ESTRO 2025 - Abstract Book

S35

Invited Speaker

ESTRO 2025

Distal edge tracking (DET) is a technique that places Bragg peaks at the distal end of the tumor target volume, without filling up the dose with more proximal Bragg peaks. A uniform dose coverage of the tumor is achieved through intensity modulation and the use of multiple beams or arc therapy. This technique is dosimetrically advantageous, but it has not been used in clinical practice because of its high sensitivity to uncertainty. Through in vivo verification, the uncertainties can be greatly reduced and DET can become a clinical reality. In addition to better dosimetric characteristics, advantages of DET include it’s use for FLASH therapy, and that it leads to greater efficiency of the treatment delivery (only one energy per beam orientation), which in turn can lead to better affordability. Other uses of DET include mini beam treatments and spatial fractionation.

4. Enabling adaptive therapy

The promise of adaptive therapy is to correct for daily anatomical variations and possibly biological variations during the treatment course. Adaptation is only possible to its full extent if the beams can be placed reliably with minimal range uncertainties – which can be afforded by in-vivo verification. The need for in-vivo treatment verification is even larger in cases where intra-fractional variations, such as breathing motion, require real-time adaptation.

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Speaker Abstracts Hard facts: Clinical experience with in vivo treatment verification Christian Richter

Medical Physics Section, OncoRay - National Center for Radiation Research in Oncology, Dresden, Germany. Institute for Radiooncology - OncoRay, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany. Department of Radiotherapy and Radiation Oncology, University Hospital Carl Gustav Carus, Dresden, Germany

Abstract:

Despite the increasing availability of in-room CBCT imaging in particle therapy (PT), there is a strong demand on independent in-vivo verification of the treatment delivery. Especially in the context of online-adaptive PT, where the treatment is adapted a few minutes or in the future seconds before the delivery, with CBCT-based range prediction and without any phantom based patient-specific QA feasible, the additional safety net functionality of in-vivo treatment verification is highly desirable. There is an increasing interest in the recent years. Among the available in-vivo treatment verification techniques, prompt-gamma based in vivo treatment verification (PGTV) is currently the most promising approach and will be the focus of the presentation. PGTV offers both a safety net functionality and the ability to detect (unexpected) anatomical changes during delivery, enabling a triggered OAPT workflow. Based on the information domain that is measured and evaluated, one can distinguish prompt gamma imaging (PGI, measuring the spatial distribution), prompt gamma ray timing (PGT, measuring the temporal distribution) and prompt gamma ray spectroscopy (PGS, measuring the spectral distribution). With improvements in detectors, different domains of information could also be combined, so-called multi-feature treatment verification. The use of prompt gamma rays for proton range verification was first proposed in 2003 (Stichelhaut and Jongen 2003). Since 2015 PGI is in clinical application (Richter et al. 2016; Xie et al. 2017), PGS since 2020 (Verburg et al. 2020), proofing the feasibility of both approaches under realistic conditions. While all approaches measure primarily the prompt gamma information per pencil beam spot, there are quite different approaches of evaluating this raw data to conclude on clinically relevant treatment deviations. Usually, first a spot-wise range shift is determined by comparing the measured PG signal with an expected PG signal. However, in real-world application that spot-wise information has limited accuracy, especially for low weighted spots, but more importantly a conclusion on clinically relevant treatment deviation is difficult from range information at the spot-