Virtual 4DCT from 4DMRI for the management of respiratory motion in carbon ion therapy of abdominal tumors.

Abstract

PURPOSE\nTo evaluate a method for generating virtual 4DCT from 4DMRI data in carbon ion radiotherapy with pencil beam scanning for abdominal tumors.\n\n\nMETHODS\nDeformable image registration is used to: (1) register each respiratory phase of the 4DMRI to the end-exhale MRI; (2) register the reference end-exhale CT to the end-exhale MRI volume; (3) generate the virtual 4DCT by warping the registered CT according to the obtained deformation fields. A respiratory-gated carbon ion treatment plan is optimized on the planning 4DCT and the corresponding dose distribution is recalculated on the virtual 4DCT. The method was validated on a digital anthropomorphic phantom and tested on 8 patients (18 acquisitions). For the phantom, a ground truth dataset was available to assess the method performances from the geometrical and dosimetric standpoints. For the patients, the virtual 4DCT was compared with the planning 4DCT.\n\n\nRESULTS\nIn the phantom, the method exhibits a geometrical accuracy within the voxel size and Dose Volume Histograms deviations up to 3.3% for target V95% (mean dose difference ≤0.2% of the prescription dose, gamma pass rate >98%). For patients, the virtual and the planning 4DCTs show good agreement at end-exhale (3% median D95% difference), whereas other respiratory phases exhibit moderate motion variability with consequent dose discrepancies, confirming the need for motion mitigation strategies during treatment.\n\n\nCONCLUSION\nThe virtual 4DCT approach is feasible to evaluate treatment plan robustness against intra- and inter-fraction motion in carbon ion therapy delivered at the abdominal site.