Abstract
In the various brachytherapy techniques the intent is to deliver as high a tumour dose as possible, limited only by surrounding normal tissue tolerance. The main feature of the techniques is very steep dose gradients, representing a potential limiting factor in accurate dose distribution measurements around sources. Dose distributions at distances less than 1 cm are therefore normally generated using either validated Monte Carlo (MC) simulations or standard dose calculation formalisms, for example that of AAPM TG 43, while dose measurements can only be performed at larger distances, normally greater than 1cm from the outer dimensions of the source encapsulation Ge-doped silica fibres are a viable thermoluminescent dosimetry (TLD) system, providing good spatial resolution of approximately 120 μm, sensitive response to ionizing radiations, large dynamic dose range, good reproducibility and reusability, dose rate independence, minimal fading, resistance to water and low cost. Dosimetric characterisation of commercial Ge-doped silica fibres have been obtained by subjecting them to kilovoltage therapeutic x-ray radiation beams, verifying their use for brachytherapy sources; dose response, reproducibility and fading at 90 kVp and 300 kVp have been investigated. Central-axis depth doses have been obtained at the two accelerating potentials using different field size applicators, measurements being made using the fibres in both water and a GAMMEX RMI 4571 solid water phantom. Comparison has been made with central-axis depth doses, measured using a 0.6 cm3 graphite-walled ionisation chamber data and British Journal of Radiology Supplement 25 tabulated values (both in water). Ge-doped optical fibre dosimeters show good dosimetric response for low photon energies. These desirable characteristics support the use of these TL fibres as dosimeters for brachytherapy applications. Ge-doped optical fibre TL dosimeters have been used to measure the dose distribution around two Low Dose Rate (LDR) 125I seeds; model 6711, the new thinner model 9011 and a High Dose Rate (HDR) 192Ir (MicroSelectron V2) source at proximal distances down to 1 mm, measured in a Perspex medium. The anisotropy has also been measured in Perspex, for distances from 10 to 100 mm from a LDR 125I seed model 6711 centre, in 10 mm increments and at angles 10 to 90 in 10 increments from the seed central axis. Measured doses have been compared with calculations and treatment planning system (TPS) predicted doses for the same locations. Monte Carlo simulations were obtained using the EGSnrc ________ DOSRZnrc codes and TPS predicted doses were obtained using the system VariSeed V8.0.2. For 125I seed model 6711, the measurements agree with simulations to within 2.3 % +- 0.3 % along the transverse and perpendicular axes and within 3.0 % +- 0.5 % for measurements investigating anisotropy in angular dose distribution. Measured and Veriseed TM brachytherapy treatment planning system (TPS) values agreed to within 2.7 % +- 0.5 %. For the new thinner 125I model 9011, dose measurements were in good agreement with simulations to within 2.1 +- 0.2 %, while dose measurements and doses obtained through use of the Variseed TPS agreed well, to within 2.2 +- 0.5%. The above work has therefore demonstrated the applicability of Ge-doped optical fibres for use in brachytherapy.