Comparison of gantry-less magnetic resonance guided proton therapy with photon therapy for hepatocellular carcinoma patients

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Background and purpose: With magnetic resonance imaging (MRI), soft-tissue tumors such as hepatocellular carcinoma (HCC) are visualised. MR guided proton therapy (MRgPT) may combine the advantages of proton therapy with soft-tissue image-guidance. Therefore, we assessed the calculated dose differences between MR-guided photon therapy (MR-LINAC) and MRgPT at a horizontal beamline for HCC patients, considering non-adapted and adapted plans. Materials and Methods: Planning computed tomography and T2-weighted MRI of ten HCC patients with 13 lesions were included. Adaptive hypofractionated MR-LINAC, gantry-based PT as well as MRgPT treatment plans at a horizontal beamline were optimized using RayStation 2023B-R on the daily MRIs. The comparison included various dose-volume-histogram (DVH) parameters of target volumes and normal tissue. Results: Clinical target volume (CTV) coverage could be fulfilled for 12/13 cases, where plan adaptation was required for 41, 36 and 15 out of 65 fractions for MRgPT, gantry-based PT and MR-LINAC, respectively. With plan adaptation, the D 98% ,CTV could be restored for 56, 60 and 59 fractions, respectively. The median adapted D avg of the uninvolved liver was reduced from 10.1 Gy for MR-LINAC to 6.1 Gy(RBE) for MRgPT, and 4.5 Gy(RBE) for gantry-based PT. For all adapted plans, the normal tissue constraints were fulfilled in all patients. Conclusions: More than half of the proton fractions needed plan adaptation for adequate target coverage. MRgPT lead to clinically acceptable treatment plans for HCC patients and reduced the dose to the uninvolved liver compared to MR-LINAC, with similar target coverage. HCC may thus be a promising indication for hypofractionated adaptive MRgPT.

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