ESTRO 2025 - Abstract Book

S343

Brachytherapy - Physics

ESTRO 2025

Figure 2. Location of the detector in relation to the radiation source – applicator a) Patient 1 (red line distance 37mm), b) Patient 2 (blue line 65mm) The observed deviations for doses calculated in TPS relative to the measurement may be caused by different phenomenal. In first, such deviations can be caused by imprecise positioning of the detector relative to the radiation source. Therefore, in the case of large dose gradients, even small change in position can result in a large uncertainty in the dose reading. Second reason observed deviations in dose measurements can be connected with optical fibre (stem effect) Conclusion: Scintillation crystals combined with optical fibers and very sensitive luminescence spectrometers create an ideal measurement system for in situ determining the dose of various ionizing radiations. However, their use in clinical practice requires the development of procedures for precisely positioning the detector in relation to the radiation target.

Keywords: in vivo; scintillation

References: [1] Akselrod, M.S., Lucas, A.C., Polf, J.C., McKeever, S.W.S., 1998. Radiat. Meas. 29, 391-399 [2] A. Sas-Bieniarz, B. Marczewska, P. Bilski, W. Gieszczyk, M. Kłosowski, Radiation Measurements, 136 (2020) 106408. [3] D. Jimenez-Rey; B. Zurro; K. J. McCarthy; G. Garcia; A. Baciero, Rev. Sci. Instrum. 79, 10E516 (2008) [4] I. Veronese, M. G. Bissogni, F. Cova, E. Ciarocci, M. Fassoli, M. Tonnely, M. Morrocchi, A. Vedda, Presented at SSD2023 conference.

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Proffered Paper Robust Treatment Plan Evaluation for Cervix Cancer Brachytherapy Using Organ Motion Simulation in RayStation Stefan Ecker 1 , Nicole Eder-Nesvacil 1 , Lovisa Byman 2 , Tom Niessen 2 , Erik Engwall 2 , Maximilian Schmid 1 , Aleksandra Winkler 1 , Dietmar Georg 1 , Christian Kirisits 1 1 Department of Radiation Oncology, Comprehensive Cancer Center, Medical University Vienna, Vienna, Austria. 2 Research and Development, RaySearch Laboratories, Stockholm, Sweden Purpose/Objective: To evaluate the robustness of clinically used high-dose-rate (HDR) brachytherapy (BT) treatment plans for locally advanced cervical cancer, by simulating organ motion in the RayStation treatment planning system (TPS). Material/Methods: A retrospective analysis was conducted on 10 treatment plans from 5 patients, treated with four fractions of HDR BT using intracavitary applicators and interstitial needles. Treatments involved magnetic resonance image-guided adaptive brachytherapy (MR-IGABT), with dose-volume parameters prescribed according to ICRU-89 and EMBRACE II objectives. Clinical plans were created using manual optimization and simple robust treatment planning principles for loading patterns. Treatment plans were analyzed in RayStation. Relative motion of organs at risk (OARs) was simulated using multiple deformed patient images generated by deformation fields driven by motions of one OAR at a time. The implant channels and primary tumor target volumes were excluded in the deformation to simulate a fixed implant. Organ motion for the bladder, rectum, and bowel was simulated by displacing them in discrete directions - superior, inferior, anterior, posterior, left, and right - with magnitudes of ±0.5, ±1, and ±2 cm (Figure 1). For each scenario, the dosimetric impact on OAR D 2cm3 (absorbed dose in Gy) was evaluated by calculating the absolute percentage difference per millimeter (ΔD %/mm ) between the clinical and shifted scenarios.