Safety of a Scout Dose Preceding Hepatic Radioembolization with 166Ho Microspheres

Before Ho-166 radioembolization, a small batch of the same type of microspheres is administered as a scout dose instead of the conventional Tc-99m-macroaggregated albumin (Tc-99m-MAA). The Ho-166 scout dose provides a more accurate and precise lung shunt assessment. However, in contrast to Tc-99m-MAA, an unintended extrahepatic deposition of this beta-emitting scout dose could inflict radiation damage, the extent of which we aimed to quantify in this study. Methods: All patients eligible for radioembolization in our institute between January 2011 and March 2014 were reviewed. Of the extrahepatic depositions of Tc-99m-MAA on SPECT, the amount and volume were measured. These were used to calculate the theoretic absorbed dose in the case a Ho-166 scout dose had been used. The extrahepatic activity was measured as the sum of all voxels of the deposition. Volumes were measured using a threshold technique including all voxels from the maximum voxel intensity up to a certain percentage. The threshold needed to obtain the true volume was studied in a phantom study. Results: In the phantom study, a threshold of 40% was found to overestimate the volume, with the consequence of underestimating the absorbed dose. Of 160 patients, 32 patients (34 cases) of extrahepatic deposition were identified. The depositions contained a median of 1.3% (range, 0.1%-19.5%) of the administered activity in a median volume of 6.8 mL (range, 1.1-42 mL). The use of a scout dose of 250 MBq of Ho-166 microspheres in these cases would theoretically have resulted in a median absorbed dose of 6.0 Gy (range, 0.9-374 Gy). The dose exceeded a limit of 49 Gy (reported in 2013) in 2 of 34 cases (5.9%; 95% confidence interval, 0.7%-20.1%) or 2 of 160 (1.3%; 95% confidence interval, 0.1%-4.7%) of all patients. In these 2 patients with a large absorbed dose (112 and 374 Gy), the culprit vessel was identified in 1 case. Conclusion: Extrahepatic deposition of a Ho-166 scout dose seems to be theoretically safe in most patients. Its safety in clinical practice is being evaluated in ongoing clinical trials.