ESTRO 2023 - Abstract Book

S714

Monday 15 May 2023

ESTRO 2023

be one of the (if not the) major causes of tumour recurrence and consequent treatment failure. The genetic heterogeneities of the tumour, the varying metabolism of the tumour cells, different cellular densities, and tumour hypoxia lead to varying levels of radiation sensitivity within the same tumour mass. Dose escalation strategies targeting those tumour sub-volumes may increase local control, provided that meaningful radiobiological targets are defined. Functional imaging, such as positron emission tomography (PET), magnetic resonance imaging, or combined multiparametric modalities, may have a crucial role not only in the identification of those tumour sub-regions of increased radioresistance for a biologically guided radiotherapy strategy but also in the monitoring of the responsiveness of the tumour to the treatment in a framework of response-adaptive radiotherapy. In the case of PET imaging of hypoxia, several dose sculpting strategies have been proposed to escalate the dose in radioresistant tumour sub-volumes. They include empirical approaches exclusively based on clinical experience, methods based on a linear conversion of radiotracer uptake into dose levels, and more sophisticated strategies guided by radiobiological modelling where the computation of the dose distribution required to achieve a desired level of tumour control probability (TCP) is considered. This talk will overview the proposed dose painting strategies, going from the "classical" dose painting by numbers, dose painting by contours and dose redistribution methods to alternative dose clustering methods more recently proposed. To date, the potential of hypoxia-PET imaging for quantitatively characterising the microenvironment and determining the required dose escalation level has yet to be fully explored. Indeed, one of the main challenges when dealing with hypoxic treatment resistance is the inherent multiscale feature of the problem where the functional imaging of hypoxia is limited to the millimetre regime, while the oxygen diffusion varies on a sub-millimetre scale. Consequently, the limitation of the PET resolution risks overshadowing the ability of this imaging technique to represent the microenvironment faithfully. Hence, developing robust dose sculpting methods to bridge the resolution gap would be timely and desirable to improve current treatment standards. As an example, two different dose prescription strategies based on hypoxia PET imaging for both photons and protons will be compared in terms of TCP by considering an in-silico modelled underlying oxygen distribution at a sub-millimetre scale. If pre-treatment functional imaging investigations could provide initial information on the dose levels required to counteract the specific radioresistance of the tumour for each patient, subsequent examinations early during the treatment could then provide information on tumour responsiveness. This information may subsequently be used to determine the need for treatment adaptation and allow for individual adjustments of the radiation dose, also taking into account the accumulated delivered dose distribution at the repeated imaging time. While biologically guided radiotherapy and individualisation of cancer treatment remain two of the ultimate aims of modern radiotherapy, their translation into clinical practice remains challenging. Abstract Text Stereotactic body radiotherapy (SBRT) is a method of external beam radiotherapy in which a high dose of radiation is delivered precisely to an extracranial target in one or a few treatment fractions. It is similar to brachytherapy (BT) in most respects, except that BT is not an external beam procedure. In this presentation, similarities and differences between BT and SBRT will be highlighted. An important issue is the comparison of physical doses, as both techniques can deliver high, often inhomogeneous doses to a small volume. Motion management is also important, as doses should be delivered with high dose gradients. Here, BT has an advantage because the applicators and radiation source are directly connected to the patient and movements do not have a major impact on the treatment. Both techniques can be used adaptively, i.e., 3D imaging is performed immediately before irradiation, and the treatment plan is adjusted accordingly. Adaptive treatment of prostate cancer and cervical cancer has been used in BT for many years; for SBRT, this is still a challenge, but is already well possible with dedicated treatment devices for adaptive irradiation. Another issue that should be considered is the integral dose. Due to the physical properties of high-energy photon beams, SBRT exposes a large volume of the patient to a low dose. This volume is lower with BT and therefore beneficial to the patient as secondary malignancies are likely to be reduced. SP-0860 Homogeneous dose as is for the whole body: A universal tool D. Thorwarth 1 1 University of Tübingen, Section for Biomedical Physics, Department of Radiation Oncology, Tübingen, Germany Abstract Text External bean radiotherapy (EBRT) has a long tradition of prescribing homogeneous radiation doses to tumor volumes for safe and efficient fractionated treatments. In this talk, the rationale for homogeneous dose prescriptions and their advantages will be discussed, also in the light of uncertainties present during radiotherapy planning and application. In addition, risks associated with lower doses to regions with unknown tumor cell density and microscopic spread will be outlined. Furthermore, current tumor control probability models informing EBRT will be discussed, seen that the vast majority of data used to train such models has been collected in homogeneous dose regimens. Symposium: What can EBRT learn from brachytherapy and vice versa? SP-0859 High dose to a small volume - Is brachytherapy the best SBRT? F. Siebert 1 1 UKSH, Campus Kiel, Clinic of Radiotherapy, Kiel, Germany

SP-0861 Image-guidance, intensity-modulation and AI in brachytherapy C. Kirisits 1 1 Medical University of Vienna, Radiation Oncology, Vienna, Austria

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