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

UNLABELLED Before (166)Ho radioembolization, a small batch of the same type of microspheres is administered as a scout dose instead of the conventional (99m)Tc-macroaggregated albumin ((99m)Tc-MAA). The (166)Ho scout dose provides a more accurate and precise lung shunt assessment. However, in contrast to (99m)Tc-MAA, an unintended extrahepatic deposition of this β-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 (99m)Tc-MAA on SPECT, the amount and volume were measured. These were used to calculate the theoretic absorbed dose in the case a (166)Ho 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 (166)Ho 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 (166)Ho scout dose seems to be theoretically safe in most patients. Its safety in clinical practice is being evaluated in ongoing clinical trials.