RT Journal Article SR Electronic T1 Safety of a Scout Dose Preceding Hepatic Radioembolization with 166Ho Microspheres JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 817 OP 823 DO 10.2967/jnumed.115.155564 VO 56 IS 6 A1 Prince, Jip F. A1 van Rooij, Rob A1 Bol, Gijsbert H. A1 de Jong, Hugo W.A.M. A1 van den Bosch, Maurice A.A.J. A1 Lam, Marnix G.E.H. YR 2015 UL http://jnm.snmjournals.org/content/56/6/817.abstract AB Before 166Ho radioembolization, a small batch of the same type of microspheres is administered as a scout dose instead of the conventional 99mTc-macroaggregated albumin (99mTc-MAA). The 166Ho scout dose provides a more accurate and precise lung shunt assessment. However, in contrast to 99mTc-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 99mTc-MAA on SPECT, the amount and volume were measured. These were used to calculate the theoretic absorbed dose in the case a 166Ho 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 166Ho 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 166Ho scout dose seems to be theoretically safe in most patients. Its safety in clinical practice is being evaluated in ongoing clinical trials.