ESTRO 2020 Abstract Book

S210 ESTRO 2020

of patients, but their applicability for predicting the response of individual patients is highly questionable. Different approaches could therefore be considered for deriving the parameters for the TCP models. The increasing role of functional imaging in conjunction with preclinical assays for the assessment of TCP model parameters will be discussed. Finally, the role of radiobiological modelling in planning evaluation and patient selection for different treatment modalities will be presented. The limitations as well as the potential of the models for TCP will be acknowledged. The modelling of the tumour control probability together with the a priory toxicity predictions could help guiding the treatment planning process towards individualised approaches to the great benefit of the patients. SP-0392 The role of functional imaging in radiotherapy planning K. Roe Redalen Norwegian University of Science and Technology, Trondheim, Norway Modern radiotherapy techniques such as IMRT, VMAT and proton therapy are all very flexible treatment techniques where we today can obtain high dose coverage in the tumor volume at the same time as we spare organs at risk. This flexibility is also giving us the opportunity to escalate the radiation dose to more radioresistant areas of the gross tumor volume without increasing the side effects. In parallel we have also seen technological developments in medical image acquisition and analysis that increasingly are providing faster and more detailed imaging for contouring of both targets and organs at risk (OARs), treatment planning, response prediction and evaluation, as well as quality assurance. Anatomical imaging has been used for a long time to identify healthy and diseased tissue, tumor size, stage and location of the tumor. On the other hand, functional imaging is imaging methods that allow us to visualize and quantify functional or radiobiological tissue characteristics such as metabolism (FDG-PET), proliferation (FLT-PET), cell density (DW-MRI), perfusion (DCE MRI) and hypoxia (FMISO-PET). These characterstics are often different for tumors with same size and stage and are often associated with different response to radiotherapy. Therefore, functional imaging is a promising tool to use for improving individualised radiotherapy. To exploit quantitative features in functional images in radiotherapy, different approaches can be used. Some imaging signals can be used directly from the scanner whereas others need normalization to various parameters such as body weight or injected dose of PET tracer. Other approaches require more complex quantification such as tracer kinetic modeling of DCE MRI. The quantitative functional images can be used for different purposes and three of these will be discussed; 1) more accurate target and organs at risk delineation, 2) treatment stratification and response monitoring for treatment adaptation, and 3) dose escalation or dose-painting of radioresistant subvolumes. Strategies for how functional images can be used in these three situations will be presented along with examples from lung cancer and head and neck cancer. For those imaging parameters which demonstrate changes during radiotherapy that is connected outcomes, there becomes a possilibity to use this information for functional adaptive therapy. Then, both anatomical and functional changes can be re-assessed with imaging during the course of therapy, and response-adaptive radiotherapy becomes

a possibility in order to achieve a better outcome and lower toxicity. To use functional imaging in radiotherapy applications require robust procedures for image acquisition and analysis. Important aspects to acquire images with high image quality and geometric accuracy suitable for radiotherapy applications will be presented along with relevant metrics to assess repeatability and reproducibility of image parameters. The presentation will end with a discussion of some current research topics, in particular on the future possibilities of merging functional imaging with artificial intelligence in radiotherapy applications. SP-0393 Audits for IMRT/VMAT: present and future P. Kazantsev 1 1 IAEA - International Atomic Energy Agency, NAHU, Wien, Austria Abstract text The International Atomic Energy Agency has been providing dosimetry audit services for radiotherapy centres around the world for over 50 years. One of the major challenges in this work is to keep up with modern developments in radiotherapy technology such as introduction of multileaf collimators (MLCs), computerized and later inverse treatment planning, and advanced on- board imaging capabilities. Clinical implementation of any new technologies should be accompanied by adequate quality assurance (QA) procedures and supported by external audits. Broad introduction of intensity-modulated radiation therapy (IMRT) into clinical practice in the early 2000s led to the development of several remote and on-site audit methodologies of these techniques. Almost simultaneously, several dosimetry audit networks (DANs) in Europe and USA developed audit methodologies for IMRT audits. They mostly utilized custom made phantoms of different geometries mimicking specific anatomical sites, with the dose delivered measured with films and thermoluminescent (TLDs) or other solid-state dosimeters placed in critical locations. The IAEA initiated the development of IMRT audits in 2012, under the coordinated research project (CRP) “Development of Quality Audits for Advanced Technology in Radiotherapy Dose Delivery”. Since the aim was to build expertise in the national DANs participating in the CRP, it gradually covered topics of auditing MLC performance, small field dosimetry, single-field IMRT checking, and finally tested an audit methodology following an end-to- end approach. The latter audit methodology utilized a custom-made solid water phantom containing the planned target volume (PTV) and the organ at risk (OAR) located close to each other and accommodating TLDs within the structures and a film in the cross-section. The audit was implemented in 6 countries with an average pass rate of 66%. An on-site audit methodology was then developed by the IAEA, which also employed the end-to-end approach. The phantom for the audit was developed and is now commercially available as the “Shoulders, Head and Neck, End-to-end” phantom (SHANE, CIRS Inc.). The audit includes a pre-visit phase which allows to the collection and analyses of relevant linac commissioning, treatment planning and MLC performance data before the on-site Symposium: Audits for advanced radiotherapy techniques