Ge-doped silica optical fibres as RL/OSL dosimeters for radiotherapy dosimetry

Abstract

Various tailor-made doped-silica optical fibres are investigated as dosimeters in support of radiotherapy, based on radioluminescence (RL) and optically stimulated luminescence (OSL) technology. Investigations focus on the development of these glassy dosimetric media, offering a number of advantages going well beyond their water impervious nature and excellent spatial resolution (∼few microns). An RL/OSL photomultiplier-tube (PMT)-based reader was assembled, providing for study of the influence on the RL/OSL signal of different Ge-dopant concentrations (3.59, 4.74 and 7.03wt%) in silica fibres exposed to medical LINAC photon beams. Among the three arbitrary choices of dopant concentration, those fibres containing the least concentration of Ge (3.59wt%), denoted as Ge-1, gave rise to the greatest RL yield, at 1.67 and 2.34 times that of Ge-2 (fibres Ge-doped at 4.74wt%) and Ge-3 (fibres Ge-doped at 7.03wt%) respectively, reducing in yield with increasing Ge-dopant concentration. At 7.3×103 counts Gy−1min at 22°C, the Ge-1fibres provided the superior sensitivity, also being found to be reusable without noticeable variation in RL signal (<1%; 1 SD) for X-ray exposures delivered at a dose-rate of 600cGy/min. The RL signal was found to be free from spectral superposition or noise, also exhibiting energy independence in the use of X-rays generated at 6- and 10MV. In regard to percentage depth-dose (PDD), in measurements made using optical fibre dosimeters and 6MV X-ray photon beams, the maximum value of PDD, dmax, was obtained at a depth of 1.5cm, in accord with ionization chamber measurements. Using green stimulation light, for all three concentrations of Ge-dopant, linearity between the OSL signal and dose was observed across the 0.5- to 8Gy dose range investigated. Results from this ongoing study are intended to assist in efforts towards improving the performance of RL/OSL fibre sensors for radiotherapy dosimetry.