ESTRO 2022 - Abstract Book

S1347

Abstract book

ESTRO 2022

Two independent dosimetry methods (silica TL beads, 1.1mm thick, and CCO4 ionisation chamber, 0.04cm 3 volume, 4 mm cavity diameter) were used to measure the dose delivered for a range of treatment applicator field sizes (2-20cm diameter) and lead cut-outs, for the Xstrahl D3300 treatment unit, over a range of energies (70-250kV). The novel aspect of this work was to experimentally determine output factors using silica beads, which have only had limited use in kV energies [Palmer et al 2017] and [Jafari et al 2014]. Due to their small size, nine beads were used for each dose measurement to reduce uncertainly. Beads were placed on the surface of a block of solid water with a stand- off of 6 mm, see Figure 1. Calibration of the glass beads was performed for each energy against a traceably calibrated ionisation chamber to generate an energy specific calibration factor. This was in addition to bead-specific individual sensitivity calibration using 6 MV photons in a reference condition factors, completed due to small variation measured across a batch of beads. The glass bead TL signal was read-out using a TLD reader (Toledo 654). Results Differences were observed between the experimentally determined output factors using the two detectors and existing plan data obtained from BJR 25, as illustrated in Figure 2 for a 4cm applicator with various lead cut-outs The experimental output factors measured using the CC04 ionization chamber and silica beads TLDs were in agreement, within experimental uncertainty. These discrepancies were higher for the smallest and largest field sizes, for all energies. The beads and CC04 results differed by up to 1% (uncertainty +/- 1.1%), the mean value of which was up to 3.1% different to BJR data.

Conclusion The output factors for a kV treatment unit have been successfully measured using silica bead TL dosimeters, verified against an ionisation chamber. Statistically significant difference was measured from the current clinically used BJR 25 data and should be adopted clinically. This work also shows that silica beads can be successfully used to verify the dose delivery of small field sizes on a kV unit, important as they are only 1.1mm thick (compared to 4mm diameter for the CC04) so can be used to measure very small field sizes, down to 1cm diameter cut-outs in this work.

PO-1566 Investigation of the required detector distance in array-based treatment plan verifications

A. Stedem 1 , J. Quast 1 , H. Gottschlag 2 , A.A. Schönfeld 3

1 Heinrich-Heine-Universität Düsseldorf, Mathematisch-Naturwissenschaftliche Fakultät, Düsseldorf, Germany; 2 Universitätsklinik Düsseldorf, Klinik für Strahlentherapie und Radioonkologie, Düsseldorf, Germany; 3 Sun Nuclear Corporation, Research and Development, Melbourne, USA Purpose or Objective The aim of the work was to determine the spatial frequency content of small radiation fields and planar dose distributions common in SRT, to ensure that the Nyquist-Shannon sampling theorem is fulfilled by detector arrays used for pre-treatment patient specific quality assurance.