Room: Stars at Night Ballroom 2-3
Purpose: To investigate the clinical need for MR-integrated proton therapy (MRiPT) systems given the availability of alternative image-guided solutions.
Methods: Imaging data for 10 patients with primary liver tumors and liver metastasis was retrospective selected. Patients had undergone multiple MRI scans before, during and after treatment as part of a clinical trial. Assuming the availability of MRiPT adaptive therapy, we quantified the residual PTV margins by evaluating tumor variations (size and deformations), contouring uncertainties, and expected motion compensation latency. Subsequently, 3 proton treatment plans were created for each patient comparing different image-guidance techniques: CBCT-guided (conventional margins), offline MRI-guided (new margin plus setup and motion margins), and real-time MRiPT (new margin). Photon plans were also created for 6 patients using the new margins for comparison with MR-linac. Normal tissue complication probability (NTCP) was calculated with the endpoint of Child-Pugh score increase, to evaluate the toxicity of the uninvolved liver.
Results: The residual PTV margin was found to be 3 mm (67% reduction from nominal) using the van Herk recipe. This included random uncertainty of 1.25 mm compensating for tumor variations in 75% of cases, 2 mm random contouring uncertainty, 1.5 mm random motion prediction uncertainty and 0.5 mm systematic setup uncertainty. The CTV expansion could be avoided when using MRI for tumor delineation due to the superior image contrast. The dosimetric implications of MRiPT included reduction in mean dose of on average 51% for liver, 70% for lung and 88% for inferior vena cava, compared to the CBCT-guided scenario. Liver NTCP was significantly decreased by up to 48%, 18% and 27% compared to CBCT-guidance, offline MRI and MR-linac, respectively.
Conclusion: MRiPT offers substantially reduced liver NTCP compared to CBCT-guidance, offline MRI, and MR-linac. Similar results are expected for other sites, especially in the abdominal region.