ESTRO 2022 - Abstract Book

S92

Abstract book

ESTRO 2022

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

Ring distances from source centre (mm) (# beads per ring)

Source Strength, t=0 (mCi)

Exposure time (days)

Template

1

5, 8, 10 …. 50 in 5 mm steps (5)

0.50

15

2

5, 8, 10 …. 50 in 5 mm steps (5)

0.28

15

3

10 (12)

0.55

21

4

10 (5), 20 (20)

0.43

14

5

20 (20), 40 (20)

0.53

35

Results 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. Conclusion 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.