ESTRO 2023 - Abstract Book

S412

Sunday 14 May 2023

ESTRO 2023

Figure 1 : Overlay of three images showing different source locations as visualized by the HDR-Vue system. The Iridium-192 source was at dwell position 1 in section A) of the image, and in positions 10 and 20 in sections B) and C) respectively. Conclusion The measurements have shown that the HDR-Vue can detect the position of an Iridium-192 source in real-time, even when the source is inside a phantom. The ability to monitor the HDR source may help to detect gross errors and enhance treatment safety. Also, the knowledge of the source position facilitates patient evacuation in the event of incidents or accidents. PD-0504 Method for regular calibration of small point detector in brachytherapy M.L. Heidotting 1 , P. Georgi 1 , S. Kynde Nielsen 2 , A. Traberg Hansen 2 , H. Spejlborg 2 , S. Bjerre Hokland 2 , K. Tanderup 2 , J.G. Johansen 3 1 Aarhus University, Department of Clinical Medicine, Aarhus, Denmark; 2 Aarhus University, Department of Oncology, Aarhus, Denmark; 3 Aarhus University, Danish Center for Particle Therapy, Aarhus, Denmark Purpose or Objective Development of a simple method for routine recalibration of small point detectors used for in vivo dosimetry (IVD) during brachytherapy (BT). Materials and Methods IVD is a standard part of BT treatments at our clinic. The IVD is performed using a small detector based on a scintillating crystal (1x0.5x0.5 mm^3) coupled to a Si-Diode through a 15 m long optical fiber. During BT, the probe is placed inside the target area through BT catheters and the dose rate is read out at 20 Hz. To protect the probe, the optical fiber is detached from the Si-diode after each treatment, and reattached just prior to the next treatment. The response of the detector varies as it is detached and attached. This gives rise to the need of recalibrating the detector before each use. A simple method, which takes <10 min and can be performed as part of the pre-treatment QA, has been developed. The calibration is performed using a 30x12x12 cm^3 phantom made from PMMA (fig. 1 (top)). In this phantom there are two straight and parallel needles placed 20 mm apart; one for the probe, which is placed at the bottom of the needle, and one which is attached to an afterloader. A measurement is performed with 15 dwell positions, spaced 5 mm apart, each with a fixed dwell time of 10 s. A function based on a modified version of the TG43 formalism is fitted to the 15 data points (fig. 1). The modifications account for the missing scatter in the phantom. The calibration factor (from voltage to dose rate) (CF) and the location relative to the bottom of the needle of the closest approach to the sensitive area of the probe ( Δ Z) are used as fitting parameters. This procedure was performed on data from 48 different calibrations, using 4 different probes between March 2019 and December 2020.