ESTRO 2025 - Abstract Book

S363

Brachytherapy - Physics

ESTRO 2025

As shown in the figure, the use of gMCO alone (CC+gMCO) increases the acceptance rate from 3.70% to 27.70% at μ = 0.1 (31.48% at μ = 0.005) for the more stringent soft constraints and from 70.37% to 79.63% for hard constraints. For optimized catheter configurations (SC+gMCO), acceptance increases up to 50.00% for soft constraints and 81.48% for hard constraints. Conclusion: The gMCO algorithm offers an overall better adherence to the EMBRACE II soft and hard constraints. Catheter number and configuration optimization (SC+gMCO) appears to provide substantial gain, but further validation is required. References: [1] P. Rogowski et al., “Clinical outcome of combined intracavitary / interstitial brachytherapy using a hybrid applicator in locally advanced cervical cancer”, eng, Gynecologic Oncology 166, 576–581 (2022). [2] C. Bélanger et al., “Simultaneous catheter and multicriteria optimization for HDR cervical cancer brachytherapy with a complex intracavity/interstitial applicator”, en, Medical Physics 51, 2128–2143 (2024). [3] R. Pötter et al., “The EMBRACE II study: The outcome and prospect of two decades of evolution within the GEC ESTRO GYN working group and the EMBRACE studies”, eng, Clinical and Translational Radiation Oncology 9, 48–60 (2018). Proffered Paper In-vivo Afterloader Integrated Electromagnetic Tracking allows accurate automatic Applicator Reconstruction in Cervical Cancer Patients Ioannis Androulakis 1 , Jérémy Godart 1,2 , Laura E. van Heerden 1 , Henrike Westerveld 1 , Remi Nout 1 , Mischa Hoogeman 1,2 , Inger-Karine K. Kolkman-Deurloo 1 1 Department of Radiotherapy, Erasmus MC Cancer Institute, University Medical Center, Rotterdam, Netherlands. 2 Department of Medical Physics & Informatics, HollandPTC, Delft, Netherlands Purpose/Objective: Applicator reconstruction is a laborious task in cervical cancer brachytherapy, despite the use of applicator dwell position model (DPM) libraries [1]. This is especially true in some of the tandem-ovoid applicators where channels can move in respect to the other two, increasing time and complexity of the reconstruction process. In previous research, we found that afterloader-integrated electromagnetic tracking (AIEMT) implemented in a Flexitron prototype afterloader (Elekta AB), is highly accurate for dwell-position model (DPM) definition of gynaecological applicators [2]. By nature, AIEMT has also the potential to automate the applicator reconstruction process using in vivo measurements and therefore reduce workload and pre-treatment preparation time. In this study, we evaluate whether AIEMT measurements in patients are accurate enough for automatic AIEMT-based reconstruction. First, we determined the DPM for the Utrecht (Elekta AB) applicator using ex-vivo AIEMT measurements. Then, we assessed the uncertainty of in-vivo AIEMT-based applicator reconstruction in patient treatments compared to the ex-vivo DPM and compared this uncertainty to the residual uncertainty of image-based applicator reconstruction. Material/Methods: Five ex-vivo AIEMT measurements of the Utrecht applicator were performed. The mean dwell-positions were used to generate the measured ex-vivo DPM. For patient evaluations, treatment plans and corresponding EMT measurements from 41 fractions of 20 patients were used. Image-based implant reconstructions were library based, using MRI(28) or CT(13). For 20 fractions, EMT measurements of the right ovoid were unavailable. To evaluate AIEMT-based reconstruction uncertainty in patients, we assumed that the geometry of each channel is Keywords: Dose optimization, Gyneacology 4131