ESTRO 2024 - Abstract Book

S81 ESTRO 2024 This presentation will delve into late effects of radiotherapy from a physics perspective. Topics covered will include methods to estimate dose, the use of substructures in risk modeling, and the application of risk models in plan optimization or patient selection for different treatment techniques. When considering dose, it can be important to not only consider the primary dose that is well-modeled by the treatment planning system, but also the stray and out-of-field doses. For photon therapy, this mostly consists of scatter and leakage photon radiation, while for proton therapy and other heavy ion therapy, it includes neutrons and other secondary charged particles and fragments. Different types of radiation have different radiobiological effectiveness, which should be corrected for with an appropriate radiation weighting factor. For historical treatments lacking detailed anatomy and dose information, techniques can be used to reconstruct the dose. These methods can include Monte Carlo simulations, analytical calculations, and measurements performed using generic/scalable phantom geometries with approximate organ locations. When detailed plan and anatomy information is available, extracting detailed dose information for organs and even organ substructures can be useful. One example is how the dose to the heart valves has been shown to be related to the risk of heart valve disease. Once the doses to different organs or substructures are estimated, they can be utilized in epidemiological studies for the creation of new risk models, or in the application of already existing models for the prediction of the risk of late effects. While these models are uncertain, applying them in a quantitative way can be useful for optimization of a given technique or for choosing between different techniques (e.g. referral for photon therapy or proton therapy). Invited Speaker

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Online adaptive RT in gynecological tumours

Liselotte ten Asbroek-Zwolsman

Medisch Spectrum Twente, Radiotherapy, Enschede, Netherlands

Abstract:

Online adaptive radiotherapy (OART) is increasingly becoming the new standard for many tumor areas. In Medisch Spectrum Twente, we have been treating patients adaptively with the Ethos system (Varian Medical Systems) since the beginning of 2020. We started with prostate treatments. After gaining considerable experience, we expanded OART to other tumor sites in the pelvic region, such as bladder and rectal tumors. We are convinced OART could also offer added value for patients with a primary gynaecological tumor. Being able to correct for a change in anatomy and inter-fractional motion, margins may in time be reduced; potentially leading to less side effects due to a lower dose in organs at risk. At ESTRO 2023 we presented the first results of OART in cervical cancer. We observed that OART resulted in increased target coverage, but not necessarily in less bladder and rectum dose. Besides correction for position changes of the target, the main advantage of OART for cervical cancer patients was the possibility to correct for target volume reductions which were considerable and resulted subsequently in lower bowel bag doses. We are currently investigating the implementation of OART in post-operative endometrial cancer. The challenge herein is that the Ethos system has to recognize the postsurgical area in order to properly propagate the CTV. In contrast to the other treatment sites, where the affected organ is still in the body. In this session the considerations for OART for gynaecological tumors will be discussed. In addition, the results of both primary cervical OART and the feasibility of post-operative endometrial OART will be presented. We will discuss