ESTRO 2025 - Abstract Book

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Invited Speaker

ESTRO 2025

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Speaker Abstracts Overview on problems of dose accumulation in combined brachytherapy and EBRT treatments Taran Paulsen Hellebust Department of Medical Physics, Oslo University Hospital, Oslo, Norway Abstract: For treatments where external beam radiotherapy (EBRT) is used in combination with brachytherapy (BT) the contribution from both modalities should be considered. This is relevant for two scenarios: 1) decision making during the daily clinical procedures and 2) for development of dose response models. For the first scenario we need clinically approved tools that is not prolonging the procedure extensively. The second scenario is most often performed in the frame of a research project and more complex procedures and methods could be used and tested. Several methods for calculation of the contribution from the two modalities exist, e.g. DVH-parameter summation or voxel wise summation of the 3D dose distribution. The latter is often referred to as dose accumulation. Clinically today the simplest method, the DVH summation, is most often used and usually the contribution from the EBRT is assumed to be equal to the prescribed dose. Such assumption is most probably sufficient for 3D conformal EBRT and parameters representing the high dose area (e.g. D 2cm3 of rectum, bladder and sigmoid during cervical cancer brachytherapy). Dose response models use statistical methods to predict outcome, either probability of tumor response or toxicity, based on clinical radiotherapy data. For cervical cancer a lot of dose response models have been developed based on the EMBRACE data and in these models the DVH-parameter summation method is used [1]. For organs at risk (OAR) the strongest predictive models are found for the high doses to small volumes. For parameters representing other parts of the DVH curves, like a volume of an organ irradiated to a certain dose (e.g. V40) or dose delivered to a certain volume of an organ (e.g. D30), strong predictive models have been more difficult to create. One may speculate that this is due that more more sophisticated dose accumulation methods is needed to calculate dose parameters that is used as input in the models. To be able to do a voxel wise summation of the 3D dose distributions from both modalities, a deformable image registration (DIR) must be performed. In such procedure the spatial relationship between the two image sets is found. A high-quality DIR is essential to have reliable dose accumulation in regions with dose gradients. Since the applicator(s) used in BT is not present during EBRT it is challenging to achieve high-quality DIR in many cases. Moreover, large changes in the shape of an organ, e.g. bowel, between the two image series will add uncertainties to the dose accumulation procedure. Even though commercial systems are available, they lack the technology to perform the procedure swift and reliable, i.e. it is very time consuming to create a high-quality deformable image registration, if at all possible. This means that it is challenging to use dose accumulation during a BT procedure today if high accuracy is needed. The relevance of the clinical use of dose accumulation is therefore related to treatment site, which structure that is analyzed and the planning goal. The value of using the voxel wise summation method for development of prediction models should be explored further. 1. Tanderup K, Nesvacil N, Kirchheiner K et al. Evidence-Based Dose Planning Aims and Dose Prescription in Image-Guided Brachytherapy Combined With Radiochemotherapy in Locally Advanced Cervical Cancer. 2020 Semin Radiat Oncol 30:311−327

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Speaker Abstracts Concomitant RT and systemic therapy in breast cancer: Focus on ADCs and CDK4/6i Birgitte Offersen Department of Experimental Clinical Oncology, Aarhus University Hospital, Aarhus, Denmark