The angular dependence and effective point of measurement of a cylindrical scintillation dosimeter with and without a radio-opaque marker for brachytherapy.

Fibre optic scintillation dosimeters, consisting of a plastic scintillator coupled to an optical fibre, are a promising dosimeter in brachytherapy applications. The combination of tissue equivalence, real-time readout and small spatial size makes them especially attractive for in vivo verification of patient treatments. Given that the orientation of the dosimeter with respect to the radioactive source changes during brachytherapy treatment, the angular dependence of the dosimeter is important. We derived the dependence of the response of a cylindrical dosimeter to a point radiation source as a function of distance along its axis and along a radius. Using the results, the effective point of measurement of a cylindrical scintillator was located for two points in the angular response curve as a function of distance between the source and dosimeter. We measured the angular response experimentally for a cylindrical scintillation dosimeter, when the source was located at a distance of 50 mm from the centre of the scintillator. A refinement of the design, in which a radio-opaque marker is incorporated into the tip for accurate localization in the patient, modifies the angular response of the dosimeter. For this new dosimeter design, we show that the dosimeter response decreases by 20% when the source is located on the axis of the scintillator, due to absorption by the marker. The dosimeter response becomes almost angle independent at 10 degrees away from the axis. Excluding this cone, a cylindrical scintillation dosimeter which incorporates a radio-opaque marker was found to be angle independent to within 2%. In most clinical brachytherapy applications, this design has an acceptable angular dependence.