ESTRO 2020 Abstract book

S456 ESTRO 2020

When the dose is delivered in a magnetic field, the Lorenz force will change the trajectories of the high energy electrons generated by the megavoltage radiation. The effect on dose distribution the depends on the magnetic field strength, its direction relative to the treatment beam and the beam energy. This can result in a change in the build-up region and shifted penumbra. Changes can also be observed in the dose distribution near interfaces of two materials with different densities, particularly near tissue- air boundaries as electrons can be curved back into the tissue (electron-return-effect). Electrons exiting the tissue can be captured by the magnetic field and will spiral outside the beam along the magnetic field lines (the electron streaming effect). The influence of the magnetic field also affects the reading of various detectors used for reference dosimetry, acceptance and commissioning, regular quality assurance (QA) and patient QA. The change in reading of a detector depends on the field strength, orientation relative to the photon beam and the magnetic field and the presence of layers of air between build-up material and detector. For reference dosimetry in a magnetic field, two issues need to be considered: the change in local dose due to the change in electron trajectories and the influence of the magnetic field on the detector reading. Ideally the change in detector reading due to the magnetic field would be proportional to the change in local dose. This is however not always the case, therefore the currently used formalisms have to be adapted to correct for the influence of the magnetic field. To avoid any possible effects of air gaps around detectors, measurements should preferably performed in water. The performance of waterproof farmer type ionization chambers in magnetic fields has been investigated thoroughly. Correction factors have been derived for the magnetic field in various geometries and orientations to obtain absolute dose measurements. Other detectors such as a diamond detector have also been investigated for use in magnetic fields. To evaluate dose distributions of clinical plan delivery, patient specific quality assurance can be performed using various dedicated 3D detectors arrays (Delta4, ArcCHECK, Octavius, Martix). The performance of a dedicated MRI compatible versions of such systems have been evaluated. The devices generally perform equally well in a 1.5 T magnetic field compared to the conventional linac use but require a recalibration of the detectors. Besides these detectors film dosimetry has been used for plan QA in MRI guided radiotherapy for it high spatial resolution. The effects of the magnetic field on film dosimetry have also been investigated. Since the MR-Linac aims for online adaptive radiotherapy, online plan QA needs to be developed additional to the offline QA procedures and devices. At the MRI-linac, an IMRT plan will be created online based on the daily anatomy while the patient is on the treatment couch. Therefore, individual plan QA via measurements can’t be performed. New methods have been developed for online plan QA such as fast independent dose calculation algorithms and plan parameter checks. After twenty years of research the magnetic field effects on the dose distribution are much better understood. Detectors have been characterized and applied in a magnetic field. New methods and protocols for QA have been developed, which ensure safe delivery of MRI guided radiotherapy.

Teaching Lecture: Overview of micro/nanodosimetry and application to particle beams

SP-0728 Overview of micro/nanodosimetry and application to particle beams H. Palmans EBG MedAustron GmbH, Wiener Neustadt, Austria

Abstract not received

Teaching Lecture: Real time adpative radiotherapy with MRlinac

SP-0729 Real time adaptive RT with MR-linac A. Betgen 1 1 Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands Abstract text Clinical implementation of MRI-guided, adaptive radiotherapy (MgRT) started about 3 years ago and since 1.5 year treatment of patients with the Elekta Unity system has been initiated in several centers in the world. In the Netherlands Cancer Institute in Amsterdam our focus is to optimize our patient treatment with advanced techniques like 4D MR- image guidance for liver SBRT, online plan adaptation using the daily MR as reference and implementation of margin reduction by using advanced MgRT. Since the whole workflow routine deviates from a conventional linac, there was lot to learn. The main difference between a treatment session on the MR-linac (Unity, Elekta AB) compared to a conventional linac is the fact that during each fraction the treatment plan needs to be adapted and approved in a short time. Besides a simple ‘shift’ of the dose, we also have the possibility of adapting the treatment plan based on the anatomy of the day. All in all this means that the traditional way of thinking about contouring / planning / treatment has changed. When a new radiation plan is computed for each fraction, it has to be evaluated and approved online, which could imply the presence of a physician and physicist each fraction. However, our goal for 2020 is to implement a standard workflow on the MR-linac led by RTTs only. After a learning period and multidisciplinary discussions, the RTTs at the MR-linac already perform online plan adaptation without the presence of a physician or physicist for a majority of the patients. This lecture will give an overview of the basics of online adaptation for different tumor sites, as well as some insights in the huge steps that have been taken.

Symposium: The latest news on FLASH: ultra-high dose rate radiotherapy

SP-0730 Regeneration of lung tissue after FLASH radiotherapy C. Fouillade 1 , M. Dutreix 1 , V. Favaudon 1 1 institut Curie, Inserm U1021/Cnrs Umr3347, Orsay, France Abstract text “FLASH” radiotherapy involves delivering large doses of radiation in a single fraction in less than 0.1 second. FLASH has been reported to spare normal tissues from dose-