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Calibration of the Al2O3:C optically stimulated luminescence (OSL) signal for linear energy transfer (LET) measurements in therapeutic proton beams
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Phys. Med. Biol. 59 4295 (http://iopscience.iop.org/0031-9155/59/15/4295) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 132.174.254.159 This content was downloaded on 22/05/2015 at 18:00
Please note that terms and conditions apply.
Institute of Physics and Engineering in Medicine Phys. Med. Biol. 59 (2014) 4295–4310
Physics in Medicine & Biology doi:10.1088/0031-9155/59/15/4295
Calibration of the Al2O3:C optically stimulated luminescence (OSL) signal for linear energy transfer (LET) measurements in therapeutic proton beams Dal A Granville1, Narayan Sahoo2 and Gabriel O Sawakuchi2 1
Carleton Laboratory for Radiotherapy Physics, Carleton University, 1125 Colonel By Drive, Ottawa, ON, K1S 5B6, Canada 2 Department of Radiation Physics, The University of Texas, MD Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX 77030, USA E-mail: [email protected] and [email protected] Received 7 May 2014, revised 17 June 2014 Accepted for publication 18 June 2014 Published 16 July 2014 Abstract
Optically stimulated luminescence (OSL) detectors (OSLDs) have shown potential for measurements of linear energy transfer (LET) in proton therapy beams. However, the technique lacks the efficiency needed for clinical implementation, and a faster, simpler approach to LET measurements is desirable. The goal of this work was to demonstrate and evaluate the potential of calibrating Al2O3:C OSLDs for LET measurements using new methods. We exposed batches of OSLDs to unmodulated proton beams of varying LET and calibrated three parameters of the resulting OSL signals as functions of fluence-averaged LET (ϕ-LET) and dose-averaged LET (D-LET). These three parameters included the OSL curve shape evaluated under continuous wave stimulation (CW-OSL), the OSL curve shape evaluated under pulsed stimulation (P-OSL), and the intensity ratio of the two main emission bands in the Al2O3:C OSL emission spectrum (ultraviolet [UV]/blue ratio). To test the calibration, we then irradiated new batches of OSLDs in modulated proton beams of varying LET, and used the OSL signal parameters to calculate ϕ-LET and D-LET under these new test conditions. Using the P-OSL curve shape, D-LET was measured within 5.7% of the expected value. We conclude that from a single 10 s readout (following initial calibration), both the absorbed dose and LET in proton therapy beams can be measured using OSLDs. This has potential future applications in the quality assurance of proton therapy treatment plans, particularly for those that may account for LET or relative biological effectiveness in their optimization. The methods demonstrated in this work may also be applicable to other particle therapy beams, including carbon ion beams. 0031-9155/14/154295+16$33.00 © 2014 Institute of Physics and Engineering in Medicine Printed in the UK & the USA 4295
D A Granville et al
Phys. Med. Biol. 59 (2014) 4295
Keywords: proton therapy, optically stimulated luminescence, linear energy transfer, Al2O3:C (Some figures may appear in colour only in the online journal) 1. Introduction In radiation therapy, treatment plans are typically optimized based on the absorbed dose distribution. However, for the same absorbed dose, the biological damage resulting from radiation therapy can differ depending on the radiation quality. The relative biological effectiveness (RBE) is a quantity that describes how effectively a given radiation quality causes biological damage. Thus, the RBE-weighted absorbed dose (ICRU 2007) is a quantity that can better describe the biological implications of a radiation therapy treatment than the absorbed dose alone. The RBE of megavoltage electrons and photons is unity, which means that the RBEweighted dose and the absorbed dose are equivalent for these particles. However, the RBE of hadrons (such as protons and carbon ions) is dependent on the linear energy transfer (LET) of the particle. Thus, knowledge of both the LET and absorbed dose is necessary to describe the biological implications of a hadron therapy treatment. Because of this, the use of RBE or LET in hadron therapy treatment plan optimization has been the subject of a number of studies (Krämer and Scholz 2000, Wilkens and Oelfke 2006, Grassberger et al 2011, Giantsoudi et al 2013). A radiation detector that can be used to perform simple, routine measurements of both absorbed dose and LET would be useful for quality assurance of such plans. Currently, no such detector exists. Many devices have demonstrated the ability to measure LET, such as plastic nuclear track detectors (PNTDs), thermoluminescent detectors (TLDs), tissue-equivalent proportional counters, and semiconductor devices (Sawakuchi et al 2010). However, a number of impracticalities and limitations exist that may limit their potential for routine clinical use. PNTDs are suited for measurements of LET above 5 keV μm−1 in water (Benton et al 2002), which is higher than the values typically found in proton therapy. The LET dependent characteristics of TLD glow curves are strongly influenced by a number of other factors, including detector batch, preparation and absorbed dose (Bilski 2006). Due to their size, tissue-equivalent proportional counters cannot provide good spatial resolution (Bradley et al 2001), and while semiconductor detectors have shown promise for LET measurements (Wroe et al 2009), their routine clinical use has not yet been demonstrated. In addition to these detectors, polymer gel dosimeters have recently shown an LET dependence that may be useful for future clinical applications (Lopatiuk-Tirpak et al 2012). Optically stimulated luminescence (OSL) detectors (OSLDs) have been well studied for dosimetric applications in radiation therapy (Aznar et al 2004, Edmund et al 2007, Jursinic 2007, Schembri and Heijmen 2007, Sawakuchi et al 2008a, Viamonte et al 2008, Reft 2009, Kerns et al 2011, 2012, Mrčela et al 2011, Omotayo et al 2012). Aluminum oxide doped with carbon (Al2O3:C) is a particularly common OSL material. There are two main bands in the OSL emission spectrum of Al2O3:C; one centered in the blue region of the visible spectrum (~420 nm; lifetime ~35 ms) (Akselrod et al 1998) and another centered in the ultraviolet (UV) region (~330 nm; lifetime
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Calibration of the Al2O3:C optically stimulated luminescence (OSL) signal for linear energy transfer (LET) measurements in therapeutic proton beams
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Phys. Med. Biol. 59 4295 (http://iopscience.iop.org/0031-9155/59/15/4295) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 132.174.254.159 This content was downloaded on 22/05/2015 at 18:00
Please note that terms and conditions apply.
Institute of Physics and Engineering in Medicine Phys. Med. Biol. 59 (2014) 4295–4310
Physics in Medicine & Biology doi:10.1088/0031-9155/59/15/4295
Calibration of the Al2O3:C optically stimulated luminescence (OSL) signal for linear energy transfer (LET) measurements in therapeutic proton beams Dal A Granville1, Narayan Sahoo2 and Gabriel O Sawakuchi2 1
Carleton Laboratory for Radiotherapy Physics, Carleton University, 1125 Colonel By Drive, Ottawa, ON, K1S 5B6, Canada 2 Department of Radiation Physics, The University of Texas, MD Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX 77030, USA E-mail: [email protected] and [email protected] Received 7 May 2014, revised 17 June 2014 Accepted for publication 18 June 2014 Published 16 July 2014 Abstract
Optically stimulated luminescence (OSL) detectors (OSLDs) have shown potential for measurements of linear energy transfer (LET) in proton therapy beams. However, the technique lacks the efficiency needed for clinical implementation, and a faster, simpler approach to LET measurements is desirable. The goal of this work was to demonstrate and evaluate the potential of calibrating Al2O3:C OSLDs for LET measurements using new methods. We exposed batches of OSLDs to unmodulated proton beams of varying LET and calibrated three parameters of the resulting OSL signals as functions of fluence-averaged LET (ϕ-LET) and dose-averaged LET (D-LET). These three parameters included the OSL curve shape evaluated under continuous wave stimulation (CW-OSL), the OSL curve shape evaluated under pulsed stimulation (P-OSL), and the intensity ratio of the two main emission bands in the Al2O3:C OSL emission spectrum (ultraviolet [UV]/blue ratio). To test the calibration, we then irradiated new batches of OSLDs in modulated proton beams of varying LET, and used the OSL signal parameters to calculate ϕ-LET and D-LET under these new test conditions. Using the P-OSL curve shape, D-LET was measured within 5.7% of the expected value. We conclude that from a single 10 s readout (following initial calibration), both the absorbed dose and LET in proton therapy beams can be measured using OSLDs. This has potential future applications in the quality assurance of proton therapy treatment plans, particularly for those that may account for LET or relative biological effectiveness in their optimization. The methods demonstrated in this work may also be applicable to other particle therapy beams, including carbon ion beams. 0031-9155/14/154295+16$33.00 © 2014 Institute of Physics and Engineering in Medicine Printed in the UK & the USA 4295
D A Granville et al
Phys. Med. Biol. 59 (2014) 4295
Keywords: proton therapy, optically stimulated luminescence, linear energy transfer, Al2O3:C (Some figures may appear in colour only in the online journal) 1. Introduction In radiation therapy, treatment plans are typically optimized based on the absorbed dose distribution. However, for the same absorbed dose, the biological damage resulting from radiation therapy can differ depending on the radiation quality. The relative biological effectiveness (RBE) is a quantity that describes how effectively a given radiation quality causes biological damage. Thus, the RBE-weighted absorbed dose (ICRU 2007) is a quantity that can better describe the biological implications of a radiation therapy treatment than the absorbed dose alone. The RBE of megavoltage electrons and photons is unity, which means that the RBEweighted dose and the absorbed dose are equivalent for these particles. However, the RBE of hadrons (such as protons and carbon ions) is dependent on the linear energy transfer (LET) of the particle. Thus, knowledge of both the LET and absorbed dose is necessary to describe the biological implications of a hadron therapy treatment. Because of this, the use of RBE or LET in hadron therapy treatment plan optimization has been the subject of a number of studies (Krämer and Scholz 2000, Wilkens and Oelfke 2006, Grassberger et al 2011, Giantsoudi et al 2013). A radiation detector that can be used to perform simple, routine measurements of both absorbed dose and LET would be useful for quality assurance of such plans. Currently, no such detector exists. Many devices have demonstrated the ability to measure LET, such as plastic nuclear track detectors (PNTDs), thermoluminescent detectors (TLDs), tissue-equivalent proportional counters, and semiconductor devices (Sawakuchi et al 2010). However, a number of impracticalities and limitations exist that may limit their potential for routine clinical use. PNTDs are suited for measurements of LET above 5 keV μm−1 in water (Benton et al 2002), which is higher than the values typically found in proton therapy. The LET dependent characteristics of TLD glow curves are strongly influenced by a number of other factors, including detector batch, preparation and absorbed dose (Bilski 2006). Due to their size, tissue-equivalent proportional counters cannot provide good spatial resolution (Bradley et al 2001), and while semiconductor detectors have shown promise for LET measurements (Wroe et al 2009), their routine clinical use has not yet been demonstrated. In addition to these detectors, polymer gel dosimeters have recently shown an LET dependence that may be useful for future clinical applications (Lopatiuk-Tirpak et al 2012). Optically stimulated luminescence (OSL) detectors (OSLDs) have been well studied for dosimetric applications in radiation therapy (Aznar et al 2004, Edmund et al 2007, Jursinic 2007, Schembri and Heijmen 2007, Sawakuchi et al 2008a, Viamonte et al 2008, Reft 2009, Kerns et al 2011, 2012, Mrčela et al 2011, Omotayo et al 2012). Aluminum oxide doped with carbon (Al2O3:C) is a particularly common OSL material. There are two main bands in the OSL emission spectrum of Al2O3:C; one centered in the blue region of the visible spectrum (~420 nm; lifetime ~35 ms) (Akselrod et al 1998) and another centered in the ultraviolet (UV) region (~330 nm; lifetime
