Clinical and Dosimetric Implications of Calculating Lung Shunt Fraction for Hepatic ⁹⁰Y Radioembolization Using...

BACKGROUND. Accurate assessment of hepatopulmonary shunting, typically performed by planar scintigraphy, is critical in planning ⁹⁰Y radioembolization. High lung shunt fractions (LSFs) may alter treatment. OBJECTIVE. The purpose of this study is to compare LSFs calculated from planar scintigraphy versus SPECT/CT in patients with high planar LSFs (> 15%) and to describe the potential clinical and dosimetric implications of SPECT/CT LSF calculations. METHODS. This retrospective study included 36 patients (29 men and seven women; mean age, 62.4 ± 9.8 [SD] years) who underwent ^99mTc-macroaggregated albumin (MAA) planar scintigraphy for planning hepatic radioembolization, had a planar LSF greater than 15%, and underwent concurrent SPECT/CT. Clinically reported planar LSFs were recorded. SPECT/CT LSFs were retrospectively calculated using automatically generated volumetric ROIs around the lungs and liver with subsequent manual adjustments. Total lung and perfused liver doses were calculated using a medical internal radiation dose model. Values derived from planar and SPECT/CT data were compared using Mann-Whitney U tests. Multivariable regression analysis was performed of factors associated with the discrepancy in LSF between the techniques. RESULTS. Mean planar LSF was 25.1% ± 11.6%, and mean SPECT/CT LSF was 16.0% ± 9.3% (p < .001). Mean lung dose was 18.8 ± 8.0 Gy for planar LSF versus 12.3 ± 7.2 Gy for SPECT/CT LSF (p < .001). Mean perfused liver dose was 92.9 ± 36.1 Gy using planar LSF versus 102.7 ± 39.1 Gy using SPECT/CT LSF (p < .001). In multivariable analysis, a larger discrepancy in LSF between planar scintigraphy and SPECT/CT was associated with a body mass index (weight in kilograms divided by the square of height in meters) of 26 or higher (p = .02), maximum tumor size of less than 9 cm (p = .05), and left hepatic intraarterial injection (p = .02). Fourteen of 36 patients did not undergo upfront radioembolization due to a planar LSF greater than 20% and instead underwent shunt-reducing embolization with subsequent radioembolization (n = 7), transarterial chemoembolization (n = 5), or no treatment (n = 2). Five of these 14 patients had a SPECT/CT LSF of less than 20% and would have been eligible for upfront radioembolization based on SPECT/CT LSF. Seven of 29 patients treated with radioembolization underwent prescribed dose reductions based on planar LSF; six of these patients would have qualified for standard radioembolization without dose reduction using SPECT/CT LSF. CONCLUSION. Planar scintigraphy yields greater LSFs compared with SPECT/CT, possibly leading to unnecessary shunt-reducing procedures and prescribed dose reductions. CLINICAL IMPACT. SPECT/CT should be considered for clinical LSF calculations before radioembolization in patients with high LSFs.