ESTRO 37 Abstract book

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ESTRO 37

are a multidisciplinary approach where patient selection and treatment selection are based not only on tumor load like number and size of metastases and patient-related prognostic factors, but also on biological tumor markers and available systemic or targeted therapy. Where an intracranial response maybe expected, no longer whole brain radiotherapy is necessary, but a focal radiation like SRS would be sufficient to control residual, new or progressive brain metastases in addition to the targeted therapy. Treatment of patients with brain metastases has developed into an individualized and tailored treatment strategy with an emphasis on high local control based on focal radiotherapy techniques. SP-0686 Against the motion J. Zindler 1 1 MAASTRO Clinic, Radiotherapie, Maastricht, The Netherlands Abstract text Stereotactic radiosurgery (SRS) is the preferred treatment for the majority of patients with a maximum of three brain metastases (BM). Recent technological advances with LINAC based SRS have enabled treatment of multiple BM (10 or more) within 20 minutes with comparable plan quality as Gamma Knife. These technological advances include fully automated single isocentre non-coplanar SRS using a frameless mask and a 6 degree of freedom correction couch. Recent studies have shown that survival is comparable after SRS for 2-4 BM versus 5 or more BM in patients with low volume BM. These studies raise the question if SRS is also the preferred treatment strategy for patients with 4 and more BM. Though, the previous mentioned studies are limited by selection bias and the main question if SRS provides clinical benefit over whole brain radiotherapy (WBRT) for the patient is not answered in these studies. In most guidelines in patients with 4 or more BM WBRT, primary systemtic therapy or best supportive care is adviced. Treatment choice depends on both patient as treatment characteristics. The potential advantages of SRS over WBRT are less hair loss, less neurocognitive damage, and a higher local tumor control probability of the BM. Potential disadvantages of SRS over WBRT are a risk of radionecrosis, a significant risk of 50-90% of new BM (distant brain recurrences or DBR) during follow-up, and higher costs. The risk of radionecrosis and DBR may impair quality of life and even survival. Moreover with Kaplan-Meyer analysis almost all patients will need WBRT during follow-up if they are initially treated with SRS and the initial number of BM is the most important prognostic factor of developing DBR during follow-up. Therefore it is likely that patients with 4 or more BM may benefit more from WBRT than patients with a limited number of BM. Though, it could be that a subgroup of patients with 4 or more BM may benefit of SRS over WBRT especially in the setting of effective systemic agents for BM. Before accepting SRS as the standard treatment for subgroups of patients with 4 or more BM, randomized phase III trials are needed. These randomized trials are currently ongoing, such as the NCT02353000. In this study the palliative value of WBRT is directly compared to the value of SRS in patients with 4-10 BM. In November 2017 22 patients were randomized and 9 centres were approved by the ethical committee for accrual in the Netherlands. The aim is to open more centres in the Netherlands and abroad to finish the trial within 2 years. Without level I evidence for the cost-effectivity of SRS over WBRT in patients with 4 or more BM and taking into account potential disadvantages of SRS, e.g. radionecrosis and high risk of DBR, WBRT or primary systemic therapy is still the preferred treatment in patients with 4 or more BM.

Joint Symposium: ESTRO-EFOMP: CBCT in radiotherapy: Improving and sharing best practice

SP-0687 CBCT in RT: Current status and standardising protocols M. Van Herk 1 1 Christie Hospital NHS, Divison of Molecular & Clinical Cancer Sciences, Manchester, United Kingdom Abstract text CBCT is the workhorse for image guided radiotherapy in most radiotherapy centres. However, there are concerns about image quality and imaging dose, and the possibility to change numerous acquisition settings doesn’t make the task of developing clinical protocols easier. In addition, there are many options for image registration and deriving optimal couch shifts that must be understood and applied in the appropriate situation. Finally, radiographers must be trained to recognize anatomical changes in CBCT such that appropriate actions are taken, e.g. adaptation. The aim of this talk is present the current status and work towards standardizing protocols for these tasks. One of the issues that frequently are raised is concern about imaging dose. However, it must be clear that dose required for a CBCT scan will depend on the imaging task. In many cases that would be image registration of the bony anatomy to derive a setup correction. The amount of dose that is needed for this task is one or two orders of magnitude smaller than for a diagnostic task because registration uses information of tens of thousands pixels and is hardly affect by noise. To be able to recognize anatomical changes in the images, often contrast between water and tissue is used, and this also remains visible at very low doses. So CBCT dose and scan time are not a given but must be optimized for each clinical task. Users should also be aware by limitations and possibilities of the technology. The ability of the IGRT system to automatically localize anatomy depends critically on deformations within the region of interest used for registration and selection of the region of interest is therefore an important clinical decision. Also the way that the IGRT system deals with rotations is important and the appropriate settings can depend on the choice of isocentre and other settings within the system. Finally, because CBCT provides volumetric information, it conveys medically important information about the patients’ anatomy that radiographers should be able to read as to alert a physician or physicist when needed. Examples are weight loss, gross postures changes or atelectasis in the lungs. Training of the radiographers is essential such that such problems are detected and reported when critical. For all of these tasks, clinical application of CBCT is highly augmented by developing clear clinical and technical protocols, such that imaging dose is appropriate, acquisition and analysis settings are setup per patient cohort, and rules are in place to detect relevant anatomical changes. These save a lot time, and reduce the likelihood of errors. SP-0688 Development of quality in CBCT: How we can get the best from the system J.P. Bissonnette 1 1 Bissonnette Jean-Pierre, Department of Medical Physics, Toronto, Canada Abstract text Linac-integrated cone-beam CT (CBCT) has enhanced the practice of radiation oncology since, immediately prior to delivery of radiotherapy (RT), systematic and random