ESTRO 2020 Abstract Book

S205 ESTRO 2020

Preliminary randomized prospective studies are confirming that OMD patients may have controllable symptoms, better outcome, and may even be cured. Incorporating OMD patients in clinical trials requires that patients are correctly “staged” and followed with appropriate imaging, to avoid inappropriate inclusion and inappropriately treating disease subsequently recognized as polymetastatic. The diagnosis of OMD is thus highly dependent on imaging modalities used to detect lesions. Modern imaging modalities are necessary. PET/CT imaging using cancer specific tracers and (whole body) MRI including diffusion weighted imaging (DWI) are currently the methods of choice, and the diagnostic strategy varies according to cancer type, tumour type, stage of the disease (newly diagnosed, recurrence, resistance,…), and timing of evaluation in relation to previous treatment. Those techniques provide a whole-body approach, allow a and reliable diagnosis of OMD compared to previously used bone scintigraphy and CT. This presentation briefly illustrates the evidence supporting the use of modern imaging and cancer specific approaches of OMD in the most frequent cancers. Clinical algorithms for integrating modern imaging methods in the care pathway to identify OMD at the various stages of these cancers are proposed. Clinical trial designs utilizing modern imaging methods are discussed for evaluating the benefits of MDT. Abstract text The response of normal tissues to irradiation is mainly determined by the survival and regenerative potential of the tissue stem cells, and modulated by inflammatory processes, vasculature damage and altered neuronal innervation and fibrosis. Indeed, transplantation of tissue specific stem cells has been shown to restores tissue homeostasis and prevent late radiation effects. Moreover, the specific sparing of localized stem cells was predicted to preserve salivary gland function in rodents and patients treated for head and neck cancer. Recently, we and others have developed methods to culture patient specific salivary gland and thyroid gland stem cell derived organoids (tissue resembling structures). These organoids contain all the glandular lineages and are able to extensively self-renew, allowing the study of their radiation response. Indeed, stem cell radiation survival curves and DNA DSB repair kinetics indicate that the response of organoids to different radiation qualities may differ from tissue to tissue, especially in the low dose regions typically delivered to the normal tissue outside the planning target volume. Interestingly, several signalling pathways involved in stemness have been associated with the sensitivity to irradiation. Moreover, many of these factors are modulated upon stem cell transplantation. In this presentation the potential of stem cell derived organoids in the prediction prevention and treatment of normal tissue side effects will be discussed. Several signalling pathways involved in stemness affect Symposium: Use of 3-dimensional organoid models in radiation research SP-0379 Radiation responses of normal tissue organoids R. Coppes 1 1 University Medical Center Groningen, Radiation Oncology & Biomedical Sciences Of Cells And Systems, Groningen, The Netherlands

radiosensitivity and may be used to optimize response. Next to this, mature organoids can be used to study cell death and regeneration responses. Some of these will also be discussed and potential means to manipulate will be addressed. Supported by the Dutch Cancer Society (KWF, grant Nrs. 4022, 5792 and 10650), the Netherlands Institute for Regenerative Medicine (NIRM, grant No. FES0908) and The Netherlands Organisation for Health Research and Development (ZonMw, Grant nrs. 11.600.1023 and 40- 43600-98-14003) SP-0380 Investigating interactions been tumors and normal tissue using normal and cancer organoids M. Vooijs Maastricht University, The Netherlands

Abstract not received

SP-0381 Using tumor organoids to predict responses to therapy F. Pajonk David Geffen School of Medicine at University of California, USA

Abstract not received

Symposium: Controversies in locally advanced breast cancer

SP-0382 Regional nodal irradiation without chestwall radiotherapy for breast cancer L. Boersma 1 1 Maastricht University Medical Centre+, Radiation Oncology MAASTRO- GROW School for Oncology and Developmental Biology-, Maastricht, The Netherlands Abstract text In the first part of this presentation, the definition of Locally Advanced Breast Cancer (LABC) will be discussed. A short review will be given on the distribution of locoregional recurrences after a modified radical mastectomy (MRM) in the absence of radiation treatment (RT), to identify regions with the highest risk on a recurrence. From these data the choice for target volumes of post mastectomy radiotherapy (PMRT) will become clear. In the second part, replacing an axillary lymph node dissection (ALND) by axillary RT (ART) is discussed, which may result in some specific situations where regional RT is applied without chestwall (CW)-RT. Definition of LABC and distribution of locoregional recurrences: The term LABC usually describes a breast cancer that has progressed locally but has not yet spread outside the breast and local lymph nodes. The exact definition varies from Stage III breast cancer 1 to patients with ≥cT3, and/or ≥cN2 2 disease, to even patients with only cT1-2N+ patients. In most guidelines, locoregional treatment of LABC consists of breast surgery, usually MRM including an ALND, and at least CW-RT, but mostly locoregional RT. The CW is always included in the target volume, since several studies have shown that after MRM the risk on developing CW-recurrences is the highest, compared to nodal recurrences: in an analysis of 1099 patients treated with an MRM and systemic treatment in four ECOG trials where adjuvant RT was not allowed, Recht et al 3 found a 10 year CW-recurrence rate of 12%, with 8% in the supraclavicular nodes, 4% in the axillary nodes, and