ESTRO 37 Abstract book

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ESTRO 37

tracking, and potentially adapting the treatment plan in real-time. The superior soft-tissue contrast provided by MR, and the ability to acquire images continuously during treatment delivery without additional dose to the patient, allow us to better understand the various sources of motion and deformation within a treatment fraction, and to more accurately address these changes. Here we will discuss the commercially available MRgRT systems, their imaging capabilities, and the various proposed and implemented techniques for monitoring and correcting motion and deformation during the treatment.

Symposium: New developments in online adaptive MRgRT

SP-0237 In room MR imaging M. Palacios 1 1 VU University Medical Center, Radiation Oncology Department, Amsterdam, The Netherlands Abstract text MR-guided radiotherapy provides superior soft-tissue visualization and provides the means for online inter- and intrafraction monitoring of the internal anatomy of the patient. During this presentation the basic principles of MRI will be first presented, thereby addressing the basic aspects of commissioning and quality assurance for in- room imaging in the implementation of an MR-guided radiotherapy program. Next, current MR protocols used for treatment planning in the realm of adaptive radiotherapy and real-time intrafraction visualization of soft-tissue structures will be discussed. Finally, perspectives for performing online functional imaging for dose response evaluation will be highlighted. 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom Abstract text In clinical practice the majority of Image Guided Radiotherapy (IGRT) applications are currently based on in-room kV Cone Beam Computed Tomography (CBCT). In recent years inter-fraction adaptation strategies ranging from protocol based (daily or offline) re-positioning of the radiation target using the treatment couch, daily selection of an optimal treatment plan from a range of pre-optimized solutions and daily or weekly plan adaptation based on dose accumulation driven by deformable image registration have been investigated. With the introduction of MR Guided Radiotherapy (MRgRT) new methods have been develop to leverage the superior soft tissue contrast of MR imaging in high precision radiotherapy. The acquired MR images allow for daily adaptation of the treatment plan to improve the sparing of the organs at risk and for potential dose escalation to the tumour. Within this presentation the following topics related to daily replanning strategies using MR images will be addressed: - How to calculate dose on MR images? A brief overview of existing methods to derive synthetic CT images will be presented. - How to derive the necessary treatment planning contours on the MR images? - What daily plan adaptation methods are currently available or being researched? - What are the requirements for online plan QA? SP-0239 Intra-fraction variability and motion management R. Kashani 1 1 University of Michigan, Radiation Oncology, Ann Arbor MI, USA Abstract text Implementation of in-room MR image-guided systems (MRgRT) provides new opportunities in management of intra-fraction motion, using the target directly for gating, SP-0238 Daily replanning strategies S. Nill 1 , U. Oelfke 1

Symposium: What could replace the PTV?

SP-0240 The challenging path towards clinical adoption of probabilistic planning M. Witte 1 1 Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands

Abstract text Introduction

Over the past decades, technological innovations allowed the incidental radiation dose delivered outside designated treatment volumes to be reduced. Conventional box fields became conformal radiotherapy, and then IMRT. Treatment room image guidance has reduced geometric uncertainties in the dose delivery, allowing smaller treatment volumes. In parallel, advanced imaging techniques have been under development, providing an increasing insight into the heterogeneous nature of tumors. Investigations about the extent of subclinical disease surrounding the tumor are being performed based on surgical series, and indirectly by correlating delivered radiation dose patterns with outcome. Potentially, modern radiotherapy can match the highly heterogeneous nature of invasive cancer growth and the geometrical intricacies of fractionated treatment delivery to a moveable patient with a high level of flexibility to modulate the dose. Nonetheless, the current day standard of care relies on the definition of binary volumes (GTV, CTV, PTV) along with uniform dose prescriptions. This practice provides a high confidence that the intended dose will in fact be delivered to the tumor, however at the cost of treating large volumes to high dose. Thus, dose limiting constraints of surrounding healthy structures prevent the delivery of high tumor doses. In the pre-IMRT era, the adequacy of this practice was dictated by the limitations of the planning and delivery systems. Nowadays, the ingrained routine of subdividing a patient volume into either target or not, and the flat dose paradigm to accompany it, may become obstacles in the further development of the field. Extending the GTV concept, dose painting initiatives have emerged to deliver intentionally non-uniform tumor dose distributions, based on biological and/or functional imaging. Initiatives to replace the CTV are still hampered by the largely unknown distributions of microscopic disease, however now that information about geometric uncertainties is abundant through IGRT, the definition of a PTV could be considered obsolete as soon as better plan optimization techniques based on probabilities become available. Reasons to deprecate the PTV Foremost, one should consider the potential clinical benefit of PTV-less planning. While the PTV is intended to provide a certain confidence of minimum tumor dose, there is very limited clinical evidence about the impact of geometric miss on local tumor control. Therefore, it is difficult to estimate the potential benefit of an alternative planning technique which trades a higher dose