ESTRO 2020 Abstract Book

S868 ESTRO 2020

Results Table 1 summarizes the results for the geometric tracking accuracy. Breaths per Minute Amplitude [mm] Geometric tracking error [mm] Predictor setting 12 5 0.40 Variable 12 10 0.45 Variable 12 10 0.80 Fixed 12 15 0.57 Variable 15 5 0.41 Variable 15 10 0.53 Variable 15 15 0.68 Variable Table 1: Results for the geometric tracking accuracy using MR images as input. The tracking error for all measurements is well below 1mm. The gamma pass rates for 2 %/1 mm / 10 % dose cutoff are 28.3 % and 98.7 % for the non-tracked and tracked delivery. Figure 1 shows a comparison for one exemplarily measured profile.

PG was used as gating structure with an additional 3mm boundary margin and 10% of the target structure were allowed outside the boundary (as currently applied for patients treatments). PLP motion was based on a real patient breathing pattern resulting in a target motion amplitude of 2.5cm and a motion cycle frequency of 10 cycles/min. The motion was tracked during irradiation with a 2D trufi cine-MR (4 images/second, sagittal orientation). PG evaluation was carried out on a 3T MR system (PRISMA, Siemens) 48h after irradiation using a multi spin-echo sequence. Results were evaluated using the 3D -criterion of 3%/1mm and 3%/3mm (dose difference with respect to the local dose/distance-to-agreement, comparing gel dose with TPS dose) and taking only dose levels larger than 10 % of the maximum dose into account. Results The PLP showed an anthropomorphic image contrast both in MR and CT. Fine lung structures were visible and could be well separated from the heart and PG-based target structure (Figure 1). Although the PLP was moved between CT imaging and irradiation, the locations of inner structures were highly reproducible allowing for a good image registration between CT and MR. The gated treatment took 9:44 mins in total. The target structure could be tracked throughout the entire treatment session resulting in a successfully gated irradiation. Evaluation of the PG target showed no significant under- or overdosage throughout the entire evaluated volume resulting in high 3D γ passing rates of 95.9 % for 3%/3mm and 93.3 % for 3%/1mm (Figure 2).

Conclusion We have demonstrated the feasibility of using MR images to track tumour motion using an MLC on the Elekta Unity system. Our framework improves the geometric accuracy compared to previously reported work (Glitzner PMB 64 15NT02) and the dosimetric evaluation does encourage further experiments for the delivery of complex IMRT treatment plans exposed to more realistic tumour movements. PO-1596 End-to-end test in MRgRT: 3D dose verification of a gated treatment with a porcine lung phantom P. Mann 1,2 , D. Stefan 1,2 , S. Katharina 2,3 , K. Christian P 1,2 1 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany ; 2 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO, Heidelberg, Germany ; 3 University Hospital Heidelberg, Department of Radiation Oncology, Heidelberg, Germany Purpose or Objective To test and validate the entire treatment chain in MRgRT for a gated treatment in a realistic treatment setup combined with 3D dose verification Material and Methods An anthropomorphic porcine lung phantom (PLP) including a heart was combined with the PAGAT polymer gel (PG) serving as a target. The PG was stored inside a BAREX container (length=40mm, diameter=27mm) and sewed onto the mediastinum of the lung. In this study, the PLP was used to verify a gated treatment on a 0.35 T MR-LINAC (MRIdian, Viewray). For treatment planning, a CT (SOMATOM, Siemens) was acquired 1h prior to irradiation. The target volume delineation and plan calculation was carried out using the Viewray TPS. A total of 7 gantry angles with 39 segments and a D 50 = 4Gy was planned homogeneously to the PTV. The PTV contained the sensitive PG volume with an additional 3mm margin. The

Conclusion The presented study has shown to be an extremely valuable tool for the verification of intra-fractional motion management techniques in MRgRT. The PLP shows anthropomorphic image contrast, well comparable to that of real patients and the PG allows validating the applied dose in 3D, while the PG itself was not influenced by the magnetic field. We therefore conclude that this setup can