99mTc-MAA overestimates the absorbed dose to the lungs in radioembolization: a quantitative evaluation in patients treated with 166Ho-microspheres

Purpose Radiation pneumonitis is a rare but serious complication of radioembolic therapy of liver tumours. Estimation of the mean absorbed dose to the lungs based on pretreatment diagnostic Tc-99m-macroaggregated albumin (Tc-99m-MAA) imaging should prevent this, with administered activities adjusted accordingly. The accuracy of Tc-99m-MAA-based lung absorbed dose estimates was evaluated and compared to absorbed dose estimates based on pretreatment diagnostic Ho-166-microsphere imaging and to the actual lung absorbed doses after Ho-166 radioembolization. Methods This prospective clinical study included 14 patients with chemorefractory, unresectable liver metastases treated with Ho-166 radioembolization. Tc-99m-MAA-based and Ho-166-microsphere-based estimation of lung absorbed doses was performed on pretreatment diagnostic planar scintigraphic and SPECT/CT images. The clinical analysis was preceded by an anthropomorphic torso phantom study with simulated lung shunt fractions of 0 to 30 % to determine the accuracy of the image-based lung absorbed dose estimates after Ho-166 radioembolization. Results In the phantom study, Ho-166 SPECT/CT-based lung absorbed dose estimates were more accurate (absolute error range 0.1 to -4.4 Gy) than Ho-166 planar scintigraphy-based lung absorbed dose estimates (absolute error range 9.5 to 12.1 Gy). Clinically, the actual median lung absorbed dose was 0.02 Gy (range 0.0 to 0.7 Gy) based on posttreatment Ho-166-microsphere SPECT/CT imaging. Lung absorbed doses estimated on the basis of pretreatment diagnostic Ho-166-microsphere SPECT/CT imaging (median 0.02 Gy, range 0.0 to 0.4 Gy) were significantly better predictors of the actual lung absorbed doses than doses estimated on the basis of Ho-166-microsphere planar scintigraphy (median 10.4 Gy, range 4.0 to 17.3 Gy; p < 0.001), Tc-99m-MAA SPECT/CT imaging (median 2.5 Gy, range 1.2 to 12.3 Gy; p < 0.001), and Tc-99m-MAA planar scintigraphy (median 5.5 Gy, range 2.3 to 18.2 Gy; p < 0.001). Conclusion In clinical practice, lung absorbed doses are significantly overestimated by pretreatment diagnostic Tc-99m-MAA imaging. Pretreatment diagnostic Ho-166-microsphere SPECT/CT imaging accurately predicts lung absorbed doses after Ho-166 radioembolization.