ESTRO 2021 Abstract Book

S420

ESTRO 2021

Conclusion Intravenous- contrast enhanced CBCT position verification is feasible and improves the visibility and registration of the PTV-surroundings.

OC-0533 Two Centre Feasibility study of single fraction lung stereotactic ablative radiotherapy (SABR) Z. Jani 1 , M. Chiu 2 , S. Petkar 3 , A. Blower 4 , H. Grimes 5 , N. Lalli 6 , R. Akasha 7 , D. Johnson 8 , S. Masinghe 9 , G. Kumar 10 , S.A.A. Moinuddin 11 , S. Dubash 12 , C. Peedell 9 , J. Wilson 13 , E. Aynsley 14 , C. Hiley 12,15 1 University College London Hospitals Foundation Trust, Radiotherapy , London, United Kingdom; 2 University College London Hospitals Foundation Trust , Radiotherapy , London , United Kingdom; 3 University College London Hospitals Foundation Trust , Radiotherapy , London, United Kingdom; 4 James Cook University Hospital , Radiotherapy, Middlesbrough, United Kingdom; 5 University College London Hospitals Foundation Trust , Radiotherapy Physics , London, United Kingdom; 6 University College London Hospitals Foundation Trust , Radiotherapy Physics , London , United Kingdom; 7 James Cook University Hospital , Oncology , Middlesbrough , United Kingdom; 8 James Cook University Hospital , Radiotherapy Physics , Middlesbrough, United Kingdom; 9 James Cook University Hospital , Oncology , Middlesbrough, United Kingdom; 10 James Cook University Hospital , Oncology, Middlesbrough, United Kingdom; 11 University College Hospitals London Foundation Trust , Radiotherapy , London , United Kingdom; 12 University College London Hospitals Foundation Trust , Oncology , London , United Kingdom; 13 University College London Hospitals Foundation Trust , Oncology , London, United Kingdom; 14 James Cook University Hospital , Oncology , London, United Kingdom; 15 University College London , Lung Oncology Unit , London , United Kingdom Purpose or Objective The coronavirus disease 2019 (COVID-19) pandemic has caused major disruptions to cancer services worldwide. Many radiation oncology departments have adapted to this by increasing the use of hypo fractionated radiotherapy across multiple tumour sites and indications, including single-fraction Stereotactic Ablative Radiotherapy (SABR). A UK expert group has also now recommended a 30-34Gy single fraction treatment for small lung tumours (<2cm) with limited mobility that are >1cm from the chest wall based on two Phase II clinical trials, the RTOG 0915 and the Roswell Park study. In accordance with the new guidance, we reviewed the feasibility of delivery of single-fraction lung SABR across 2 UK radiotherapy centres. Materials and Methods All patients diagnosed with an early lung tumour, fulfilling the UK expert group’s single-fraction SABR criteria, were considered eligible for this study. All patients had a pre-treatment 3D cone-beam computer tomography (3DCBCT) scan for position verification, using soft tissue image matching, followed by a post-treatment 3DCBCT to confirm reproducibility and accuracy of the original match , the coverage of the tumour and intra- fraction motion. A pre-correction 4DCBCT was used for 1 patient with a lower lobe tumour. Three sets of criteria was used to document and analyse the data: 1) The intra-fraction movements 2) treatment time calculated from when the patient was on the treatment couch to off the couch 3)acute treatment side effects according to CTCAE v5.0. Results Using each department's planning and treatment delivery systems,13 patients, between May 2020 and January 2021, with early stage lung cancer (T1a-T1b) were found to be treated with 34Gy single-fraction SABR .A review of the data found that in the data set, across all patient data, there was limited intra-fraction movement with sub-millimetre corrections in any axis; pre-treatment corrections were: -0.14+/-0.26, 0.16+/-