Feasibility study of in vivo MRI based dosimetric verification of proton end-of-range for liver cancer patients

Purpose: To investigate the feasibility of using MRI to verify proton beam distal range for liver tumor treatment in a retrospective study. Methods and materials: Because the follow-up hepatocyte-specific functional MR imaging can detect the radiobiological change of liver tissue after radiation, we firstly registered the contrast-enhanced MR images to the planning CT images from 5 liver patients, then overlaid the prescribed dose distribution on the MR images. Since dose calculation is most accurate at the penumbra dose region, we correlated the MR signal intensity (SI) to the radiation dose at the superior/inferior penumbra region. This dose-SI correlation was finally employed on registered MR images to estimate the proton end-of-range. Results: Statistically significant correlations between radiation dose and MR SI were observed in superior/inferior penumbra regions, with correlation coefficient ranging from 0.93 to 0.99. By applying the dose-SI correlation to the distal region of each proton beam, the mean difference between MR-estimated and the planned dose range was -2.18 +/- 4.89 mm for anterior-posterior beams and -3.90 +/- 5.87 mm for lateral beams. Conclusions: This feasibility study proved the principle that proton dose range can be verified in vivo by follow-up MR images after proton liver treatment. (C) 2013 Elsevier Ireland Ltd. All rights reserved.