ESTRO 36 Abstract Book

S475 ESTRO 36 2017 _______________________________________________________________________________________________

Material and Methods The planning CT and delineated structures of four rectum cancer patients were selected. For each patient, a MRL treatment plan was generated with Monaco using a 7- beam IMRT technique (25 x 2.0 Gy) including all MRL- specific properties (7MV, 1.5 T magnetic field, collimator 90°, FFF, SAD: 143.5 cm). Patient setup errors of 1.0 cm and 2.0 cm in the CC and LR directions were simulated by shifting the planning CT with respect to the isocenter position. For each setup error, the initial plan was adapted by first adjusting the leaves of each segment to approximate the shift and second re-optimize the weight of each segment. Also, a reference plan was generated by adapting the initial plan with a 0.0 cm shift, as the second phase of plan adaptation was observed to introduce dose changes even for a 0.0 cm shift. All plans were rescaled (PTV V 95% = 99%). The reference and adapted plans were irradiated on the MRL on a slab phantom with a 2D detector array (PTW Octavius 1500 MR ) inserted parallel to the couch at the center position of the PTV. For each plan, the phantom position was changed according to the introduced shift. Patient setup errors in the AP direction cannot be evaluated using this measurement setup. The measured 2D dose distribution of the reference plan was rigidly registered to the measured 2D dose distribution of the adapted plans in order to assess the positional accuracy of the simple dose shift. After alignment, the similarity between the 2D dose distributions of the reference plan and the adapted plans was evaluated using a 3%/3mm γ analysis (local dose, 20% low dose threshold). Results For all adapted plans, the measured positional accuracy was within 0.1 cm. The γ analysis between the dose distributions of the reference plan and the adapted plans resulted in an average pass rate (γ≤1) of 96.2% (range: 83.3% – 99.9%). Smaller values of γ mean were observed for dose shifts in the CC direction compared to the LR direction as well as for 1.0 cm dose shifts compared to 2.0 cm dose shifts (Table 1). Figure 1 shows an example of a 2D γ distribution. High γ values are measured in the low dose area mainly. A simple dose shift to correct for a 1.0 cm setup error in the CC direction resulted in limited dose differences. Various γ hotspots were observed for the 2.0 cm setup error in the LR direction.

protons, IMPT PR plans were the most robust and only 4/26 (15%) decreased in coverage to below 99%. The CTV coverage for all patients and plans are shown in Fig 1. The most common anatomical changes are lateral target deformations, enlargement of mediastinum and changes in diaphragm position. The posterior proton plans are sensitive to target deformations, while the multiple photon fields are sensitive to all three types of changes (see Fig. 2).

Conclusion Treating oesophageal cancer with protons has the advantage of decreasing dose to organs at risk and at the same time it improves the robustness towards common anatomical changes. Frequent imaging is still needed to identify patients with target deformations requiring adaptive treatment planning.

PO-0878 Plan adaptation on the MR-Linac: first dosimetric validation of a simple dose shift R. Koopman 1 , A.J.A.J. Van de Schoot 1 , J. Kaas 1 , T. Perik 1 , T.M. Janssen 1 , U.A. Van der Heide 1 , J.J. Sonke 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Patient positioning on the MR-Linac (MRL; Elekta AB, Stockholm) requires online plan adaptations to correct for setup errors due to the fixed couch position. The aim of our study was to validate the size and direction of such plan adaptations (simple dose shifts) and evaluate dosimetric differences for rectum cancer patients.