Thermoluminescent dosimetry (TLD) for megavoltage electron beam energy determination Vinod Nelson a, b, * , Donald McLean b , Lois Holloway a, c, d a Liverpool and Macarthur Cancer Therapy Centres, Campbelltown, NSW 2560, Australia b Discipline of Health Sciences, University of Sydney, Sydney, Australia c Institute of Medical Physics, University of Sydney, Sydney, Australia d Centre for Medical Radiation Physics, University of Wollongong, Australia article info Article history: Received 17 August 2009 Received in revised form 29 October 2009 Accepted 28 December 2009 Keywords: Thermoluminescent dosimetry Electron energy abstract Megavoltage electron beams are used in many institutions for the treatment of superficial malignant diseases. Accurate dosimetry of electron beams is a fundamental requirement for patient safety. To ensure this, national and international regulatory bodies conduct electron dosimetry audits utilising thermoluminescent dosimeters such as LiF:Mg, Ti (TLD100). TLD100 has been shown to exhibit energy dependence which can introduce errors in dosimetry. In this study commercially available TL materials have been used for a practical method developed to ascertain electron beam energy. This method could be applied in these dosimetry audits to reduce inaccuracies due to energy dependence of TLD100. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Many institutions use megavoltage electron beams for the treatment of superficial malignant diseases. The rapid fall of dose with depth allows sparing of the deep-seated organs at risk. Accurate and precise dosimetry of these electron beams, including measure- ment of beam energy, is fundamental in ensuring a patient’s radia- tion safety and in optimising radiation therapy practices. It is generally agreed that 5% accuracy in clinical dosimetry is required for radical radiation therapy (Nisbet and Thwaites, 1997). However, a higher accuracy of 3–3.5% has also been proposed, which is based on consideration of the consequences of dose variability on loss of tumour control and limiting the incidence of an unacceptable increase in normal tissue complication risks (Brahme, 1984 and Mijnheer et al., 1987, respectively, in Nisbet and Thwaites, 1997). To ensure standards are met many national and international organi- sations such as, RPC (USA), ESTRO, (Europe), and IAEA/WHO have conducted or advised on dosimetry audits utilising thermolumi- nescent dosimeters such as LiF (TLD100). These dosimeters have been shown to exhibit energy dependent response to electron beams (Bartolotta et al., 1995; Robar et al., 1996; Mobit et al., 1996, 1998; Marre et al., 2000), which can introduce error of up to 5% in dosimetry audits (Nisbet and Thwaites, 1997). A number of TL materials exhibiting energy dependent TL response have been described in literature (McKeever et al., 1995). Sutcliffe (1983) and Kron (1995) suggested utilising this property for energy estimation in photon dosimetry. The purpose of this study was to explore the possibility of using TLDs with different energy dependent response to estimate electron beam energy. This was undertaken by investi- gating the difference in response to electron energy of different TLD types and establishing materials for which the ratio of response could be utilised. 2. Materials and method To establish a consistent set of TL dosimeter, fifty dosimeters of different types (10 each of type TLD100, TLD100H, TLD200, TLD400 and TLD500, Table 1) were initially annealed in an oven according to the manufacturer’s recommendations. After this the TLDs were cooled to room temperature before being subjected to irradiation from a 6 MV photon beam from a Siemens Primus linear accelerator (Siemens Medical Solutions, Oncology Care Systems Group, USA). All TLDs were irradiated in a polystyrene tray, specially fabricated to hold thermoluminescence chips, at 5 cm depth in a solid water phantom to a dose of 1 Gy. The output of the linear accelerator was verified prior to irradiation according to departmental quality procedures to an accuracy of better than 1%. The TL response stabilised after eight irradiation and read-out cycles. The maximum coefficient of variation of readings of each TLD batch was below 7% for all thermoluminescence types. From these, five dosimeters of each TLD type were selected on the basis of their similarities in * Corresponding author at: Liverpool and Macarthur Cancer Therapy Centres, Campbelltown, NSW 2560, Australia. Tel.: þ61 246344341; fax: þ61 246344350. E-mail address: veekay58@yahoo.com (V. Nelson). Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas 1350-4487/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2009.12.040 Radiation Measurements 45 (2010) 698–700