Time‐resolved MRI for off‐line treatment robustness evaluation in carbon‐ion radiotherapy of pancreatic cancer

PURPOSE: In this study, we investigate the use of magnetic resonance imaging (MRI) for the clinical evaluation of gating treatment robustness in carbon‐ion radiotherapy (CIRT) of pancreatic cancer. Indeed, MRI allows radiation‐free repeated scans and fast dynamic sequences for time‐resolved (TR) imaging (cine‐MRI), providing information on inter‐ and intra‐fraction cycle‐to‐cycle variations of respiratory motion. MRI can therefore support treatment planning and verification, overcoming the limitations of the current clinical standard, that is, four‐dimensional computed tomography (4DCT), which describes an “average” breathing cycle neglecting breathing motion variability. METHODS: We integrated a technique to generate a virtual CT (vCT) from 3D MRI with a method for 3D reconstruction from 2D cine‐MRI, to produce TR vCTs for dose recalculations. For eight patients, the method allowed evaluating inter‐fraction variations at end‐exhale and intra‐fraction cycle‐to‐cycle variability within the gating window in terms of tumor displacement and dose to the target and organs at risk. RESULTS: The median inter‐fraction tumor motion was in the range 3.33–12.16 mm, but the target coverage was robust (‐0.4% median D (95%) variation). Concerning cycle‐to‐cycle variations, the gating technique was effective in limiting tumor displacement (1.35 mm median gating motion) and corresponding dose variations (‐3.9% median D (95%) variation). The larger exposure of organs at risk (duodenum and stomach) was caused by inter‐fraction motion, whereas intra‐fraction cycle‐to‐cycle dose variations were limited. CONCLUSIONS: This study proposed a method for the generation of TR vCTs from MRI, which enabled an off‐line evaluation of gating treatment robustness and suggested its feasibility to support treatment planning of pancreatic tumors in CIRT.