Safety of holmium-166 microsphere scout dose during radioembolization work-up

Objectives A holmium-166 (¹⁶⁶Ho) scout dose is superior in calculating the lung shunt fraction compared with technetium-99m-macroaggregated albumin (^99mTc-MAA) (1). The isotope has both beta and gamma emission, allowing ¹⁶⁶Ho loaded microspheres to be used as a scout dose (250 MBq) as well as a therapeutic dose (>4 GBq) for radioembolization treatments. However, the beta emissions of ¹⁶⁶Ho may cause concerns on the safety of using ¹⁶⁶Ho microspheres as a scout dose (250 MBq) (2). All patients who received work-up with a ¹⁶⁶Ho scout dose in our institute were reviewed for safety.

Methods A total of 69 patients were included in prospective phase 1-2 studies on ¹⁶⁶Ho radioembolization (3-5). In patients with an extrahepatic deposition of the ¹⁶⁶Ho scout dose, the radiation absorbed dose to extrahepatic tissue was calculated and medical records were reviewed for potential complaints related to the extrahepatic deposition of the ¹⁶⁶Ho scout dose. Based on a phantom study, a threshold value of 30% of the maximum activity in the deposition was chosen to conservatively (under-) estimate the volume of the deposition on SPECT/CT. Images were reconstructed using an in-house developed Monte Carlo based quantitative SPECT reconstructor. Absorbed dose (in Gy) was calculated using: D = E_absorbed [asterisk] A / (ρ [asterisk] V), in which E_absorbed = 15.87 mJ/MBq is the energy absorbed per MBq ¹⁶⁶Ho, ρ = 1.06 g/cm³ is the tissue density (6), A the activity in MBq and V the volume in ml.

Results Five patients had an extrahepatic deposition of a ¹⁶⁶Ho scout dose (median administered activity 265 MBq; range 215 - 272 MBq). The extrahepatic depositions (median activity 3.7 MBq; range 1 - 33 MBq) were located in duodenum (three times), gastric fundus and the lesser curvature of the stomach, deposited in a median volume of 15.3 ml (range 10 - 36 ml), which resulted in an estimated median absorbed dose of 3.6 Gy (range 1.4 - 13.8 Gy). No (clinical) complications related to the extrahepatic deposition of the ¹⁶⁶Ho scout dose occurred after a median follow-up of 4 months (range 1 - 6 months).

Conclusions These results support the safety of a 250 MBq ¹⁶⁶Ho scout dose in a clinical setting.