Application of the fundamental parameter method to the in vivo x-ray fluorescence analysis of Pt

The application of the fundamental parameter method (FPM) to the in vivo x- ray fluorescence (XRF) analysis of Pt has been investigated. The FPM is con ventionally used to carry out elemental analysis of samples in vitro withou t the need to use standard samples of accurately known composition for syst em calibration. The present work has involved the use of the FPM to calcula te the concentration of Pt solutions in phantoms, with concentrations rangi ng from 25-1000 ppm. The phantoms simulate the measurement of Pt-based chem otherapy drugs in head and neck rumours. The radiation sources were a 150 k V tungsten-anode x-ray tube and the isotope Tc-99m. The minimum detection l imit measured for Pt was in the range 8-30 ppm (depending on radiation sour ce and geometry), using a narrow (5 mm) diameter beam. Dose rates in the ph antom were 0.1-5 mGy h(-1). Average differences between nominal and calcula ted values of Pt concentration were < 8% using the phantoms in air to simul ate measurement of Pt in superficial body sites. If the phantoms were place d in a water bath, to simulate measurement at greater depths of overlying t issue, higher systematic differences (15-20%) were observed. This effect is probably due to multiple scattering processes in the surrounding medium.