ESTRO 2022 - Abstract Book

S618

Abstract book

ESTRO 2022

Despite this central role in cancer treatment, today’s radiation therapy treatments suffer from a major problem: whilst they routinely image patients prior to treatment, no anatomical information is typically available during treatment. Tumours are not static during treatment, so methods to monitor tumour motion during radiation targeting are essential to ensure prescribed dose coverage. This is even more critical with high-dose treatments such as stereotactic body radiotherapy (SABR) where a large radiation dose must be delivered in a small number of treatment sessions. To monitor the target motion in real-time, X-ray imaging are often utilised, including add-on X-ray imaging systems to the treatment room such as the ExacTrac system (BrainLab). The Real-time Tracking Radiotherapy (RTRT) system was one of the room-mounted system to be developed and deployed clinically. Room-mounted dual X-ray imaging systems have deployed for dedicated linear accelerators systems such as the Vero or CyberKnife, the latter of which uses X-ray imaging in conjunction with an optical system for target motion monitoring. The standard linear accelerator is equipped with one gantry-mounted kilovoltage X-ray imager, which can be used to monitor the target motion. In this case, at least some predictive or probabilistic algorithm is required estimate the 3D motion as each image only contain 2D information of the target. Typically, the inter-dimensional correlation of the patient 3D motion is utilised as a priori for these algorithms, particularly for respiratory-related motion. Motion monitoring using gantry-mounted kV imager was shown to achieved sub-mm accuracy and used clinically at a few centres around the world for prostate cancer treatment and recently, liver SBRT, using implanted fiducial markers as surrogates to the targets. In the future, markerless kV monitoring, particularly for high contrast targets such as lung tumour and the spine, is highly promising. Finally, the MV imager can be used to capture target motion information in the Beam’s eye view, which is highly advantageous as there is no additional imaging dose is required, the images containing the target information are created by the MV beam. However, MLC occlusion and modulation, together with low imaging contrast in the MV images are technical challenges that need to be overcome to obtain high motion monitoring accuracy. Abstract Text The demand for hypofractionation and higher plan conformality since the last ten years resulted in an inevitable increase in plan complexity and concerns about their robustness. This pushed to an escalation in frequency and doses of the imaging procedures, in order to ensure a submillimetre patient setup and a constant monitoring along the treatment delivery. The requirement for three-dimensional, frequent, and even time-resolved imaging led to the spread of non-radiographic imaging modalities, like optical or thermal surface-based techniques. Optical surface imaging can be performed using different technologies like laser scanners, time-of-flight-, stereovision-, or structured-light-cameras, and is already part of the routine of many departments. Their main use has been patient positioning and motion monitoring [Hosaik,2020]. The major benefits are the non-invasive real-time feedback of the patient surface, thanks to a high spatial and temporal resolution, ally to an interface with the main linac vendors. The large field- of-view and the constant development of new characteristics (as e.g. Cherenkov imaging [Jarvis, 2021]) promises an acceptance and adoption as standard imaging technique. In contrast, thermal imaging uses the temperature pattern of the patient’s surface and is still a recent technology in the field of radiotherapy. The two main applications known are for the monitoring of patient’s motion and physiologic- processes. This technology promises motion detection without the impact of room lighting, skin tone, or clothing. Regarding the analysis of the treatment efficiency, the main reported application, due to its anatomical position, is breast radiotherapy. Here variations in temperature and vascular patterns might be used as feedback of the treatment effectiveness [Baic, 2021; Hoffer,2018]. In this session, these two modalities, with a focus on motion management applications, are going to be explored, from the overview of the current state of the art, challenges, and clinical examples. SP-0701 Optical and thermal surface tracking V. Batista 1 1 Heidelberg University Hospital, Department for Radiation Oncology and Radiation Therapy, Heidelberg, Germany

SP-0702 A theoretical framework for treatment margins for online adaptive radiotherapy

T. Janssen 1 , U. van der Heide 1 , P. Remeijer 1 , J. Sonke 1 , E. van der Bijl 2

1 Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands; 2 Radboud University Nijmegen Medical Center, Radiotherapy, Nijmegen, The Netherlands

Abstract Text Aim and introduction

Margin recipes to deal with treatment uncertainties are commonly used in radiotherapy. The common margin recipes are based on the classic work of van Herk et al. and build upon a theoretical framework, assuming an idealized dose distribution, perturbed under random and systematic normally distributed errors. Modern radiotherapy aims to increase delivery precision by different means, including online motion management and online plan adaptation. Not only does this shrink the width of the underlying error distribution, it also changes some of the assumptions underlying the margin recipes. The aim of this work is to present a theoretical framework to discuss treatment margins for these developments.