ESTRO 36 Abstract Book

S452 ESTRO 36 _______________________________________________________________________________________________

Sweden) for both initial treatment planning and online plan adaptation. Next to the presence of a magnetic field, also several MRL-specific beam and collimator properties need to be taken into account that could influence plan quality. Our aim was to investigate the influence of MRL- specific characteristics on plan quality for rectum cancer and benchmark MRL plans against current clinical practice. Material and Methods Eight rectum cancer patients treated on a conventional CBCT-based linac (25 x 2.0 Gy) were included in this retrospective study. For each patient, the clinically acquired planning CT, delineated structures and treatment plan generated with Pinnacle 3 (dual-arc VMAT, 10MV, collimator 20°, SAD: 100.0 cm) were available. The same CT and structure set were used to create two MRL treatment plans with Monaco: one plan with (MRL + ) and one plan without (MRL – ) the presence of a 1.5 T magnetic field. Both MRL plans were created using a 7-beam IMRT technique incorporating MRL-specific properties (7MV, collimator fixed at 90°, FFF, SAD: 143.5 cm). Plan optimization was based on a class solution and objective values were individually optimized. Also, a quasi MRL plan was generated with Pinnacle 3 using a 7-beam IMRT technique and comparable MRL properties (6MV, collimator 90°, FFF, SAD: 143.5 cm). After rescaling (PTV V 95% = 99.2%), plans were accepted when the clinical acceptance criterion was fulfilled (PTV D 1% < 107%). Quality differences between MRL + , MRL – and quasi MRL plans were assessed by calculating PTV D mean , PTV D 1% , bowel D mean and bladder D mean . Also, D mean and D 1% to the patient excluding PTV 2cm (i.e. PTV + 2.0 cm) were determined. All MRL plans were benchmarked against the clinically delivered treatment plans and tested for significance (Wilcoxon signed-rank test). Results All MRL plans were clinical acceptable after rescaling. Figure 1 shows an example of dose distributions for the MRL plans and the clinical plan of one patient. The 7-beam IMRT technique used for all MRL plans resulted in a minor decrease in plan homogeneity, indicated by an increased PTV D mean (Table 1). Also, all MRL plans showed a significant increase in D mean for the bladder, bowel and body compared to clinical practice. However, the clinical relevance of these differences is expected to be limited. Given the similar quality of MRL – and quasi MRL plans, differences between MRL + plans and clinical practice are mainly induced by the MRL-specific properties. The small difference between MRL + and MRL – plans indicated limited influence of the magnetic field on plan quality.

Conclusion The use of DWA for APBI improved the dose distribution compared to that of non-coplanar 3D-CRT and coplanar VMAT; this may reduce the risk of toxicity without prolonging treatment time. PO-0838 Treatment planning for the MR-linac: plan quality compared with current clinical practice A.J.A.J. Van de Schoot 1 , C. Carbaat 1 , B. Van Triest 1 , T.M. Janssen 1 , J.J. Sonke 1 1 The Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Clinical introduction of the MR-linac (MRL) involves treatment planning using Monaco (Elekta AB, Stockholm,