ESTRO 2025 - Abstract Book

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Invited Speaker

ESTRO 2025

6. Spencer, K., Defourny, N., Tunstall, D., Cosgrove, V., Kirkby, K., Henry, A., Lievens, Y., & Hall, P. (2022). Variable and fixed costs in NHS radiotherapy; consequences for increasing hypo fractionation. Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology, 166, 180–188. https://doi.org/10.1016/j.radonc.2021.11.035

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Speaker Abstracts Transferring experience from dose accumulation in EBRT into combined EBRT+BT treatments Joakim Jonsson Department of diagnostics and intervention, Umeå University, Umeå, Sweden

Abstract:

This presentation aims to provide a comprehensive overview of dose accumulation techniques, with a focus on adapting established methods from external beam radiotherapy (EBRT) for use in combined EBRT and brachytherapy (BT) treatments. The lecture is designed to offer an educational review of the methodologies and quality assurance processes central to effective dose mapping in clinical practice. The presentation begins with an introduction to image registration, which is the cornerstone of dose accumulation. It covers the basic principles and practical aspects of registration, including the various coordinate systems and the significance of voxel size, position, and orientation. A variety of registration techniques are discussed, ranging from simple translation and rigid transformations to more complex affine and non-rigid methods. We discuss how the choice of transform should be matched to the specific clinical problem, such as correcting for patient position or compensating for anatomical variations over time. We also discuss the evaluation of registration quality, and the common metrics used to assess the degree of overlap between images during image registration, such as mean squared difference for single modality comparisons and mutual information for multi-modal cases. We highlight the importance of verifying registration accuracy using landmark comparisons and inverse consistency error analysis, ensuring that both fixed and moving images are properly aligned before proceeding to dose accumulation. Moving into the domain of dose summation, we outlines the strategies used to combine dose distributions from multiple treatment fractions. It compares two primary approaches: direct dose mapping (DDM) and energy/mass transfer (EMT). DDM is a method that utilizes pull interpolation to maintain a smooth dose distribution, while EMT is discussed in terms of its physical basis in conserving energy and mass, despite potential challenges such as generating gaps in the output dose distribution. Practical examples demonstrate how these techniques are applied in EBRT and the adjustments needed for the additional complexities of BT. The final section of the talk addresses brachytherapy-specific considerations. It reviews the challenges posed by the presence of applicators, such as the disappearing volume phenomenon, and discusses tailored registration approaches that may be required when intensity-based methods are not sufficiently robust. Overall, the session aims to equip attendees with a solid understanding of both the theoretical and practical aspects of dose accumulation and image registration. By summarizing established methods and quality assurance measures, the lecture aims to provide an introduction to the techniques that underpin accurate and effective dose accumulation, and what difficulties we encounter when trying to combine EBRT and BT doses.