Safety analysis of holmium-166 microsphere scout dose imaging during radioembolisation work-up: A cohort study

Radioembolisation is generally preceded by a scout dose of technetium-99m-macroaggregated albumin to estimate extrahepatic shunting of activity. Holmium-166 microspheres can be used as a scout dose (+/- 250 MBq) and as a therapeutic dose. The general toxicity of a holmium-166 scout dose (Ho-166-SD) and safety concerns of an accidental extrahepatic deposition of Ho-166-SD were investigated. All patients who received a Ho-166-SD in our institute were reviewed for general toxicity and extrahepatic depositions. The absorbed dose in extrahepatic tissue was calculated on SPECT/CT and correlated to clinical toxicities. In total, 82 patients were included. No relevant clinical toxicity occurred. Six patients had an extrahepatic deposition of Ho-166-SD (median administered activity 270 MBq). The extrahepatic depositions (median activity 3.7 MBq) were located in the duodenum (3x), gastric fundus, falciform ligament and the lesser curvature of the stomach, and were deposited in a median volume of 15.3 ml, which resulted in an estimated median absorbed dose of 3.6 Gy (range 0.3-13.8 Gy). No adverse events related to the extrahepatic deposition of the Ho-166-SD occurred after a median follow-up of 4 months (range 1-12 months). These results support the safety of 250 MBq Ho-166-SD in a clinical setting.