Session Item

Micro-Silica beads have been used as thermoluminescent (TL) dosimeters for radiotherapy with photon energies in the range 0.7 to 25 MV [1,2].  This study is the first to characterise their use with I-125 seeds, at a mean photon energy of 27 keV. Beads have potential to be valuable dosimeters in brachytherapy (BT) due to their small size (1.6 mm diameter) allowing high spatial resolution. 

An I-125 BT seed (AgX100, BXTAccelyon) was positioned vertically into a custom-made solid water (Gammex RMI 457) jig. A template holding silica bead (TOHO, Japan) dosimeters was positioned, with the centre of the beads level with the centre of the source. The beads were arranged to avoid any attenuation shadowing of other beads (Figure 1).  The solid water jig and template were positioned centrally in full scatter conditions (40 x 30 x 30 cm). Five different concentric ring bead templates were designed (table 1):

·   Templates 1 and 2 to check the linearity of the bead’s TL response with dose and dose rate. 

·  Templates 3 – 5 to evaluate repeatability and uncertainty. 

To convert TL signal to nominal dose, an individual bead sensitivity calibration factor (CF_S:D) was determined, by linac irradiation at 6MV.  Preparation, annealing and read-out of the beads followed previously published work.  Raw counts were corrected for background and sensitivity before applying CF_S:D to get nominal dose from calibrated beads (D_b). TG43U1 formalism was used to calculate the expected dose to water (D_w) at each of the ring distances used.

Table 1.  Details of the bead configurations, source strength and exposure time.

Figure 2a shows the ratio of bead dose to water dose (D_b/ D_w) for all beads on templates 1 and 2 as a function of distance from the source.  The results for beads furthest from the source, shows significantly bigger variability in response than those closer to the source. A possible explanation is that the limit of detectability from the beads has been reached or increased uncertainty at low dose levels. The average D_b/ D_w was 5.48 ± 0.16 (k = 2) (range 1.53 to 6.74).  Two anomalies circled in Figure 2b have been excluded from the average and the range.  Repeatability results from templates 3-5 were used to calculate the uncertainty.

The potential for use of silica beads in BT source dosimetry has been demonstrated. There is a need for reduced uncertainty in the readout process.  Further work including Monte-Carlo modelling of dose deposition in silica from I-125, as a function of energy change with distance from the source, will be conducted.