AE 0.38 mm and 0.30 mm AE 0.38 mm in the radial and longitudinal directions, respectively. Caudal retraction of the detector did not lead to reduced accuracy as long as the detector was located superior to the most inferior active dwell positions in all needles. However, reduced accuracy was observed for detector positions inferior to the most inferior active dwell positions which corresponded to detector positions in and inferior to the prostate apex region. Detector positions in the prostate apex and 12 mm inferior to the prostate resulted in mean AE SD (k = 1) ST accuracy of 0.7 mm AE 1 mm and 2.8 mm AE 1.6 mm, respectively, in radial direction, and −1.7 mm AE 1 mm and −2.1 mm AE 1.1 mm, respectively, in longitudinal direction. The largest deviations for the configurations with those detector positions were 2.6 and 5.4 mm, respectively, in the radial direction and −3.5 and −3.8 mm, respectively, in the longitudinal direction. Conclusion: This phantom study demonstrates that ST based on IVD during prostate BT is adequately accurate for clinical use. The ST yields submillimeter accuracy on needle positions as long as the IVD detector is positioned superior to at least one active dwell position in all needles. Locations of the detector inferior to the prostate apex result in decreased ST accuracy while detector locations in the apex region and above are advantageous for clinical applications.

A high‐Z inorganic scintillator–based detector for time‐resolved in vivo dosimetry during brachytherapy

“…1 This has led to the development of complex real-time error-detection and treatment-verification algorithms. 2,3 Furthermore, time-resolved IVD has made it possible to perform source tracking [4][5][6][7] and dwell-time verification 7,8 during treatment. For these reasons, timeresolved IVD has great potential for providing clinical staff with valuable information during or after BT treatment.…”

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

“…Detector calibration method using clinical sources and its implications on in vivo detector characterization and dosimetry accuracy are further questioned in Paper IV. There we also address how phantom size can affect detector calibration in small water-equivalent phantoms that are employed for convenience (Kertzscher et al 2014a, Jørgensen et al 2021b. In-phantom scatter conditions are not the same in patients and the determined calibration coefficient may not be valid.…”

“…It was not the case for Al 2 O 3 which is more water-equivalent. Since neither detector response is affected by in-phantom scatter up to r 0 = 2 cm, pre-treatment constancy check and calibration of detectors in a limited scatter water-equivalent phantom could be performed as advised (Kertzscher et al 2014a, Jørgensen et al 2021b. The change in photon spectrum due to the presence of bones did not affect energy deposition in ZnSe for clinically relevant cases.…”

MaxiCalc: A tool for online dosimetric evaluation of source-tracking based treatment verification in HDR brachytherapy

“…Several groups with well-developed IVD systems have recognised the value of source tracking, either by measuring the individual 3D source positions or by measuring implant movements relative to the in vivo dosimeter. [4][5][6][7][8] Other groups have performed source tracking via other methods of directly measuring the source position with detectors including fluoroscopy, pinhole cameras, magnetic resonance, or 2D arrays. [3,[9][10][11][12][13][14] Some of the source tracking approaches have been demonstrated clinically, [3,7,9,15] or in principle [5,6,8,[10][11][12][13]16].…”

“…[4][5][6][7][8] Other groups have performed source tracking via other methods of directly measuring the source position with detectors including fluoroscopy, pinhole cameras, magnetic resonance, or 2D arrays. [3,[9][10][11][12][13][14] Some of the source tracking approaches have been demonstrated clinically, [3,7,9,15] or in principle [5,6,8,[10][11][12][13]16].…”