ESTRO 2020 Abstract Book

S410 ESTRO 2020

Tuesday 1 December 2020

fibrillation (VF). Current best practices for management of VT include (1) placement of implantable defibrillator, (2) medical management, and (3) invasive catheter ablation. Invasive catheter ablation (CA) has a high (50%) risk of recurrence, and significant complications which increase with repeat CA and medical comorbidities. Coupling noninvasive mapping of VT with noninvasive ablation using stereotactic ablative radiotherapy (SABR) has enabled a new treatment paradigm in the form of noninvasive cardiac radioablation. In this talk, we will identify the patient population at risk, methods for delivery, and early results from our off-label and clinical trial experiences. SP-0724 Is it safe to treat central lung lesions with SBRT? N. Haasbeek 1 1 VU University Medical Center, Department of Radiation Oncology, Amsterdam, The Netherlands Abstract text Stereotactic ablative radiotherapy (SABR/SBRT) is the current standard of care for unfit patients with peripheral lung cancer. The dose should be to a biologically equivalent tumor dose of at least 100 Gy prescribed to the encompassing isodose. However, with limited data available, European Society of Medical Oncology (ESMO) guidelines suggest more conventionally fractionated radiotherapy or accelerated schedules for central NSCLC, based on higher toxicity described after ultra-short, especially 3 fraction SABR schedules. However, many elderly, frail patients are unfit for longer schedules of fractionated radiotherapy or even chemo-radiotherapy. Population studies show that up to one quarter receives no anticancer treatment, resulting in short median overall survival. As the requirement for travel for 4-6 weeks to undergo conventional radiotherapy may discourage frail patients, more fractionated SABR regimens have been advocated. These longer SABR schedules using 5-12 fractions have been used for many years in central tumors, with inconsistent results. Results are dependent on the fractionation schedules used, the definition of “central”, and the location of tumors that were accepted in relation to organs at risk (OAR). “Central” tumors are often defined according to a “no fly zone” were the tumor is located within 2 cm of the proximal bronchial tree, but many different definitions of the no fly zone have been used. Recently, the term ‘ultra-central’ has been introduced to describe tumors with the highest risk, where the gross tumor volume (GTV) directly abuts the major airways, or where the planning target volume (PTV) overlaps the trachea or main bronchi. Ultra-central tumors should be distinguished from ‘moderately central’ tumors, which are adjacent to other central structures. SABR of ultra-central tumors may be associated with an increased risk for hemorrhage and strictures of central airways. However, ultra-central tumors may also have an inherent higher tendency of bleeding, irrespective of the type of treatment. The used normal tissue constraints for SABR differ widely between studies, and are mostly based on experience in what appears relatively safe, and not on concrete SABR toxicity data. MRI based real time tracking of normal tissue during treatment with dose accumulation may offer some answers in the near future. Until better toxicity data is available, central and especially ultra- Teaching Lecture: Is it safe to treat central lung lesions with SBRT?

Teaching Lecture: Spatially fractionated GRID radiotherapy - rationale and promise

SP-0722 Spatially fractionated GRID radiotherapy - rationale and promise E. Malinen 1 1 Oslo University Hospital / University of Oslo, Department of Medical Physics, Oslo, Norway Abstract text Spatial fractionation is the delivery of radiotherapy beams through a systematic 2D grid pattern, giving a highly heterogeneous dose profile with valleys and peaks. Other names referring to the technique are among others “lattice radiotherapy” and “minibeam therapy”. The spatial periodicity of such grid patterns varies between studies, but is typically in the order of millimeters down micrometers. Spatially fractionated radiotherapy with X- rays was first used as early as 1909, and was seen to give less skin reactions compared to conventional broad beam treatment. One current motivation for the use of spatial fractionation is to induce significant tumor shrinkage without any normal tissue toxicity. The technique, typically delivered in a single fraction with high (> 10 Gy) peak dose, has been used for palliative treatment of large bulky tumors and has in some patients led to elimination of the whole tumor. The treatment has been used both as a monotherapy and neoadjuvant to conventionally fractionated radiotherapy. Similar experience has been gained in preclinical models systems. Since the treatment implies that not all tumor cells are directly irradiated, the observed control rates may potentially be attributed to local bystander effects mediated via gap-junction communication or through secretion of soluble factors such as cytokines and reactive oxygen/nitrogen species. Moreover, the immune system may also play a role as spatial fractionation results in both sparing of immune cells and the release of damage associated molecular patterns from directly irradiated tumor cells. This can trigger an immune response, affecting the whole tumor. An account of the historical development of spatially fractionated radiotherapy will be given alongside different technical solutions. Clinical and experimental findings from the literature will be presented, both in terms of tumor and normal tissue effects. The potential of using spatial fractionation to stimulate an immune response will be discussed alongside the possibility of using different radiations.

Teaching Lecture: Noninvasive cardiac radioablation for ventricular tachycardia

SP-0723 Noninvasive cardiac radioablation for ventricular tachycardia C. Robinson 1 1 Washington University, Radiation Oncology, St. Louis, USA Abstract text Cardiovascular disease remains the primary cause of death around the world. Many patients will ultimately succumb directly or indirectly from development of arrhythmias such as ventricular tachycardia (VT) or ventricular