Proton Therapy Treatment Planning With Mri Only: Dose Calculation Accuracy In Mri-Derived Substitute Computed Tomography Images For Brain Tumor And Prostate Cancer Treatments

MRI is increasingly used for radiation therapy (RT) target delineation, image guidance, and treatment response monitoring. An MRI-only workflow has the potential to avoid redundant CT imaging, coregistration uncertainties, and unnecessary exposure of healthy tissues. Recent studies have shown that x-ray beam RT workflow based solely on MRI is feasible, with the advantage of soft tissue discrimination and real-time image guidance. Adapting the same workflow to intensity modulated proton therapy (IMPT) would maximize its clinical potential for enhanced therapeutic ratio. In this contribution, the feasibility of MRI-only RT dose calculation for IMPT is demonstrated using substitute CT (sCT) of the head and the pelvis. sCT images were generated for 10 prostate cancer and 10 brain tumor patients. A dual model conversion technique was used to transform intensity values from in-phase MR images into Hounsfield units (HU) using fitted models applied within and outside of autosegmented bones. IMPT plans were generated with robust optimization. The dose differences in sCTs were quantified against actual CTs using dose-volume indices and gamma analysis. The dual model method of sCT generation gave high-fidelity representations of patient anatomy. Global mean absolute HU differences in heterogeneous sCTs were 34 HU in the head (soft tissues: 13, bones: 92) and 42 HU in the pelvis (soft tissues: 9, bones: 97). In IMPT plans for the head and prostate respectively, 95.9% and 95.0% of dose points were within 1% and 1 mm gamma criteria. Mean proton dose differences in clinical target volume dose-volume histogram parameters between the sCTs and actual CTs for head IMPT plans were -0.2% (range : -1.4% to 0.9%) and for prostate plans 0.1% (range: -0.5% to 0.6%). The corresponding evaluations with x-ray VMAT in head and pelvis were -0.1% (range: -0.4% to 0.7%) and -0.1% (range: -0.5% to 0.3%), respectively. This study concludes that dose calculation with protons can be conducted accurately in MRI-derived substitute CT images to a comparable level of uncertainty as X rays. MRI-only RT dose calculation is feasible for IMPT of brain tumor and prostate cancer.