99m Tc-macroaggregated albumin ( 99m Tc-MAA) scintigraphy is utilized in treatment planning for Yttrium-90 ( 90 Y) Selective Internal Radiation Therapy (SIRT) of liver tumors to evaluate hepatopulmonary shunting by calculating the lung shunt fraction (LSF). The purpose of this study was to evaluate if LSF calculation using SPECT/CT instead of planar gamma camera imaging is more accurate and if this can potentially lead to more effective treatment planning while avoiding excessive pulmonary irradiation.
LSF calculation was obtained using two different methodologies in 85 cases from consecutive patients intended to receive 90Y SIRT. The first method was based on planar gamma camera imaging in the anterior and posterior views with geometric mean calculation of the LSF from regions of interest of the liver and lungs. The second method was based on segmentation of the liver and lungs from SPECT/CT images of the thorax and abdomen. The differences in planar imaging versus SPECT/CT derived LSF values along with the estimated absorbed lung mean dose (LMD) were evaluated. The LSF values were higher in planar imaging versus SPECT/CT in 81/85 cases, with a mean value of 8.5% vs. 4.6% respectively; the difference was statistically significant using a paired t-test (alpha = 0.05). In those patients who received SIRT, the estimated absorbed LMD calculated with planar imaging was significantly higher than with SPECT/CT (t-test, P<0.005). Repeated phantom experiments using an anthropomorphic torso phantom with variable 99m- Tc activity concentrations for the liver and lungs were performed with the standard patient protocol, demonstrated improved accuracy of the LSF calculation based on SPECT/CT than planar imaging (mean overestimated value of 6% vs. 26%).
This study demonstrates that LSF calculation using planar imaging can be significantly overestimated while calculation using SPECT/CT imaging and appropriate segmentation tools can be more accurate. A lower calculated LSF could allow for a higher dose prescription which can potentially lead to more effective treatments for hepatic tumors.