Abstract
The need for better characteristics of a radiation dosimeter arises from the requirement to verify the advanced techniques now employed in radiotherapy such as Intensity Modulated Radiation Therapy (IMRT), Intensity Guided Radiation Therapy (IGRT) and Cyberknife. To-date, one of the most promising dosimeter is thermoluminescent (TL) Ge-doped silica dioxide (SiO2) optical fibre. It has been established that it provides excellent spatial resolution, flexibility, modest cost, a nonhygroscopic nature, and excellent radiation response characteristics. In this study, the TL yield of 9 μm Ge-doped optical fibres (Ge-9 μm) has been investigated, establishing their key dosimetric characteristics: for verification of dose distributions in IMRT prostate dosimetry; for measuring out-of-field photon. These results show that the fibres offer consistent linearity between TL yield and dose for doses from 0.05 Gy up to 10 Gy for photon and electron beam energies, with reproducibility of better than 5%. For all investigated megavoltage photon and electron beam energies, the fibres also offer angular-, dose rate-, and temperature-independence, while a small energy-dependent response was found, of between 6 to 11%. However, at kilovoltage potentials there is significant energy dependence. When held at room temperature results show fading of 11% 133 days post-irradiation. In addition, Ge-doped optical fibre was observed to verify doses to within 3% of the IMRT radiotherapy treatment planning system predicted doses and LiF TLDs (TLD-100 and TLD-700) for the 6 MV and 15 MV energy photon beams used. The fibres have demonstrated potential for use in measuring IMRT out-of-field photon dose when using 6 MV photons, however when conducting 15MV irradiations, the fibres’ response needs to be corrected to account for the activation neutron dose. Ge-doped fibres also represent a viable system for use in mailed audit radiotherapy programmes; in particular measuring beam output under reference conditions as demonstrated in a postal dosimetry audit at selected Malaysian radiotherapy centres. The audit methodology has been developed with an expanded uncertainty of 4.22 % at 95% confidence interval for the energy photon beams used.