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Rossi G, Failing T, Gainey M, Kollefrath M, Hensley F, Zink K, Baltas D. Determination of the dose rate around a HDR 192Ir brachytherapy source with the microDiamond and the microSilicon detector. Z Med Phys 2023; 33:463-478. [PMID: 36038432 PMCID: PMC10751698 DOI: 10.1016/j.zemedi.2022.07.004] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2022] [Revised: 07/23/2022] [Accepted: 07/25/2022] [Indexed: 12/01/2022]
Abstract
PURPOSE To employ the microDiamond and the microSilicon detector (mDD and mSD, both PTW-Freiburg, Germany) to determine the dose rate around a HDR 192Ir brachytherapy source (model mHDR-v2r, Elekta AB, Sweden). METHODS The detectors were calibrated with a 60Co beam at the PTW Calibration Laboratory. Measurements around the 192Ir source were performed inside a PTW MP3 water phantom. The detectors were placed at selected points of measurement at radial distances r, ranging from 0.5 to 10 cm, keeping the polar angle θ = 90°. Additional measurements were performed with the mSD at fixed distances r = 1, 3 and 5 cm, with θ varying from 0 to 150°, 0 to 166°, and 0 to 168°, respectively. The corresponding mDD readings were already available from a previous work (Rossi et al., 2020). The beam quality correction factor of both detectors, as well as a phantom effect correction factor to account for the difference between the experimental geometry and that assumed in the TG-43 formalism, were determined using the Monte Carlo (MC) toolkit EGSnrc. The beam quality correction factor was factorized into energy dependence and volume-averaging correction factors. Using the abovementioned MC-based factors, the dose rate to water at the different points of measurement in TG-43 conditions was obtained from the measured readings, and was compared to the dose rate calculated according to the TG-43 formalism. RESULTS The beam quality correction factor was considerably closer to unity for the mDD than for the mSD. The energy dependence of the mDD showed a very weak radial dependence, similar to the previous findings showing a weak angular dependence as well (Rossi et al., 2020). Conversely, the energy dependence of the mSD decreased significantly with increasing distances, and also showed a considerably more pronounced angular dependence, especially for the smallest angles. The volume-averaging showed a similar radial dependence for both detectors: the correction had a maximal impact at 0.5 cm and then approached unity for larger distances, as expected. Concerning the angular dependence, the correction for the mSD was also similar to the one previously determined for the mDD (Rossi et al., 2020): a maximal impact was observed at θ = 0°, with values tending to unity for larger angles. In general, the volume-averaging was less pronounced for the mSD due to the smaller sensitive volume radius. After the application of the MC-based factors, differences between mDD dose rate measurements and TG-43 dose rate calculations ranged from -2.6% to +4.3%, with an absolute average difference of 1.0%. For the mSD, the differences ranged from -3.1% to +5.2%, with an absolute average difference of 1.0%. For both detectors, all differences but one were within the combined uncertainty (k = 2). The differences of the mSD from the mDD ranged from -3.9% to +2.6%, with the vast majority of them being within the combined uncertainty (k = 2). For θ ≠ 0°, the mDD was able to provide sufficiently accurate results even without the application of the MC-based beam quality correction factor, with differences to the TG-43 dose rate calculations from -1.9% to +3.4%, always within the combined uncertainty (k = 2). CONCLUSION The mDD and the mSD showed consistent results and appear to be well suitable for measuring the dose rate around HDR 192Ir brachytherapy sources. MC characterization of the detectors response is needed to determine the beam quality correction factor and to account for energy dependence and/or volume-averaging, especially for the mSD. Our findings support the employment of the mDD and mSD for source QA, TPS verification and TG-43 parameters determination.
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Affiliation(s)
- Giulio Rossi
- Division of Medical Physics, Department of Radiation Oncology, Medical Center, Faculty of Medicine, University of Freiburg, Germany; German Cancer Consortium (DKTK) Partner Site Freiburg, German Cancer Research Center (DKFZ), Heidelberg, Germany.
| | - Thomas Failing
- University of Applied Sciences Giessen, Institute of Medical Physics and Radiation Protection, Giessen, Germany; University Medical Center Göttingen, Department of Radiation Oncology, Göttingen, Germany
| | - Mark Gainey
- Division of Medical Physics, Department of Radiation Oncology, Medical Center, Faculty of Medicine, University of Freiburg, Germany; German Cancer Consortium (DKTK) Partner Site Freiburg, German Cancer Research Center (DKFZ), Heidelberg, Germany
| | - Michael Kollefrath
- Division of Medical Physics, Department of Radiation Oncology, Medical Center, Faculty of Medicine, University of Freiburg, Germany; German Cancer Consortium (DKTK) Partner Site Freiburg, German Cancer Research Center (DKFZ), Heidelberg, Germany
| | - Frank Hensley
- University Hospital of Heidelberg, Department of Radiation Oncology, Heidelberg, Germany
| | - Klemens Zink
- University of Applied Sciences Giessen, Institute of Medical Physics and Radiation Protection, Giessen, Germany; University Medical Center Giessen-Marburg, Department of Radiotherapy and Radiation Oncology, Marburg, Germany; Marburg Ionbeam Therapycenter (MIT), Marburg, Germany
| | - Dimos Baltas
- Division of Medical Physics, Department of Radiation Oncology, Medical Center, Faculty of Medicine, University of Freiburg, Germany; German Cancer Consortium (DKTK) Partner Site Freiburg, German Cancer Research Center (DKFZ), Heidelberg, Germany
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Kaveckyte V, Jørgensen EB, Kertzscher G, Johansen JG, Tedgren ÅC. Monte Carlo characterization of high atomic number inorganic scintillators for in vivo dosimetry in 192 Ir brachytherapy. Med Phys 2022; 49:4715-4730. [PMID: 35443079 DOI: 10.1002/mp.15674] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2021] [Revised: 03/01/2022] [Accepted: 04/06/2022] [Indexed: 11/05/2022] Open
Abstract
BACKGROUND There is increased interest in vivo dosimetry for 192 Ir brachytherapy (BT) treatments using high atomic number (Z) inorganic scintillators. Their high light output enables construction of small detectors with negligible stem effect and simple readout electronics. Experimental determination of absorbed-dose energy dependence of detectors relative to water is prevalent, but it can be prone to high detector positioning uncertainties and does not allow for decoupling of absorbed-dose energy dependence from other factors affecting detector response. PURPOSE To investigate which measurement conditions and detector properties could affect their absorbed-dose energy dependence in BT in vivo dosimetry. METHODS We used a general-purpose MC code penelope for the characterization of high-Z inorganic scintillators with the focus on ZnSe (Z¯=32). Two other promising media CsI (Z¯=54) and Al2 O3 (Z¯=11) were included for comparison in selected scenarios. We determined absorbed-dose energy dependence of crystals relative to water under different scatter conditions (calibration phantom 12 × 12 × 30 cm3 , characterization phantoms 20 × 20 × 20 cm3 , 30 × 30 × 30 cm3 , 40 × 40 × 40 cm3 , and patient-like elliptic phantom 40 × 30 × 25 cm3 ). To mimic irradiation conditions during prostate treatments, we evaluated whether the presence of pelvic bones and calcifications affect ZnSe response. ZnSe detector design influence was also investigated. RESULTS In contrast to low-Z organic and medium-Z inorganic scintillators, ZnSe and CsI media have substantially greater absorbed-dose energy dependence relative to water. The response was phantom-size dependent and changed by 11 % between limited- and full-scatter conditions for ZnSe, but not for Al2 O3 . For a given phantom size, a part of the absorbed-dose energy dependence of ZnSe is caused not due to in-phantom scatter but due to source anisotropy. Thus, the absorbed-dose energy dependence of high-Z scintillators is a function of not only the radial distance but also the polar angle. Pelvic bones did not affect ZnSe response, whereas large and intermediate size calcifications reduced it by 9 % and 5 %, respectively, when placed midway between the source and the detector. CONCLUSIONS Unlike currently prevalent low- and medium-Z scintillators, high-Z crystals are sensitive to characterization and in vivo measurement conditions. However, good agreement between MC data for ZnSe in the present study and experimental data for ZnSe:O by Jørgensen et al (2021) suggest that detector signal is proportional to the average absorbed dose to the detector cavity. This enables an easy correction for non-TG43-like scenarios (e.g., patient sizes and calcifications) through MC simulations. Information that should be provided to the clinic by the detector vendors. This article is protected by copyright. All rights reserved.
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Affiliation(s)
- Vaiva Kaveckyte
- Department of Health, Medicine and Caring Sciences, Linköping University, Linköping, SE-581 85, Sweden
| | - Erik B Jørgensen
- Department of Clinical Medicine, Aarhus University, Aarhus, DK-8000, Denmark.,Department of Oncology, Aarhus University Hospital, Aarhus, DK-8000, Denmark
| | - Gustavo Kertzscher
- Department of Oncology, Aarhus University Hospital, Aarhus, DK-8000, Denmark
| | - Jacob G Johansen
- Department of Clinical Medicine, Aarhus University, Aarhus, DK-8000, Denmark.,Department of Oncology, Aarhus University Hospital, Aarhus, DK-8000, Denmark
| | - Åsa Carlsson Tedgren
- Department of Health, Medicine and Caring Sciences, Linköping University, Linköping, SE-581 85, Sweden.,Department of Medical Radiation Physics and Nuclear Medicine, Karolinska University Hospital, Stockholm, SE-171 76, Sweden.,Department of Oncology-Pathology, Karolinska Institute, Stockholm, SE-171 76, Sweden
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