ESTRO 2023 - Abstract Book

S1500

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ESTRO 2023

Table1: Results for each target including diameter (cm), Volume (cc), Distance from Iso (cm), Conformity Index (CI), Gradient Index (GI Paddick), and Dose Cube Offset (mm)

Results Table 1 shows the estimated offset (mm) between the geometric centers of the delivered and planned 100% IDL for each PTV. The average offset for the deliveries is 0.8 mm for the VersaHD ET-D system. We note that the CT-CT and MR-CT fusions used images with 0.625 mm and 1mm slice thickness for the CT and MR scans respectively. Furthermore, our analysis did not consider any possible geometric deformations during MR scanning. Table 1 shows a weak correlation between the volume of the PTV and its distance from the isocenter when compared to the estimated offset. Conclusion Gel measurement has demonstrated that the system, VersaHD ET-D, has an average offset of nearly 1.0mm of the PTV's dose coverage for up to 5.0 cm from the isocenter which is well within clinical recommendation and expectation.

PO-1779 Along-away table with a plastic scintillator detector for in vivo dosimetry in HDR brachytherapy

J. Tarrats-Rosell 1 , A. Herreros 1 , J. Neves 2 , J. Melo 2 , L. Moutinho 2

1 Hospital Clínic de Barcelona, Servei d'Oncologia Radioteràpica, Barcelona, Spain; 2 NU-RISE, S.A., Aveiro, Portugal

Purpose or Objective Plastic scintillator detectors (PSDs) fulfil the different requirements for in vivo HDR brachytherapy dosimetry, such as small size, near water equivalence, good dynamic range and energy, temperature and angular independence. For the PSD BCF-12 (Saint Gobain Crystals) an along-away table has not been reported yet in Ir-192 HDR brachytherapy. The purpose of this study was to measure an along-away table with a PSD system and comparing data with those published in the AAPM-ESTRO Report No. 229. Materials and Methods BCF-12 is a 2-mm long organic scintillator attached to a polymethyl methacrylate (PMMA) optical fibre with a core diameter of 0.5 mm. Along-away characterisation of the BCF-12 was performed in a home-made PMMA phantom (NU-RISE) (Figure 1). The PSD and HDR Ir-192 microSelectron v2 source were inserted along the z-axis parallelly. The centre of the PSD corresponds to the z-axis origin. The phantom was completely submerged in a water phantom to emulate the full scatter conditions of AAPM TG-43. We evaluated the response to radial source-to-PSD distance, r, from 1 cm to 7 cm, for z = 0. For each radial distance, the z-axis dependency was evaluated for z ranging between –6 cm and +6 cm. The same measurements were made with a dummy probe (without the scintillator) for a manual subtraction of the stem effect. Signal was acquired by the PRO-DOSE in vivo dosimetry system (NU-RISE). Measurements were analysed with a home-made script to remove the dark noise, identify abrupt changes in the signal to recognize positions where the source stopped and obtain the mean count rate. All measurements were corrected for the current RAKR and normalised at a radial distance of 4 cm in the z-origin to the value of the published along-away table.