ESTRO 2020 Abstract Book

S413 ESTRO 2020

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.

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- limiting toxicity whilst keeping the antitumor efficiency unchanged. Though the FLASH effect has been reproduced in different tissues the underlying mechanism involved in the sparing of the healthy tissues remains to be demonstrated. To uncover the molecular and physiological mechanisms that underlie the differential response in the lung, we used a combination of transcriptomic, biochemical, immunochemical and histological in vivo and in vitro approaches and analysed the acute wound healing phase as well as late stages when fibrosis developps. Our results indicate that, compared to conventional radiotherapy, FLASH (i) induces less DNA damage, (ii) minimizes the induction of pro-inflammatory genes, (iii) limits the increase of senescent cells in the months following thoracic radiotherapy, and (iv) reduces the proliferation of progenitor cells after injury. Altogether, these results suggest that FLASH preserves the regenerative capacity of the lung. Consistent with this hypothesis, the beneficial effect of FLASH was lost in Terc - /- mice which have a deficiency in telomerase activity and harbour critically short telomeres preventing lung regeneration. The observations performed in irradiated mouse lung were confirmed in different models of primary human cells suggesting that FLASH properties could be exploited to reduce the toxicity of some radiotherapy protocols in the clinic. SP-0731 Physics treatment planning and delivery issues for FLASH Radiotherapy W. Verbakel 1 1 Amsterdam Umc, Radiation Oncology Department, Amsterdam, The Netherlands Abstract text Flash is a promising new treatment modality, where preclinical research has shown a reduction in toxicity for single fraction, high dose treatments at ultra-high dose rates. Most preclinical work has been done using open electron beams. The disadvantage of electron beams is the limited transmission depth and field size to treat with a homogeneous dose. Currently, linacs with photon beams cannot produce high enough dose rates for clinical Flash irradiation. Therefore scanning proton beams have been suggested as a modality that can be available on a relatively short term for clinical Flash irradiations. Ultra high dose rates up to 400 Gy/s have been achieved in a single spot of a proton beam, and delivery could be done using a scanning proton beam. However, when a plan consisting of multiple fields is delivered with such beam, the tissue is subject to a wide distribution of dose rates from the scanning beam. Although more pre-clinical work is needed to confirm that organs at risk still benefit from a Flash effect for treatment with multiple beams, a mixture of dose rates, and scanning beam that does not irradiate all tissue at once, it is important to look at the dose rate distributions of potential Flash plans. Furthermore, in order to compare different Flash experiments, we need to standardize our nomenclature in Flash to ensure that for all types of Flash delivery, whether using electrons, photons, protons, or scanned beams, it

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