ESTRO 2022 - Abstract Book

S91

Abstract book

ESTRO 2022

expected dose rates every 50 ms, fig. 1. After the irradiation of each needle, an IVD-based source tracking methodology was used to determine the position of the needle. The discrepancy between expected and tracked position of the given needle was displayed. The irradiation was repeated ten times while the treatment verification software was giving real-time feedback. Furthermore, two irradiations with induced errors were performed: A swap of guide tubes for channel 5 and 6, and a 5 mm radial shift of channel 3. Results The procedures related to IVD, including detector calibration, exporting and loading the treatment plans, were performed in <20 minutes. The mean±1SD deviation between measured and expected dose were -3.28±1.13% for the total dose and - 0.66±8.21% for the dose pr. needle. The needle positions were tracked within 5 s, which is significantly shorter than the period between the irradiation of two subsequent needles (~25 s). The tracked position of each needle differed from the expected with 0.72±0.46 mm (mean±1SD) across the ten irradiations. The swap of guide tubes resulted in positional differences of +10.22/-9.32 mm for channel 5/6. The 5 mm radial shift of a single channel was determined to -4.88 mm, fig. 1.

Conclusion Real time IVD can be performed and provide online comparison of dose rates and tracking of the source. This can be provided to the users in an easy-to-interpret way and with <20 min additional workload. The next step is to utilise the real- time IVD monitoring in patients. [1] Jorgensen EB et al. Med. Phys. 2021:1-17, http://doi.org/10.1002/mp.15257

OC-0117 Evaluation of the use of micro-silica bead thermoluminescent detectors for brachytherapy dosimetry.

S. Wilby 1 , S. Jafari 1 , W. Polak 1 , A. Palmer 1

1 Portsmouth Hospital University Trust, Medical Physics, Portsmouth, United Kingdom

Purpose or Objective 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. Materials and Methods 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.