Room: Karl Dean Ballroom C
Purpose: Functional avoidance treatment planning minimizes radiation exposure to highly functioning liver regions to preserve overall organ function. Various surrogates exist to measure liver function. This study compares maps of hepatic perfusion and hepatocyte uptake of gadoxetic acid, and their affects on treatment planning.
Methods: Ten patients with hepatocellular carcinoma received gadoxetic acid enhanced MRI scans of the liver prior to radiation therapy. Perfusion and uptake rate maps were created as measures of localized liver function. A representative normal functioning region was manually selected in each patient. Voxels with uptake and perfusion >36% of normal were designated function sparing regions (FSRs) and perfusion sparing regions (PSRs), respectively. Voxels were scored as â€œmatchedâ€? if they appeared in sparing or non-sparing regions of both maps. The percentage of voxels â€œmatchedâ€? was the agreement. These regions were rigidly registered to the treatment planning CT. Three plans were optimized per patient: (i) a clinical plan (CP) minimizing dose to all liver tissue outside the tumor volume, (ii) a function based plan (FBP) minimizing dose to FSRs, and (iii) a perfusion based plan (PBP) minimizing dose to PSRs. Each was designed to deliver 55 Gy, in 5 fractions, without exceeding OAR dose constraints.
Results: The surrogate maps varied considerably, with 64.9% mean agreement (range: 6.6-94.5%). Compared to the CPs, the PBPs and FBPs, respectively, resulted in average reductions of 14.0% and 10.5% in mean dose to PSRs, and 11.5% and 12.7% to FSRs. For the most mismatched case, the FBP provided an additional 10.1% reduction in mean dose to FSRs.
Conclusion: Functional avoidance treatment planning with perfusion or uptake both result in dose reduction to regions with moderate perfusion or uptake. However, there is potential for large mismatches that limit functional sparing. A clinical trial is required to determine which method best preserves liver function.