TH‐E‐218‐08: In‐Vivo Dosimetric Verification of Hypo‐Fractionated Proton Radiation Therapy of the Liver with Hepatocyte‐Specific Functional MRI

Purpose: To investigate if MRI of the liver can be used for in‐vivo dose verification in proton therapy. Recently it was shown that irradiated healthy liver tissue shows a strong systematic decrease in uptake of a hepatobiliary‐directed contrast agent (Gd‐EOB‐DTPA) six weeks after brachytherapy. In this study it is investigated, if the radiation‐related effect is also detectable for hypo‐fractionated proton therapy. Methods: For patients who receive liver lesion directed hypo‐fractionated proton therapy (5 fractions within 2 weeks) Gd‐EOB‐DTPA enhanced MRI is performed 10–12 weeks after treatment. MR images are registered to the planning CT and the planned dose map by non‐rigid image registration. The reviewer contours the border of hypointensity on T1‐w images that indicates the hepatocyte function loss. The threshold dose for this function loss is evaluated as the D90, the dose achieved in at least 90% of the pseudolesion volume. Moreover, irradiated‐ to‐non‐irradiated liver contrast and the correlation of detected signal change in MRI with the planned dose map are analyzed. Results: Gd‐EOB‐DTPA enhanced T1‐w MR images taken after hypo‐fractionated proton therapy show a hypo‐intense area that is correlated to the area of high dose deposition in shape and volume with small deviations in location (up to 5–6 mm) giving information on the actual distal edge position. With Gd‐EOB‐DTPA enhanced MRI, irradiated‐to‐non‐irradiated contrast is superior to MRI enhanced with extracellular, nonspecific contrast agents used in a previous study. Conclusions: A biomarker for radiation induced changes in liver tissue was identified and promising post‐treatment MRI data have been acquired and are currently evaluated. For the next step, a pilot patient study has been set up to investigate, if radiation‐induced changes using Gd‐EOB‐DTPA enhanced MRI can be detected prospectively during fractionated proton therapy and therefore would allow for an immediate assessment of the proton therapy with direct impact on patient safety. The work was partly supported by the German Federal Ministry of Education and Research (BMBF) under contract no. 03ZIK445.