ESTRO 2020 Abstract book

S384 ESTRO 2020

SP-0630 Against the motion (there is a future for ART without an MR-linac) M. Scorsetti 1,2 1 humanitas Clinical And Research Hospital Irccs, Radiotherapy And Radiosurgery, Rozzano - Milan, Italy ; 2 humanitas University, Biomedical Science, Rozzano Milan, Italy Abstract text Adaptive radiation therapy (ART) is an evolving paradigm that could overcome some intrinsic limitations of current radiation therapy, i.e. the ongoing changes in the patient’s anatomy and physiology and in the tumor during the treatment course. Although the idea of adapting the treatment plan on daily modification has been debated for long time, some major obstacles have limited the clinical adoption of ART on a large scale. Most of available data derive from offline ART, mostly in the head and neck and thoracic cancers, and show small advantages, mainly from a dosimetric point of view with a still limited clinical impact. Having the possibility of an online treatment adaptation on a daily basis, MR-linac could fill the gap between the theoretical need for ART and its real life clinical implementation, without a significant impact on treatment times. However, I feel there are still a lot of gray zones that need to be clarified. First of all, the identification of patients who really deserve ART is still unclear. There are clinical situations in which no benefit can be expected adapting the treatment plan, like breast cancer or brain tumors. On the contrary, there are clinical scenarios, like prostate cancer, head and neck cancer or locally advanced lung cancer, in which we can expect dosimetric advantages and possibly clinical improvement with the adoption of ART. However, also in these cases, we are still missing data about the frequency of plan adaptation required to benefit the patients. Moreover, there are sites, like pancreatic cancer, liver tumors, upper and lower GI tumors, in which there are too few clinical data to support the implementation of ART on a large scale, although a theoretical benefit could be awaited. Beyond these uncertainties, I would like to highlight that the use of ART has been studied well before the introduction of MR-linac in the clinical arena. The real innovation coming with MR-systems could be the possibility to acquire images during the delivery but this is related to motion management and not to adaptive re-planning which is done “prior” to delivery. The two different concepts should not be mixed. Moreover, modern AI based systems might not require daily MR imaging and be able to extract the anatomy of the day from high quality CBCT and reference multimodality images. Quality of soft tissue resolution from diagnostic MR can be incorporated via AI into segmentation of daily CBCT. MR-linac will not be the only technology who will allow daily adaptation of treatment plan. Another issue is the real proportion of patients that could be treated with MR-linac, since patients with severe claustrophobia and contraindications to MRI cannot be considered for ART with this technology. However, they could be safely treated with other AI based systems, combined with more conventional Linacs. MRI guidance strategy for ART requires a radiation oncologist, medical physicist and radiation therapist to spend additional time at the machine, as tumor and normal organ changes are likely to be detected. Although rapid, adaptation of the treatment plan will require longer treatment times. During this treatment time, there may be an increase need to account for the movement of normal organs and comfort of the patients could become in issue. Due to all these reasons, I think that value of MR-

linac daily ART must be proven, with solid clinical evidences to distinguish patients who need this approach from patients who will not derive a significant benefit. I believe that there is a future for ART, but we need to generate evidences on its use and to define frequency and clinical scenarios in which this approach is more valuable. In every case, this can be done with and without a MR- linac. SP-0631 For the motion rebuttal (there is no future for ART without an MR-linac) C. Gani 1 1 University Hospital Tübingen, Radiation Oncology Department, Tübingen, Germany Abstract text In my rebuttal I will present arguments for the motion, that there is no future for adaptive RT without an MR- Linac. SP-0632 Against the motion rebuttal (there is a future for ART without an MR-linac) S. Korreman Aarhus University Hospital. Aarhus, Denmark OC-0633 The dose response of high-resolution diode detectors in magnetic field T. Tekin 1 , B. Delfs 1 , I. Büsing 1 , A. Schönfeld 1 , B. Poppe 1 , H.K. Looe 1 1 University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital - Carl von Ossietzky University, Oldenburg, Germany Purpose or Objective The clinical establishment of MR linacs for magnetic resonance guided radiotherapy (MRgRT) with online soft tissue contrast imaging introduces new challenges in dosimetry due to the modification of secondary electrons’ trajectories in static magnetic field by the Lorentz force. Alteration of dose response of air-filled ionization chambers has been frequently reported. Nevertheless, similar studies on the change of dose response of semiconductor detectors are scarce and the related mechanism is not fully understood yet. Since these detectors are used at MR Linacs whenever high spatial resolution is required, such as during device commissioning, the magnetic field dependent dose response of semiconductor detectors has been studied in this work. Material and Methods The measurements were carried out with five commercially available semiconductor detectors from three manufacturers (PTW 60017, PTW 60023, PTW 60019, IBA Razor Diode and Sun Nuclear Edge detector) at a linac equipped with an electromagnet using 6 MV photon beam. The strength of the magnetic field that is oriented orthogonal to the chamber and beam axis was varied from 0 to 1.4 T. Thereby, the change of the detector´s response in a magnetic field has been evaluated by the magnetic field correction factor, k B,Q . The experimental magnetic field correction factors were validated by Monte Carlo simulations using the EGSnrc code. Detailed step-wise Abstract not received Proffered Papers: Proffered papers 33.2: Dosimetry and QA

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