ESTRO 2021 Abstract Book

S1269

ESTRO 2021

Figure 2: Dosimetric comparison. Colors indicate matching fractions.

Conclusion The preservation of target coverage metrics during the adaptive process is promising for treatment improvements either by margin reductions to reduce sequelae or improved target coverage with maintained margins. PO-1545 Adaptation of dual isocentre plans for cervix treatments on the 1.5T MR-Linac R. Chuter 1,2 , P. Hoskin 3,2 , A. McWilliam 2,1 1 The Christie NHS FT, Medical Physics and Engineering, Manchester, United Kingdom; 2 The University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom; 3 Mount Vernon Cancer Centre, Oncology, Middlesex, United Kingdom Purpose or Objective Cervix patients exhibit large inter-fraction anatomical changes. The MR-Linac (MRL) can image these patients with MR prior to & during treatment. Due to the improved image contrast and lack of imaging dose compared to CBCT, daily plan adaptation can be achieved. However, the MRL has a limited treatment field in the Sup/Inf direction of 22 cm. This can restrict the treatment of patients who require longer treatment fields such as cervix patients with nodes. Here we explore adaptive workflows in combination with a dual isocentre approach, to widen the range of cervix patients that can benefit from this treatment. Materials and Methods Four cervix cancer patients were retrospectively planned with a dual isocentre technique delivering 45Gy in 25 fractions. 2 node negative (NN) and 2 node positive (NP) patients were planned using the EMBRACE II study protocol on research Monaco (Elekta AB, Sweden) with an MRL beam model and 1% statistical uncertainty. A 2 cm overlap region between the two isocentres was created, positioned entirely in the nodes (Fig 1a). To