This thesis consists of several individual studies concerning radiation dosimetry applications that involve the relatively low medical doses encountered in diagnostic procedures (dental, mammography, and chest x-ray) or those coming from scattered radiation during gamma knife radiosurgery (this will be discussed in more detail later in this chapter). Use has been made of the different types of novel silica materials reviewed above, that have been fabricated for this purpose. We have fabricated and investigated tailor- made doped photonic crystal fibres (PCFc, with the subscript indicating collapsed), either Ge- doped or co-doped with boron. The PCFs are fabricated using the modified chemical vapour
deposition (MCVD) and fibre-pulling technique as described above, also generating strain-related defects (Rozaila et al., 2016: Dermosesian et al., 2015). In addition,we have examined GeB-FF, fabricated to produce two different types; one has been generated by applying low pressure to cause collapse of the initial hollow doped silica tube to create additional defects while the other has been produced as a Ge-doped disc, both used for detecting the typically low doses of diagnostic radiology applications, including dental, mammography, and chest x-ray images. The typical dose values for the radiological images involved in chest–x radiography, dental, and mammography are 0.005 mSv, 0.1 mSv, 0.4 mSv respectively (radiologyinfo.org, 2018). These are doses comparable to being exposed to natural background radiation for 1, 7 and 10 days respectively.
In addition, the use of Ge-doped SiO2 telecommunication fibres, and glass beads have been utilized in an audit encompassing 20 hospitals, seeking to detect the doses received by the eyes during Gamma Knife brain tumour radiosurgery. The latter constitutes the first such project of its kind, the idea emerging as a result of new regulations based on international guidance to seek reduction in lens dose, down to 0.5 Gy (ICRP, 2012). An extension study, concerning paediatric cases, has been unique in its use of GeB-FF (this representing the most sensitive form of doped silica to have been produced to-date) (Abdul Sani et al., 2014: Begum et al., 2015: Hashim et al., 2015; Alawiah et al., 2013; Mahdiraji et al., 2015). It is to be stressed here that all previous work by the various groups using the novel doped silica dosimeters have been in radiotherapy applications. The doses evaluated herein are generally at least a thousand-fold smaller than those typical of radiotherapy, representing a considerable challenge.
To date, a number of studies have been performed to investigate the potential of using commercially available Ge-doped SiO2 telecommunication fibres as a 1-D dosimeter for therapeutic applications due to due to its high spatial resolution within relatively small diameters (∼120 μm) (see for instance, Abdul Rahman et al., 2012). Use has been made in verification of Intensity Modulated Radiation Therapy (IMRT) three-dimensional (3D) dose distributions (Noor et al., 2011), synchrotron microbeam radiation therapy (Rahman et al., 2010), brachytherapy dosimetry (Issa et al., 2012), external beam radiotherapy dosimetry (Abdul Rahman et al., 2012), UV radiation dosimetry (Abdul Rahman et al., 2014). kilovoltage X-ray therapy irradiations (Issa et al., 2011), MV radiotherapy dosimetry (Noor et al., 2014), alpha particles detection (Ramli et al., 2009). In previous
including fading, linearity, energy dependence, reproducibility, dose response, reciprocity between TL yield and dose rate and energy. These have covered a wide range of incident energies, from a few eV to several MeV. In particular, the fibres have been observed to produce a flat response to fixed photon and electron doses to within better than 3% of the mean TL distribution (Rahman et al., 2010; Abdul Rahman et al., 2012). This form of dosimeter has been shown to provide low fading, reusability following thermal annealing and acceptable energy dependence for radiotherapy purposes (Abdul Rahman et al., 2011). The response of telecommunication Ge-doped fibres in radiotherapeutics dosimetric applications has been compared against other competing forms of passive dosimeter, TLD or otherwise, typically being found to offer advantageous performance (Yaakob et al. 2011; Hashim et al. 2009; Abdulla et al. 2001). Previous studies have also focused on the possibility of using such doped SiO2 optical fibres dosimeter in detection of both photons and charged particles (Hashim et al., 2009). In these, it has been shownthat Ge-doped fibres offer spatial resolution at useful TL yield, providing an ability to measure the radiation doses in high dose gradient fields as for instance in Intensity Modulated Radiation Therapy (IMRT) (Noor et al., 2010, 2011), also being suggested to be useful in in vivo radiotherapy measurements due to its hygroscopic nature.
Further to the above, pure silica (SiO2) glass fibres doped with Ge have been demonstrated to provide TL yields significantly greater than that of telecommunication fibres doped with Al, Nd, Yb, Er, and Sm (Noor et al., 2011; Fadzil et al., 2014; Abdul Sani et al., 2014). This is suggested to relate to the presence of the same number of outer electrons in Ge as silicon. Consequently, it is expected to form the basis of a sensitive radiation dosimeter, suitable for diagnostic dose audits, approaching or perhaps exceeding that of TLD-100, the latter being widely used as a passive dosimeter, comprising a phosphor-based system of LiF doped with Mg and Ti (MatNawi et al., 2015). Other work within the group developing doped silica fibres have focused on fabricating novel silica materials that are able to detect different types of radiation and energies. Alawiah et al. (2013, 2015) have reported pure silica flat fibres (FFs) to represent a promising TL material for use as a dosimetric system for high-dose electron and photon therapy. Undoped FFs have been shown to provide excellent linear radiation response within the range of therapy energies, independence of radiation energy and dose-rate, good
reproducibility and low fading, being competitive with the widely used LiF: MgTi dosimeter in medical therapeutic applications. Moreover, studies have been carried out on several types of tailor-made flat Ge-doped silica fibres, differing in only their external dimensions.
The advantages of using flattened fibres for applications in medical radiotherapy have also been reported by others (Alawiah et al., 2013; Abdul Sani et al., 2014; Bradley et al., 2015; Saeed et al. 2012). In addition, studies have been conducted on several sizes of Ge- doped flat fibres as a dosimeter for a range of doses, using different photons and electrons energies, demonstrating superiority of TL response over the more popular standard TLD- 100. The TL fading of FFs has been found to be around 20% over a period of thirty (30) days (Begum et al., 2015). In several studies, use of germanium has been favoured as the dopant in enhancing the flat fibre response and sensitivity at high energy and high dose in radiotherapy applications. Moreover, GeB-Flat Fibre have shown excellent dosimetric features including, good reproducibility, a very low rate of fading, low variation background signal, and an excellent TL response for high energy. The new technique of producing flat fibre is based on collapsing down hollow capillary optical fibres (COF) producing fused inner walls and consequent defect generation, a number of studies demonstrating the greater performance of the fabricated FFs over the COFs. To-date, the TL response of GeB-FF shows an enhancement of a factor of 12 over that of Ge-COF. The glow curve analysis showed the generation of an additional peak in the FFs compared to that obtained in the COFs, and the TL intensity value of the new peak is considerably enhanced in the doped FFs compared to the undoped FFs (Mahdiraji et al. 2015; Ramli et al. 2015).There is an increasing demand of the dosimetry in diagnostic radiology. It has been realized among health organization members that the x-ray radiation dose received by patients needs to be optimised. Failure to justify and optimise can lead to induction of cancer across a population and in some unfortunate circumstances can cause acute damage to particular body organs such as the skin and eyes. Measurements and procedures for diagnostic radiology dosimetry has been standardized through international codes of practice to explain the essential methodologies. A fundamental need in diagnostic dosimetry is to measure the air kerma from the X-ray device under specific conditions (IAEA, 2007: AAPM, 1990: NRPB, 2002: Rosenstein, 2008). In order to achieve the appropriate accuracy and precision of dosimetric determination such
traceable to a standards laboratory. Proper dosimetric procedures are of considerable importance, not least in paediatric cases children and pregnant patients as well as for the quality control of X-ray equipment and procedures (Meghzifene et al., 2010; United Nations Scientific Committee on the Effects of Atomic Radiation, 2008). It is important for all scientific and health professional communities to understand the required dose quantities and magnitude and the relevant associated risks. Even though the diagnostic radiation doses usually do not approach thresholds for deterministic effects, considerable potential exists to do so in respect of interventional fluoroscopy. Even in the absence of deterministic effects, exposure to diagnostic radiation can cause induction of cancer after a significant latency period. While the interactions of radiation with matter are well known, the challenges of diagnostic radiology dosimetry are associated with the variations and complexity of X-ray delivery, the instruments used in dosimetry and the interaction of the radiation with the different constituent parts of the human anatomy. As mentioned above, even though doses from diagnostic radiological examinations are typically considered small and usually insufficient to reach thresholds for deterministic effects, interventional procedures represent an exception, cardiology in particular potentially involving the use of elevated doses. This could lead to serious skin injuries (Berlin 2001; Koenig et al. 2001). As such, it must be realized that doses delivered in diagnostic radiological procedures should be accurately measured in order to ensure acceptable balance between quality of image and patient exposure. Dosimetric methods should be used that ensure appropriate levels of accuracy and optimisation (International Atomic Energy Agency, 2007). X-ray diagnostic examinations encompass various techniques, including CT scans, mammography, fluoroscopic and interventional radiological procedures, dental and general radiography (herein, general radiography is used to cover all X ray imaging modalities other than dental radiography, fluoroscopy, mammography and CT). Tube voltages producing diagnostic x-ray beams are typically between 20 kVp to 150 kVp generally being different for each technique. As instances, while it is generally within 50– to 150 kVp in fluoroscopy, CT, dental and general radiography, in mammographic examinations it is generally between 22- to 40 kVp. In regard the anode material, tungsten is usually used, a particular exception is for mammography where different combinations of anode and filtration materials are employed, the most popular materials being a molybdenum anode and molybdenum filtration. Also important in diagnostic radiology dosimetry is to specify the radiation
qualities as all dosimeters depend on the spectral distribution of the X rays. Radiation qualities are usually known in terms of the X-ray tube voltage and half-value layer (HVL).