Liver Radioembolization: Predicting Lung Shunt Fraction with Contrast-Enhanced CT, Eliminating the Need for 99mTc-MAA

Accurate estimation of the Lung Shunt Fraction (LSF) is a standard of care in yttrium-90 ( ⁹⁰ Y) radioembolization treatment planning to prevent excessive lung irradiation due to arterio-venous shunting in the liver. LSF is assessed using ^99m Tc macroaggregated albumin ( ^99m Tc-MAA) imaging, but this approach adds risk, complexity, and expense to the treatment planning. This study investigates the potential of Contrast-Enhanced Computed Tomography (CECT) as a non-invasive alternative for LSF estimation.

We developed a novel metric to estimate the LSF from differences between CECT phases, based on the premise that LSF results from tumor angiogenesis and arterio-venous shunting that create excessive local opacity in the CECT arterial phase. Hypervascular volumes were identified by subtracting the portal phase from the arterial phase and contouring the corresponding regions using an density threshold. The perfused volume corresponded to the lobe and to the volume injected with ^99m Tc-MAA. We conducted a retrospective analysis involving 30 liver cancer patients who underwent 4-phase liver CECT and ^99m Tc-MAA imaging prior to ⁹⁰ Y radioembolization to study several markers based on the subtraction images. The patient cohort included several types of liver tumors (primary or metastatic), predominantly hepatocellular carcinoma. Correlations between the hypervascular-to-perfused volume ratio from CECT and LSF values from standard-of-care ^99m Tc-MAA planar imaging were assessed.

The strongest correlation was obtained between the hypervascular-to-perfused volume ratio from CECT and LSF values from ^99m Tc-MAA planar gamma imaging (R²=0.95). The best correlation was demonstrated when the hypervascular volume was corrected by the hypoxic volume and then normalized by the perfused volume. In contrast, the tumor volume did not show a strong correlation with LSF (R²=0.38). Moreover, the predictive power of the hypervascular-to-perfused volume ratio on LSF demonstrated a 1-sigma uncertainty of 3%.

CECT-based LSF estimation shows promise as a non-invasive alternative to ^99m Tc-MAA imaging in ⁹⁰ Y treatment planning. Further validation with prospective studies on larger cohorts is necessary to confirm the accuracy and safety of this approach.