Optical fibre dosimeters can have many advantages over existing systems such as TLDs, diodes or MOSFETs. A significant advantage is that optical fibres are composed of silica (glass) or plastic resulting in a material perfectly suited for the use in an MRI environment, as they are non-magnetic and do not cause interference of the image, immune to intense magnetic fields and radiofrequency (RF) present in this environment (Raaymakers et al. 2009). Optical fibres can also be summated together (multiplexed) to a single controller which in turn can form an array of detectors for 2d measurements. The development of radiation-resistant fibres has also meant that optical fibres can be utilised in areas of high levels of radiation. The small size of optical fibre sensors offers significant advantages for application in prostate brachytherapy. The small dimensions of the sensor (as low as 250 µm diameter) allows it to be easily guided within existing brachytherapy equipment; for example, within the seed implantation needle for direct tumour dose analysis, in the urinary catheter to monitor urethral dose, or within the biopsy needle holder of the transrectal ultrasound probe to monitor rectal wall dose. The measured radiation dose can be used to verify the calculated dose distribution that describes the treatment received by the patient. The availability of real-time radiation dose measurements during the brachytherapy procedure will allow for optimisation of the brachytherapy seed insertion during the procedure and result in high quality treatments. The quality of a brachytherapy treatment is directly linked to patient survival and outcomes (Hinnen et al. 2010). Radiation dosimetric methods are used for the estimation of dose absorbed by radiation in a detector material using either thermoluminescence (TL) technique or optically stimulated luminescence (OSL) technique or radioluminescence (RL) or any other technique using passive solid state detectors.
Luminescence dosimetry
Luminescence occurs when a material is subjected to radiation and that material absorbs some of the radiation and as a result, emits light with a different wavelength. The wavelength of the emitted light is dependent on the luminescence material. Different types and forms of radiation can be used to excite a material and it is these types of radiation which give rise to different types or methods of luminescence. These are thermoluminescence (excitation due to heat), photoluminescence (excitation due to optical or ultra violet light) and radioluminescence (excitation is due to alpha, beta, gamma or x-rays).
Thermoluminescence dosimetry
Thermoluminescence (TL) is the emission of light from a solid due to heating, after it has previously been excited by radiation. When exposed to radiation, the TL material absorbs energy, which it then stores until it is heated. Germanium-doped silica (Ge-doped SiO2) type optical fibres have been demonstrated as potential novel forms of thermoluminescence dosimeters for radiation therapy dosimetry (Ramli et al. 2009). Abdul Rahman et al. (2012) performed an investigation in November 2011 into the ability of a high spatial resolution (~120 µm) Ge-doped SiO2 fibre thermoluminescence dosimeter to measure radiation. The optical fibres demonstrated good reproducibility (±1.5 %), at fixed dose rate, the dosimeters were found to produce a flat response of better than 4 % (1 S.D) of mean TL distribution and showed good linearity (r
2 = 0.998) of response up to a dose of 50 Gy for photon and electron beams. Entezam et al. (2016) reviewed the response behaviour, the fibres showed good reproducibility, energy, and field-size response, it was demonstrated that dependency on the field size for the most sensitive sample (the 42 µm core size fibre) was determined for photons produced at 6- and 10 MV and field sizes of 3, 6, 8, 10, 20, 25 and 30 cm. For each field size, measurement was carried out for five fibre segments of the 42 µm core size fibre, the results being normalized for all field sizes using the obtained value for 10 × 10 cm2 field size (Fig. 1).
Fading, the reduction of TL response as a function of time post-irradiation, depends primarily on parameters such as storage temperature and radiation type. The analysis of fading for flat and cylindrical fibres, fabricated by members of this group has been determined by Ghomeishi et al. (2015). For flat and cylindrical fibres irradiated simultaneously to a dose of 8 Gy using 6 MV photons, 15 days post-irradiation the TL response for the flat and cylindrical fibres had reduced by 17 and 27 %, respectively, compared to that obtained a day after irradiation.
One advantage of Ge-doped Si optical fibre dosimeters is that they are water resistant, and therefore, it becomes possible to locate the fibre dosimeter within a particular tissue of interest; this suggests the possible use of Ge-doped SiO2 optical fibres in a variety of interface dosimetry situations, as in the application of radiation synovectomy (Karavida and Notopoulos 2010). The potential radiotherapy dosimetric applications for doped silica fibres as TLDs have been reviewed by Bradley et al. (2012). The use of Ge-doped silica fibres has also been proposed by Issa et al. (2012) in brachytherapy, the optical fibres dosimeters have been employed in obtaining doses at distances very close to the source (2 mm). Dose measurements, obtained for separations from 2 up to 20 mm, were found to be in good agreement with simulations of photon-mediated dose obtained using the DOSRZnrc Monte Carlo code with agreement within 3 and 1 % for the 133Ba and 60Co sources, respectively. A significant disadvantage of thermoluminescent dosimetry is that the dose information is dependent on the post-irradiation of the material and so real-time dosimetry is not possible.
Optically stimulated luminescence
In a similar process to thermoluminescent techniques, photoluminescence, or optically stimulated luminescence (OSL), emits the energy stored owing to irradiation, upon exposure to light. When the insulator or semiconductor is subjected to radiation, electron hole pairs are generated, defects in the OSL material trap these electron hole pairs. This illumination of the material, frees the trapped electron hole pairs with the result of luminescence from the material transmitted through the fibre and measured with a photomultiplier tube.
Carbon-doped aluminium oxide (Al2O3:C) (Yukihara et al. 2014) showed how the response from OSL is linear and independent of energy and dose rate, exhibits little fading, temperature dependent and is sensitive to light. Europium-doped potassium bromide (KBr:Eu) were reviewed by McKeever (2011). It was discovered that the signal from KBr:Eu is unstable due to fading at room temperature, rapid OSL decay and its simpler process of OSL production enables the material to be used in real-time monitoring of radiation. Enhanced aluminium oxide doped with carbon and magnesium (Al2O3:C,Mg) (Rodriguez et al. 2011) were investigated for optically stimulated luminescence (OSL) in radiation dosimetry. Findings show that the intensities of TL, OSL and RL signals of the samples were similar to that of regular carbon-doped aluminium oxide (Al2O3:C).
Dunn et al. (2013) commissioned optically stimulated luminescence dosimeters (OSLDs) as a replacement for thermoluminescence dosimeters (TLDs) for application within radiotherapy, a product from Landauer Inc. known as “nanoDots” showed supra-linearity, reproducible fading (3 %) and little signal depletion per readout (0.03 %). Marckmann et al. (2006) developed a novel idea to overcome Cerenkov radiation by coupling OSL (Al2O3:C) to the end of a polymethyl methacrylate (PMMA) optical fibre to result in simultaneous RL/OSL signals providing real-time radiation monitoring using RL and post-radiation using OSL. The characterisation of a fibre based Al2O3:C OSL dosimeter by Anderson et al. (2009) for its response to 192Ir, demonstrates the suitability of such a device for HDR brachytherapy. The system demonstrated excellent linearity in the tested dose rage (0–4.3 Gy), with reproducibility of approximately 1.3 %. It was also estimated that measurements with a 5–50 mm source to probe distance would be associated with a 5 % uncertainty.
Radioluminescence
Plastic scintillating fibre dosimeters
Scintillating fibres work by converting incident radiation energy into visible light, as they are exposed to X-ray radiation, electrons in the fibre are excited to higher energy levels through either Compton or photoelectric effect. The fibre core is doped with scintillating fluorescent particles, which fluoresce when irradiated by ionising radiation, and cladded with PMMA. A major advantage of these dosimeters in radiotherapy is their water equivalence making them an ideal dosimeter in radiotherapy dosimetry. However, recent studies (Buranurak et al. 2013) identified the effect of temperature on a fibre coupled organic plastic scintillator in applications such as external beam radiotherapy and brachytherapy. The study showed that the light yield in the peak regions of the scintillators decreases linearly with increasing temperature. Temperature coefficients of −0.15 ± 0.01 and −0.55 ± 0.04 % K−1 for blue BCF-12 and green BCF-60 from Saint-Gobain Crystals were, respectively, observed in the study. Another study (Beddar 2012) showed significant differences from measurements inside patients to those measurements in anthropomorphic phantoms due to the similar effects of temperature.
Figure 2 shows the properties of four scintillating organic fibres which were used from Saint-Gobain Crystals (2015). These were BCF10, BCF12, BCF20 and BCF60. It shows that BCF10, BCF12 emit a blue colour whereas BCF20, BCF60 both emit a green colour with a resultant higher emission peak. They range from 0.25 to 5.00 mm in diameter, have a polystyrene core with fluorescent dopants and a PMMA cladding.
Suchowerska et al. (2011) showed a fibre optic scintillating dosimeter, consisting of a plastic scintillator coupled to an optical fibre for brachytherapy. These sensors were small enough (0.5 mm) to be inserted into a No. 16 French urinary catheter, to perform in vivo dosimetry to determine the urethral dose during high dose rate (HDR) treatment to the prostate. The background signal created by Cerenkov and fibre fluorescence was 0.1 % of the signal and the sensor had the capability of real-time readout. Klein et al. (Klein et al. 2012) demonstrated a plastic scintillating fibre (BCF-60) mounted onto an endorectal balloon to verify doses in vivo during intensity-modulated radiation therapy (IMRT) and volumetric arc therapy (VMAT) for prostate cancer. The sensor measured doses that correlated with ionization chamber measurements and it was found that treatment planning system calculations were within 1 % of expected values. A proposed dosimeter for in vivo dosimetry in HDR brachytherapy was investigated (Therriault-Proulx et al. 2013), whereby a single fibre with multipoint plastic scintillators was developed for Iridium-192 HDR brachytherapy treatment verification in a water phantom. It contained a three-point detector containing BCF-10, BCF-12 and BCF-60 scintillating elements. A comparison of measured doses at different source-to-detector distances were investigated, with the result that the system was suitable for measuring source position uncertainty to less than 0.32 ± 0.06 mm.
A clinical trial of a plastic scintillating fibre dosimeter, BrachyFOD, (Suchowerska et al. 2011) enrolled 24 patients receiving HDR brachytherapy to the prostate. After 14 patients, the dosimeter design was improved for more accurate readings to improve clinical reliability: a dosimeter self-checking facility; a radiopaque marker to determine the position of the dosimeter, and a more robust optical extension fibre as depicted in Fig. 3. The results demonstrated a maximum measured dose difference of 9 % from the calculated dose from the TPS for the remaining patients in the trial indicating the importance of in vivo dosimetry in brachytherapy.
In a further study (Gagnon et al. 2012), the performance of a plastic scintillator BCF-60 was compared with a range of traditional small field dosimeters for stereotactic QA, the results compared output factors and dose profiles with a good level of agreement with diodes and EBT2 Gafchromic film. Currently, the only commercial optical fibre dosimeter for radiotherapy is the Standard Imaging Exradin W1 Scintillator (2014), a 1 mm core polystyrene-based fibre that is coupled to a PMMA optical fibre for transmission of the optical signal.
Inorganic scintillating fibre dosimeters
Inorganic scintillators are generally in crystal form grown at high temperatures. They are made of alkali halides, or oxides, and often require an activator impurity, e.g. Na(Tl), CsI(Tl). Due to the crystal form of the scintillator, it is possible to incorporate the material into the sensing region in a number of different ways, e.g. coating the fibre, coupling it to the time, or embedding it within the fibre. Distinct advantages include real-time dosimetry, small size and good spatial resolution. In evaluating the scintillation efficiencies of different phosphors, a radiation dosimeter (Jang et al. 2011) to detect tritium in real time was developed, composed of a scintillator material, an optical fibre bundle and alight measuring device, as illustrated in Fig. 4. Each scintillator interacts with electron or beta radiation and generates scintillation photons between 455 nm and 550 nm wavelength of light. Three kinds of inorganic scintillators were tested at different distances between the fibre optic sensor and source. These were Gd2O2S:Tb, cerium-doped YAG (Y3Al5O12:Ce) and CsI:Tl. The results show that the scintillation efficiencies of CsI:Tl, Y3Al5O12:Ce and Gd2O2S:Tb are 8, 5 and 15 %, respectively. The Gd2O2S:Tb type scintillator was found to give the greatest scintillation response of photons.
An optical fibre dosimeter has been developed by McCarthy et al. (2011; O’Keeffe et al. 2013) by coating the end of an exposed PMMA optical fibre, after the cladding has been removed, with Gd2O2S:Tb. The scintillating phosphor, supplied by Phosphor Technologies Ltd (2014) is mixed with an epoxy mix and injected into a cylindrical mould containing the exposed PMMA fibre optic core and allowed to cure. The radiation-sensitive scintillating material tip of the sensor fluoresces on immediate exposure to ionising radiation. The resultant emitted fluorescent light penetrates the PMMA optical fibre and propagates along the fibre to a distal scientific grade spectrometer from Ocean Optics (Dunedin, FL), where the intensity of the peak wavelength of the fluorescent light is measured. Initial characterization measurements of this sensor have been carried out, with its response being evaluated in water equivalent phantoms to assess whether it would be suitable for potential in vivo applications in either brachytherapy or external beam radiotherapy dosimetry. The results demonstrate that the fibre has a high sensitivity and good repeatability across a range of beam energies and types, and demonstrate a linear response from low doses of the order of centigray up to at least 16 Gy in a single delivery.
An optical fibre sensor (Woulfe et al. 2016) based on radioluminescence, whereby radiation-sensitive scintillation material is embedded in the core of a plastic optical fibre, is illustrated in Fig. 5. Three sensors were fabricated, using different inorganic scintillators, identified as being most suitable for brachytherapy applications: thallium doped caesium iodide (CsI:Tl), terbium doped gadolinium oxysulphide (Gd2O2S:Tb, GOS) and europium-doped lanthanum oxysulphide (La2O2S:Eu, LOS). Terbium doped gadolinium oxysulphide (Gd2O2S:Tb, GOS) demonstrated the highest sensitivity to the 125I brachytherapy seeds (Woulfe et al. 2016). The sensor is designed for in vivo monitoring of the radiation dose during radioactive seed implantation for low dose rate brachytherapy, in prostate cancer treatment, providing oncologists with real-time information of the radiation dose to the target area and/or nearby critical structures. The radiation from the brachytherapy seeds causes emission of visible light from the scintillation material, which penetrates the fibre, propagating along the optical fibre for remote detection using a multi-pixel photon counter. The sensor demonstrates a high sensitivity to Iodine-125, the radioactive source most commonly used in brachytherapy for treating prostate cancer. The developed optical fibre based sensor has a number of significant advantages for application in brachytherapy. The small dimensions of the sensor allow them to be guided within existing brachytherapy equipment; for example, within the seed implantation needle (see Fig. 5b), in the urinary catheter to monitor urethral dose, or along the transperineal ultrasound probe to monitor rectal wall dose. This allows for real-time monitoring of the radiation dose to the target area or nearby critical structures. Furthermore, the construction of the sensors is such that they are completely biologically separate from their monitoring environment, and therefore, offer no possibility of contamination or other form of threat to their target operating environment, i.e. internal human tissue.
Fibre Bragg gratings
Fibre Bragg grating (FBG) based sensors work by monitoring the wavelength shift of the returned Bragg signal which changes as a function of the measured. The Bragg wavelength is related to the refractive index of the material and the grating pitch. The light incident on the grating reflects a narrow spectral component at the Bragg wavelength, and hence in the transmission spectrum this component is missing. Work (Mihailov 2012) has concentrated on developing radiation-resistant FBGs for use in temperature and strain measurement applications in nuclear environments.
The outcome of a project investigating the use of FBGs as high dose radiation sensors was first presented by Krebber et al. (2006). The work is based on the Kramer–Kronig dispersion relations, which can be used to show that an increase of attenuation has to be accompanied by a change of refractive index. The FBGs were written in a hydrogen doped Ge-doped fibre for wavelengths of 650, 820, 1285 and 1516 nm. The radiation induced refractive index change was calculated from the Bragg wavelength shift and a wavelength shift from 850 to 1216 nm was demonstrated to be independent of dose rate for radiation doses greater than 2 kGy. Although small changes in temperature are accounted for within the sensor system, the sensor requires a highly stable setup, with stress-free attachment of the FBG along with a constant, steady temperature. Fibre Bragg gratings, written in Ge‐doped silica fibres, have been shown to be capable of monitoring high‐radiation doses (Avino 2014).