title: Optimisation of a polarised X-ray source for the in vivo measurement of platinum in head and neck tumours.
abstract: There is a continuing need to improve the understanding of kinetics of platinum-based chemotherapeutic agents, such as cisplatin and paraplatin. Although these agents demonstrate highly effective anti-cancer activity, they also have associated, often dose-limiting, side-effects such as nephrotoxocity. In vivo X-ray fluorescence (XRF) has been proven to be a suitable technique for measuring the uptake of these agents in tumour and critical organs, but radioisotope based systems have not found their way into routine clinical use due to their rapid increase in minimum detection limit (MDL) with depth. Polarised X-rays offer a solution to this problem by reducing the scattered background, which not only reduces the MDL, but also allows the intensity of the source to be increased without saturating the detector electronics. This paper describes the development and optimisation of a polarised XRF system for in vivo measurement of platinum in head and neck tumours, whose response to cisplatin is often unpredictable. The polarised X-ray source comprises a clinical X-ray therapy unit (the Pantak DXT-300) with removable purpose-built collimators. Optimisation studies have concentrated upon the operating voltage, polariser material and additional filtering. The optimum voltage was found to lie within the range 200-300 kV for all polarising materials. There was no significant difference using copper, aluminium or iron as the polariser. Increasing the additional filtering improved the MDL for a preset number of counts, but decreased the count-rate significantly, resulting in unacceptably long counting times. An MDL of 9 ppm was achieved for a phantom depth of 2 cm, using a copper polariser, 0.25 mm of tin filtering and an operating voltage of 220 kV. TLD measurements showed that the corresponding skin dose was 6 mGy. These results indicate a factor of improvement in the MDL from the previous 99mTc system, for a factor of two lower skin dose. The detection limit achieved is the lowest reported to date, and is considered adequate for a comprehensive patient study. It is anticipated that this will yield better information and the pharmacokinetics of platinum compounds and will lead to optimisation of both chemo and radiotherapy treatment. Additionally this technique can be easily integrated into any radiotherapy based department.
authors.affiliation: Department of Medical Physics and Clinical Engineering, Singleton Hospital, Swansea, U.K.
authors.forename: P A
authors.initials: PA
authors.lastname: Ali
authors.forename: C
authors.initials: C
authors.lastname: Bennet
authors.forename: A M
authors.initials: AM
authors.lastname: el-Sharkawi
authors.forename: D A
authors.initials: DA
authors.lastname: Hancock
mesh_terms: D004867:Equipment Design; D006258:Head and Neck Neoplasms / Q000737:chemistry*; D006801:Humans; D019047:Phantoms, Imaging; D010984:Platinum / Q000032:analysis*; D013052:Spectrometry, X-Ray Emission / Q000295:instrumentation* / Q000379:methods; D014965:X-Rays
chemical_list: D010984:Platinum