ESTRO meets Asia 2024 - Abstract Book

S305

Physics – Detectors, dose measurement and phantoms

ESTRO meets Asia 2024

Scott B Crowe 1,2,3 , Rachael Wilks 1,2 , Hanieh Hajnorouzi 1 , Tanya Kairn 1,2,3

1 Cancer Care Services, Royal Brisbane & Women's Hospital, Brisbane, Australia. 2 School of Electrical Engineering & Computer Science, University of Queensland, Brisbane, Australia. 3 School of Chemistry & Physics, Queensland University of Technology, Brisbane, Australia

Purpose/Objective:

The Xoft Axxent is an electronic brachytherapy system featuring a miniaturised 50 kV x-ray tube. The system can be used for intraoperative radiotherapy (IORT) of the breast when combined with a saline-filled balloon applicator. The delivery of the prescribed dose for breast IORT treatments is dependent on the accuracy of source calibration at time of treatment, TG-43 reference data used for dose calculations, and applicator specific correction factors, and the constancy of source output. This study validated the dosimetric accuracy of the system through physical measurements.

Material/Methods:

Reference air-kerma rate (RAKR) values measured using the vendor-supplied integrated well chamber were verified using a PTW TN34013 chamber, placed in an in-house designed jig, consisting of a 1 m long 20x20 mm v slot aluminium extrusion with 3D-printed source and detector holders. Measurements were made at multiple distances from 16 to 32 cm from source, with scatter-corrected RAKR at 50 cm calculated via regression. The accuracy of the TG-43 reference data and atlas plan dose calculations was established using a simple slab phantom containing a vendor-provided PMMA source holder, sheet bolus and water-equivalent plastics. End-to end verifications were performed using a balloon applicator inflated with 30 ml of water, within a breast phantom printed using PLA at a density mimicking breast tissue. The treatment plan used 5 dwell-positions to deliver a prescription of 20 Gy to the balloon surface. This measurement was repeated with a rigid shield present, to verify vendor-provided attenuation factors. For all phantom measurements, dose was measured with EBT3 film calibrated with a 50 kV superficial beam, and irradiations were performed with flexible shielding draped over the phantom, consistent with clinical treatments. Phantom geometries are shown in Figure 1.

Figure 1. Measurement geometries showing (a) a slab phantom with PMMA, (b) the printed breast phantom, and (c) the same phantom with rigid shielding.

The constancy of RAKR as measured using the well chamber was monitored for three sources over the course of 1 year, and tolerance limits established using control charts.

Results:

The RAKR measured with the soft x-ray detector was 11.2±0.3 mGy/min at 50 cm distance, which agreed within uncertainties with the mean well chamber measurement of 11.11±0.02 mGy/min.

Agreement was observed within uncertainty in the slab phantom geometry, with -0.3% difference between the calculated and measured doses, 10.8±0.3 Gy and 10.83±0.3 respectively. The dose fall-off measured orthogonal