Functional Avoidance Liver 4π Stereotactic Body Radiation Therapy Informed by Quantitative Gadoxetic Acid Contrast-Enhanced Magnetic Resonance T1 Mapping.
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PURPOSE: Recent studies have identified the T1 reduction rates (k1) from gadoxetic acid-enhanced magnetic resonance (MR) imaging as a biomarker for liver function. In this study, we validate k1 maps as a functional biomarker and develop 4π noncoplanar treatment plans using k1 maps to guide the optimization for liver functional avoidance stereotactic body radiation therapy (FA-SBRT). METHODS AND MATERIALS: One hundred six patients underwent precontrast and postcontrast T1 mapping MR. Mean k1 values extracted from liver mask excluding gross tumor volume (liver-GTV) were correlated with Child-Pugh and albumin-bilirubin scores. The high-function (HF) liver region was identified and masked using a patient-specific threshold from the Gaussian decomposition of the k1 histogram. Twenty patients were retrospectively planned with coplanar and noncoplanar 4π-SBRT with and without functional avoidance. Tumor coverage was maintained at a minimum of 90% planning target volume to receive prescription, and standard organ at risk constraints were set for all plans. Dose metrics included mean dose to the HF liver and HF liver volume receiving 6 Gy, which was shown to impact patient liver function. A paired, 2-tailed t test was used to determine the statistical significance. RESULTS: k1 values were inversely correlated with Child-Pugh and albumin-bilirubin scores. The 4π FA-SBRT plans reduced the mean dose to the HF liver volume by 21.8% (from 9.2 Gy to 6.5 Gy) (P < .0001) and the volume of HF liver receiving >6 Gy by 39.5% (from 507.5 cm3 to 302.2 cm3) compared with the 20-beam coplanar geometry (P < .0001). All reductions were statistically significant (P < .01). CONCLUSIONS: This study validates k1 derived from free-breathing T1 mapping MR as a liver function biomarker in a cancer patient cohort. Gaussian decomposition can threshold the k1 distribution to create patient-specific HF liver masks. The 4π FA-SBRT planning guided by k1 maps significantly reduced the mean dose to the HF liver, as well as the volume of HF receiving >6 Gy.