Abstract Book

S290

ESTRO 37

SP-0546 MR-LINAC technological advances and potential usability in clinical setting O. Jäkel 1 1 University Hospital Heidelberg, Radiation Oncology and Radiotherapy, Heidelberg, Germany Abstract text Radiation therapy today has been developed to a point, where excellent conformal dose distributions can be delivered using various techniques from conventional IMRT, to rotational RT, robot-based systems and particle RT. The next big gain in conformation can therefore only be made, when conformation is achieved not only in 3 dimensions, i.e. for static targets, but can also be maintained in 4 dimensions, i.e. including temporal changes in the target volume. This has long been the goal of image guided RT (IGRT). Conventional IGRT has been mostly based on X-ray cone beam CT imaging and to a lesser extent on in-room CT, MV-CT, planar X-ray systems, ultrasound-and optical surface imaging. These developments, however, have mainly improved the daily setup accuracy and to some extent also enabled modest adaption for inter-fractional changes. Due to the invasive nature of X-rays, a continuous monitoring of tumor motion during the delivery of RT was in most cases not possible. Therefore, MRI - being non-invasive and yielding excellent soft tissue contrast - is an optimal tool for continuous monitoring of tumor motion and tissue changes during delivery and for allowing intra-fractional adaption of treatment plans. The idea to integrate a linear accelerator into an MRI was put forward by the Utrecht group already in 2000 (1), but due to the technical complexity involved, it took until 2017 until arriving at a clinically usable system. The company Viewray was the first to realize a commercial system based on a low field MRI (0.35T, double donut system) and 3 Cobalt sources rotating in a ring around the patient (2). This system is less complex technically and has been used since early 2014 for patient treatment in 9 hospital centers. The Utrecht group in a collaboration with Phillips and Elekta, developed several prototypes of a high field MRI (1.5T, split coil) combined with a 6MV Elekta Linac. In mid 2017, the Utrecht group demonstrated for the first time the clinical feasibility of using the MR imaging system of their latest prototype, while delivering the treatment to the patient with a linac (3). By July 2017 first patients were also treated at Henry Ford Cancer Center, US, with a modified version of Viewray system, relying on the same magnet but with a 4MV linear accelerator replacing the cobalt sources. Several of these commercial systems are currently being installed in late 2017. Also Elekta has started to install several pre-clinical hybrid machines, which will be used in a research setting, prior to the certification, which is expected in 2018. In parallel, there are also 2 development projects going on at the Ingham institute in Sydney, Australia and the University of Alberta, Canada. The Alberta system is the only system orienting the beam direction along the magnetic field by rotating the MR- magnet along with the linac. This system is also available commercially, but not yet certified (4). The availability of commercial hybrid system is, however, just a prerequisite to start investigating the clinical potential of these hybrid MR-Linac systems in different tumor sites. There are still several problems, that have to be solved before the full clinical potential of these systems can finally be explored. In order to use the imaging information directly for controlling and adapting the beam delivery, all steps of the RT process have to be performed ideally in real-time: acquisition and reconstruction of images, segmentation and/or extraction of important imaging features, adaption of the treatment plan, QA and finally adapting the delivery. Additional problems are the dose calculation and dosimetry in magnetic fields and the use of MRI for treatment

well as the status of its clinical implementation [6]. Furthermore, additional applications in radiotherapy, e.g. for photon treatment planning, delineating and material differentiating will be briefly discussed. References [1] Möhler C, Wohlfahrt P, Richter C, Greilich S. Range prediction for tissue mixtures based on dual-energy CT. Phys Med Biol 61:N268-N275, 2016. [2] Möhler C, Wohlfahrt P, Richter C, Greilich S. Methodological accuracy of image-based electron- density assessment using dual-energy computed tomography. Medical Physics 44:2429-2437, 2017. [3] Möhler, C, Russ, T, Wohlfahrt P, Elter, A, Runz, A, Richter, C, Greilich S. Experimental verification of particle-range prediction in biological tissue by single- and dual-energy computed tomography. Phys Med Biol, accepted. [4] Wohlfahrt P, Möhler C, Richter C, Greilich S. Evaluation of stopping-power prediction by dual- and single-energy computed tomography in an anthropomorphic ground-truth phantom. IJROBP 100: 244-253, 2018. [5] Wohlfahrt P, Möhler C, Stützer K, Greilich S, Richter C. Dual-energy CT based assessment of patient-specific range prediction uncertainty in proton treatment planning. Radiother Oncol 125: 526-533, 2017. [6] Wohlfahrt P, Möhler C, Hietschold V, Menkel S, Greilich S, Krause M, Baumann M, Enghardt W, Richter C. Clinical implementation of dual-energy CT for proton treatment planning on pseudo-monoenergetic CT scans. IJROBP 97:427-434, 2017. SP-0545 Ultrasound imaging in radiotherapy: ‘Old’ technology with new applications in RT? E. Harris 1 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Radiotherapy and Imaging, Sutton, United Kingdom Abstract text Medical ultrasound was first introduced in the 1940’s and has become a powerful diagnostic tool throughout the world. Its real-time, soft-tissue imaging capability and non-ionizing nature have contributed to its ubiquitous use in disease diagnosis and image guidance in interventional and surgical procedures. Ultrasound guidance of radiotherapy is a relatively new field with the first commercial systems becoming available in the late 1990’s. Despite possessing clear benefits for imaging a variety of tumour sites, challenges associated with implementing ultrasound in the radiotherapy clinic initially impeded its widespread adoption. The influence of effective training on interobserver variation and operator performance was an early barrier. Contemporary ultrasound image guidance techniques have done much to overcome these problems and modern systems are used clinically for interfraction motion estimation in prostate and breast and for intrafraction prostate motion monitoring. Other promising clinical applications under investigation include intrafraction motion estimation in liver, pancreas and kidney, and augmentation of cone beam CT data for the purpose of adaptive radiotherapy of the prostate and gynecological cancer. This talk will give an overview of the clinical use of ultrasound in radiotherapy, discuss the advantages and limitations of ultrasound and present future perspectives for ultrasound guided radiotherapy technology and practice. Finally, some exciting new applications of ultrasound functional imaging for monitoring of tumour response to radiotherapy will be introduced.