ESTRO 2021 Abstract Book

S479

ESTRO 2021

varied experience of MRIgRT. Respondents from six centres participated in Focus Group Interviews (FGIs). FGIs were chosen to allow researchers to obtain several perspectives and reach consensus and were conducted with two facilitators using a semi-structure interview guide. Four researchers independently familiarised themselves and coded the data using framework analysis. A consensus thematic framework of codes and categories was agreed and systematically applied. Results Thirty participants took part (Radiographers: n=18, Physicists: n=9 and Clinicians: n=3). Three key themes were identified: ‘Current MRIgRT’, ‘Training’ and ‘Future Practice’. Current MRIgRT identified a variation in radiographers’ roles and responsibilities with pathways ranging from radiographer-led, clinician-light-led and MDT-led as seen in Table 1.

The consensus was to move towards radiographer-led with the need to have a robust on-call service heavily emphasised. The importance of continuity and relevant staff only to be present in designated areas when treatment is undertaken was highlighted. The breadth of knowledge required by radiographers including MRI, contouring, planning and dosimetry, and treatment experience was highlighted. Debate was presented over timing and length of training required. There was dispute over whether training should be delivered pre- MRIgRT exposure or on-the-job, but it was agreed that a blended approach was preferred by all. Future Practice identified the need to have radiographers solely deliver MRIgRT, to reduce staff present which was a main driver by all interviewed. Radiographers reported their enthusiasm for this role and physicists envisioned this happening. Clinicians and Physicists disclosed they had sufficient confidence in radiographers to do this. Time and resources to train radiographers were the main barriers with an emphasis placed on the difficulty of arranging consistent training and lack of MDT availability to deliver it. Conclusion Radiographer-led MRIgRT is an exciting development because of the potential radiographer role development. Roles must be created with sufficient support and robust governance to enable evaluation of effectiveness, impact, ongoing sustainability and responsiveness. A national training framework created collaboratively with all stakeholders and professions involved would ensure consistency in skills and knowledge. OC-0616 First VMAT delivery with MLC-tracking for single and multi fraction lung SBRT on a Unity MR- linac P. Uijtewaal 1 , P. Borman 1 , P. Woodhead 1,2 , C. Kontaxis 1 , S. Hackett 1 , J. Verhoeff 1 , B. Raaymakers 1 , M. Fast 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands; 2 Elekta AB, Elekta AB, Stockholm, Sweden Purpose or Objective Conventional lung SBRT requires large treatment margins to cover tumor motion resulting from respiration. This may avoid underdosage but increases toxicity risks. To maximize healthy tissue sparing, we previously developed MRI-guided MLC tumor tracking for the 1.5 T Unity MR-linac (Elekta AB, Stockholm, SE) in combination with IMRT. Recently, we also piloted VMAT deliveries on Unity to further maximize plan conformality and delivery efficiency. In this study, we demonstrate the feasibility of a first experimental setup on an MR-linac that combines VMAT with MLC-tracking for a range of lung SBRT indications. Materials and Methods All experiments were performed on a 1.5 T Unity MR-linac in research mode. A Quasar MRI4D phantom (ModusQA, London, CA) was used to generate: no motion (static reference), Lujan motion (cos 4 , peak-to-peak amplitude A = 20 mm, f = 0.25 Hz), and subject-derived real respiratory motion (average A = 11 mm, average f = 0.33 Hz) with an average baseline drift of 0.6 mm/min. The phantom contained a film insert with a 3 cm spherical target (GTV) that could be positioned centrally or 10 cm off- center (peripheral) in a water-filled body oval. Target positions were continuously estimated from 2D cine-MR (4 Hz). A linear regression prediction filter compensated for system latency. Predicted positions were used continuously to realign the MLC with the target position. We created three VMAT treatment plans with 3 mm GTV-to-PTV margins following the clinical planning template for lung SBRT: a central plan (8x7.5 Gy) and two peripheral plans (3x18 Gy and 1x34 Gy). Gafchromic