ESTRO 2022 - Abstract Book

S416

Abstract book

ESTRO 2022

Figure 2: Dosimetric results for the angulated setup with the real motion trace

Conclusion We have shown that MLC-tracking based on 3D-cine MRI for prostate SBRT on the MR-linac is feasible and beneficial. Slow motion and drifts could be compensated for resulting in dosimetric benefit and the possibility of further hypofractionation. Though sufficient for tracking slow motion, the latency may be further reduced by optimizing the MRI acquisition and reconstruction.

MO-0469 Extensive evaluation of dosimetric robustness for respiratory motion of lung tumor radiotherapy

N. Olofsson 1 , K. Wikström 2,3 , A. Flejmer 2,3 , A. Ahnesjö 4 , A. Dasu 4,3

1 Uppsala University, Immunology, Genetics and Pathology (IGP), Uppsala, Sweden; 2 Akademiska university hospital, Medicinsk fysik, Uppsala, Sweden; 3 Skandionkliniken, Skandionkliniken, Uppsala, Sweden; 4 Uppsala University, Immunology, Genetics and Pathology (IGP) , Uppsala, Sweden Purpose or Objective Intrafraction respiratory motion is a concern for lung tumor radiotherapy, but full evaluation of its impact is hampered by the lack of images representing the full spectrum of motion states. Typically it is estimated with the same 4DCT images used for treatment planning, normally with 10 pseudo-respiratory phases captured over a short time with only a few breathing cycles. It may therefore misrepresent both the motion amplitude and possible irregular breathing patterns. This study presents a novel evaluation using extensive free-breathing image data acquired over realistic clinical treatment times to study the impact of respiratory motion on the robustness of radiotherapy plans with special focus on the planning and prescription approaches employed. Materials and Methods Cine-CT images from 14 lung cancer patients acquired during 8 minutes of free-breathing taken at three different occasions were used. The 300 images per patient were registered to a reference set to enable dose tracking over a time period corresponding to a full treatment, (Figure 1). Photon plans were created using four different methods aiming to deliver a dose of 54 Gy in three fractions to D50% of the target: robust CTV planning based on 10-phase 4DCT, homogeneous fluence to an ITV, an isodose-based prescription to the ITV with a high central dose concurrently normalized to the D50% of the CTV. All treatment plans were recalculated on the extensive 300-image sets and the resulting doses summed for the CTV. Target coverage was evaluated in terms of the dose coverage probability over the whole set of patients. The resulting differences in dose homogeneity and coverage probability between the tested planning methods were evaluated by the Wilcoxon signed-rank test.