ESTRO 36 Abstract Book

S209 ESTRO 36 2017 _______________________________________________________________________________________________

D. Jaffray 1 1 Princess Margaret Cancer Centre University Health Network, Toronto, Canada The sensitivity and definition of magnetic resonance (MR) imaging makes it a compelling technology to guide the delivery of highly conformal radiotherapy (RT). Technological advances have now made this concept a reality in a variety of forms ranging from its use in the design of the treatment plan to enabling on-line planning and/or real-time verification of plan delivery. While the paradigm of MR-guided radiotherapy already is operating in various forms, including dedicated machines that combine MR imaging and delivery, there are numerous challenges that prevent the full exploitation of this technology to maximize the therapeutic ratio for today’s cancer patient. These challenges can be broadly grouped as biological, physics / computational, operations / procedural, and cost / value. While the implementation of MR-guided RT is advancing, clinical, research, and industry teams are exploring the radiobiological effects, addressing computation of dose and deformation, developing adaptive workflows, and seeking evidence of the value of these new processes will bring to the patient and health system as a whole. In this presentation, the full scope of these challenges will be presented to provide the audience with an understanding of their interplay and to allow them to identify areas of priority. SP-0396 Can we perform RCTs evaluating MR guided radiotherapy? V. Valentini 1 1 Università Cattolica del Sacro Cuore - Policlinico A. Gemelli, Gemelli ART, Rome, Italy Image guided radiotherapy (IGRT) has offered in the last years great opportunities to reduce treatment related uncertainties and errors and to develop new dose escalation protocols with local control improvement and toxicity reduction. The recent technological introduction of MRI in RT treatment rooms, offer new IGRT paradigms, which will allow an adaptive therapy, approach through the analysis of organ motion 3D and 4D features and volume changes. To define the metric to evaluate the assumed benefit is of utmost importance in order to achieve the best clinical outcomes in the daily management of radiotherapy (RT) treatments delivery. The issue to apply the RCT methodology to measure the assumed clinical benefit of this new technology is challenging the appropriateness of this approach, as any time that an innovative technology has to be evaluated, and the cost and time needed to get a reliable evaluation. On the other side the cost effectiveness benefit of this new technology needed to have a proper metric to allow a full understanding of its role. The possibility to manage RCT, in term of aim, feasibility and expected outcome, and to use a similarity large data base approach will be addressed. Symposium with Proffered Papers: Novel approaches in head and neck tumour control SP-0397 State of the art in head and neck tumour radiobiology B. O'Sullivan 1 1 Princess Margaret Cancer Centre University Health Network, Toronto, Canada Much contemporary knowledge and application of radiobiology to cancer treatment derives from experience and management of head and neck cancers (HNCs). In part, this relates to the more accessible nature of these lesion compared to other cancers for observation and tissue access, and because radiotherapy (RT) has provided

which combines a 1.5 Tesla MRI with a high-end linear accelerator. The MR linac will provide real-time high- quality MRI guidance, not only before, but also during treatment. With the introduction of the MR linac, radiotherapy will enter a new era of high precision treatment with a wide range of opportunities. MRI guidance will improve tumor targeting accuracy, allow for smaller PTV margins and thus result in a reduction of normal tissue exposure. Consequently, highly accurate tumor targeting with small PTV margins will enable hypofractionation and dose escalation up to ablative dose levels, potentially omitting the necessity of surgery to control the macroscopic tumor. In addition, daily and even intrafraction plan adaptation and dose painting based on anatomical changes, tumor regression and functional MR imaging will further refine dose escalation and might provide an organ-sparing treatment strategy for a growing number of indications. The excellent soft tissue contrast in combination with advanced online motion-compensation of the MR linac will also broaden the potential indications for radiotherapy. Besides its numerous opportunities, the MR linac also brings challenges. Its clinical implementation requires thorough revision of workflows and clinical protocols, training of personnel , adaptation of QA procedures, etc. In addition, a standard assessment methodology for the evaluation of innovations in radiotherapy such as the MR linac is required. In conclusion, the MR linac has the potential to provide a high-precision, high-dose, adaptive, non-invasive therapy, with improved local control, less toxicity and the possibility of omitting surgery in an extended field of radiotherapy indications. SP-0394 MR guided brachytherapy - successes and potential future developments P. Hoskin 1 1 Mount Vernon Hospital, Northwood Middlesex, United Kingdom Brachytherapy has led the way in the development of MR guided radiotherapy. The technical requirements in brachytherapy differ from external beam planning in that the source being placed within the CTV means that the problems of tissue inhomogeneity and beam entry and exit profiles do not need consideration. The main challenge in brachytherapy is to design applicators which can be readily visualised on MR so that accurate definition of applicator geometry within the CTV is achieved. In the two most common indications for brachytherapy, gynaecological cancers and prostate cancer MR guided techniques are well established and considered standard practice. The development of MR guided gynaecological brachytherapy demonstrates well the advantages of the superior anatomical information obtained from MR and the ability to accurately delineate tumours defining the CTV. The evolution can be traced from conventional point A based dosimetry using orthogonal x-rays to 3-D image guided brachytherapy using MR which has been estimated to account for a 13% increase in survival with modern treatment techniques. Similarly in prostate cancer, whilst ultrasound-based implantation is the standard for definition of the CTV registration with MR provides additional information. As the practice of focal therapy and dose painting using functional MR information evolves then MR based planning is seen to have significant advantages which will again translate into patient benefit. For the future refinement of MR compatible applicator design, access to MR guided implant techniques and MR based planning algorithms will further enhance the role of MR in brachytherapy.

SP-0395 Challenges associated with MR guided radiotherapy