Session Item

To validate a method for virtual four-dimensional computed tomography (4DCT) generation from four-dimensional magnetic resonance imaging (4DMRI) in a porcine lung phantom, and to evaluate its use in respiratory gated particle therapy using patient data.

Deformable image registration was used to: (a) register each respiratory phase of the 4DMRI to the end-exhale MRI phase; (b) register a static 3DCT at end-exhale (used as reference) to the end-exhale MRI volume; (c) generate the virtual 4DCT by warping the registered CT according to the obtained deformation fields (see figure 1). A carbon ion and a proton treatment plan were optimized on the end-exhale reference CT and the corresponding RBE-weighted dose distributions were recalculated over the virtual and ground truth (GT) 4DCTs. The method was validated on a physical phantom and 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.

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.

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.