Dosimetric characteristics of fabricated germanium doped optical fibres for a postal audit of therapy electron beams

Highlights

• A preliminary study of electron beam therapy postal audits using a fabricated Ge-doped fibres is performed.

• The dosimetric characteristics of optical fibres fabricated for electron audit are established

• The results of the electron beam therapy audit are within the tolerance limit of 5%

Abstract

The dosimetric characteristics of germanium (Ge) doped optical fibres are investigated as a potential dosimetric alternative for electron beam therapy postal audits. The dosimetric characteristics of 6 mol% Ge-doped optical fibres fabricated as cylindrical fibres (CF) and flat fibres (FF) are established in terms of signal fading, linearity of dose-response, beam energy- and dose rate dependence. Pilot electron beam therapy audit study irradiations are made with a linear accelerator located at the Royal Surrey County Hospital, applying International Atomic Energy Agency (IAEA) standard irradiation procedures for reference and non-reference conditions. Results for the CF and FF show fading of 26% and 20% respectively at 120 days post-irradiation. For a 6 MeV electron beam the dose-response is observed to be linear over the dose range 1–3 Gy, the least determination coefficient, R², being 0.985. The results of the electron beam therapy audit are within the tolerance limit of 5% recommended by the International Commission on Radiation Units and Measurements (ICRU) Report No. 24, with a maximum deviation of 4% for FF at a 6 MeV electron beam under non-reference conditions. In conclusion, the fabricated Ge-doped optical fibres are seen to offer suitability for use as an alternative dosimeter to TLD-100 in electron beam therapy postal audit.

Introduction

The postal dosimetry audit is a well-accepted part of an overall radiotherapy quality assurance programme, in good part to verify accuracy of dosimetry measurements; the International Commission on Radiation Units and Measurements (ICRU) Report 24 recommends a limit on tolerance of 5% at the 95% confidence level (ICRU, 1976). In radiotherapy, under- or overdose is clearly to be avoided, with potential for impact on the effectiveness of cancer treatment and the likelihood of radiation injury to patients (Healy et al., 2020; Kry et al., 2018; van der Merwe et al., 2017). The postal dosimetry audit also forms an essential control indicator of the competency of the in-service physicists, with traceability of measurements provided by national dosimetry laboratories (Podgorsak, 2005). A good number of national institutes provide postal dosimetry audit services to radiotherapy centres, including dosimetry audits of irradiations made under reference and non-reference conditions (Da Rosa et al., 2008; Kroutilíková et al., 2003; Rahman et al., 2008), on-site visit audits (de Prez et al., 2018; Lye et al., 2019; Park et al., 2017), and more complex issues such as treatment planning system audits (Okamoto et al., 2018; Rutonjski et al., 2012) and in-vivo dosimetry audits (Kamomae et al., 2017). For radiotherapy centres in IAEA member states, the International Atomic Energy Agency (IAEA) in collaboration with the World Health Organisation (WHO) offers an important role of conducting a postal dosimetry audit based on use of mailed thermoluminescence dosimeters (TLD). To-date this has focused most particularly on reference condition for high energy photon beams, with more than 50 years of experience accrued (Izewska et al., 2020).

There are many approaches to the dosimetry audit that are being practised at national and regional level worldwide. For instance, within the United Kingdom (UK), the audits cover various levels from basic reference dosimetry to advanced radiotherapy techniques (Clark et al., 2015). In the reference dosimetry audit, the accuracy of absolute dose calibrations for megavoltage photon beams, electron beams and kilovoltage X-ray beams are verified following the relevant UK Code of Practise (CoP) (Thomas et al., 2017). For advanced radiotherapy techniques involved validation of treatment delivery from total skin electron beam therapy (TSEBT) (Misson-Yates et al., 2015), Intensity-Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) (Tsang et al., 2017) and brachytherapy system (Humbert-Vidan et al., 2017).

In regard to audit methodologies, the IAEA has introduced nine steps, focusing more so on photon beam audits. These involve beam output measurement with various irradiation setups (steps 1–3) to more complex arrangements (steps 4–6) and advanced radiotherapy treatments (steps 7–9) involving intensity modulated radiotherapy (IMRT) (Wesolowska et al., 2019), treatment planning systems (TPS) and small field beams (Lechner et al., 2018). However, with more than 50 years of expertise, the IAEA has mostly concentrated on the step 1 audit of reference conditions for high-energy photon beams with participation of IAEA member states (Izewska et al., 2020). It is estimated that the dosimetry audit covers slightly more than 10% of the needs (Izewska et al., 2018). This is certainly to be considered insufficient. Therefore, national remote dosimetry audits should be established with the aim of catering to all nine steps of audit methodologies for all radiotherapy centres.

As a member state of the IAEA/WHO Network of Secondary Standard Dosimetry Laboratories (SSDLs), over the past 36 years Malaysia has participated in the IAEA/WHO TLD Postal Dose Quality Audit Service (Abdullah and Dolah, 2021; Samat et al., 2009). In 2011 the IAEA designated the Malaysian SSDL a moderator in coordinating the IAEA/WHO TLD Postal Dose Quality Audit Service for radiotherapy centres within Malaysia (Noor et al., 2017; Abdullah et al., 2018). Malaysia has also conducted national postal dosimetry audits for reference condition dosimetry for high energy photon beams based on IAEA Technical Report Series (TRS) No. 277 (Rassiah et al., 2004) and TRS No. 398 (Noor et al., 2014; Abdullah et al., 2016). The national dosimetry audit has subsequently been extended to include high energy photon beams non-reference conditions (Ahmad Fadzil, 2020) and intensity-modulated radiotherapy (IMRT) involving treatment planning and on-site audits (Diyana et al., 2020).

In recent years other than TLD-100, various types of transfer detector have also been introduced in postal dosimetry audits, including RPL (radio-photoluminescence) glass dosimeters (Mizuno et al., 2014; Okamoto et al., 2018), optically stimulated luminescent (OSL) dosimeters (Alvarez et al., 2017; Lye et al., 2014), radio-chromic film (Okamoto et al., 2018) and alanine dosimeters (McEwen et al., 2015; Yamaguchi et al., 2020). A number of workers have examined the feasibility of use of more novel high spatial resolution TLDs, including silica beads for lung radiotherapy postal dosimetry audits (Jafari et al., 2017) and Ge-doped optical fibres for high energy photon beam audits (Fadzil et al., 2014; Noor et al., 2014).

In present work, we report on the potential for use of fabricated Ge-doped optical fibres in postal dosimetry audits of high energy electron beams. The optical fibres provide convenient properties such as ease of handling, reusability and cost effectiveness (Bradley et al., 2012), also offering sufficient sensitivity and dose response linearity when subject to electron irradiation (Nurasiah et al., 2020) as well as well-controlled signal fading over time (Noor et al., 2012). In addition, a special characteristic of optical fibres is their high spatial resolution, demonstrated in the studies of radiotherapy dosimetry of small-field radiation (Lam et al., 2020) and in-vivo dose verification measurements (Alyahyawi et al., 2021), offering evidential support that the optical fibres can be a good candidate for utilisation in advanced remote dosimetry audits.

Prior dosimetric investigations of these fibres have looked at signal fading, linearity of dose-response, and energy- and dose-rate dependence, finding Ge-doped dosimeters to offer desirable performance, comparable to TLD-100 (Entezam et al., 2016). Using 6% mol Ge-doped optical fibres, optimal in sensitivity and excellent in linearity of dose response within the intended dose range for use in the radiotherapy dosimetry audit (Nurasiah et al., 2020), the work herein now represents the first postal dosimetry audit study of high-energy electron beams, with irradiations covering both reference and non-reference conditions.