ESTRO 2023 - Abstract Book

S162

Saturday 13 May

ESTRO 2023

tested on three lung tumor patients (6 acquisitions consisting in planning and re-evaluation 4DCTs) treated using respiratory gating at end-exhale. For the phantom, a ground truth dataset was available to assess the method performance from the geometrical and dosimetric standpoints. For the patients, the clinical treatment plans adopted at the National Center for Oncological Hadrontherapy (CNAO, Pavia, Italy) were considered and the dose distributions were recomputed over all respiratory phases of the re-evaluation and virtual 4DCTs. By comparing the two datasets and the corresponding dose distributions, the geometrical and dosimetric impact of organ motion was assessed.

Results The phantom validation exhibited a geometrical accuracy within the voxel of the MRI and mean dose deviation, with respect to the prescription dose, of 2.3% for protons and 3.2% for carbon ions in terms of target D95%, with a mean gamma pass rate of 98% (gamma criterion of 3 mm/3%, figure 2). For patients, the virtual and the re-evaluation 4DCTs showed good agreement, with limited errors on tumors D95% within the gating window up to 2% for protons and carbon ions (mean gamma pass rate of 94%). For one patient, dose variations up to 20% were observed inside the gating window due to relevant inter- fractional changes that occurred between the acquisition of the planning and re-evaluation 4DCT. Organ at risks (OARs) D2% were quantified and the highest variations were found in the heart (up to 20%) mainly because our approach accounts for respiratory motion and not for cardiac movement.

Conclusion In this study we demonstrated that the virtual 4DCT method was accurate, allowing its application on patient data for testing within a clinical scenario. Dosimetric differences between the re-evaluation and virtual 4DCTs were found to correlate with significant anatomical changes. MO-0228 Evaluation and correction of geometric accuracy in diffusion-weighted imaging on a 0.35T MR-LINAC B. Pouymayou 1 , P. Wallimann 1 , M. Piccirelli 2 , S. Nowakowska 3 , M. Mayinger 1 , M. Guckenberger 1 , S. Tanadini-Lang 1 , A. Boss 3 , N. Andratschke 1 , A. Bink 2 1 University Hospital Zürich, Department of Radiation Oncology, Zürich, Switzerland; 2 University Hospital Zürich, Department of Neuroradiology, Zürich, Switzerland; 3 University Hospital Zürich, Department of Radiology, Zürich, Switzerland

Purpose or Objective