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

Abstract

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. Radi oembolization is a minimally invasive treatment for hepatic malignancies. Millions of radioactive microspheres are injected in the hepatic artery to radiate and embolize malignancies (1). Deposition of microspheres in gastrointestinal organs can result in ulceration or inflammation of tissue by a combination of embolization and radiation damage (2-4). To prevent this, vessels leading to gastrointestinal organs may be coil-embolized in patients scheduled for radioembolization treatment, during a pretreatment session using contrastenhanced images (i.e., digital subtraction angiography complemented by C-arm CT) to identify the culprit vessel (5). 99m Tc-macroaggregated albumin ( 99m Tc-MAA) is administered in this pretreatment session to simulate intrahepatic treatment biodistribution and to assess lung shunting and the possibility of extrahepatic deposition of the microspheres (6). This procedure is safe because 99m Tc-MAA emits 140-keV g photons (detecta...

Insights into the Dose–Response Relationship of Radioembolization with Resin 90Y-Microspheres: A Prospective Cohort Study in Patients with Colorectal Cancer Liver Metastases

“…This requires several changes to the current treatment paradigm. Using a scout dose of radioactive microspheres during the preparatory angiography may prove better suited to predict the therapeutic microsphere distribution than 99m Tc-MAA (13). Once the therapeutic microsphere distribution can be predicted, pretreatment activity calculation can be tailored to the individual patient with the previously described artery-specific partition model, yielding a distribution-specific maximum tolerable treatment activity (14).…”

Feasibility of imaging 90Y microspheres at diagnostic activity levels for hepatic radioembolization treatment planning

“…The phantom had a LSF of 5.3% and a tumor to nontumor ratio (T/N) of 7.9. Based on the results of a previous study by Prince et al, the activity concentration of the extrahepatic depositions was chosen to be 1.3% of the total activity in the phantom divided by 6.8 mL …”

“…Observed differences in 99m Tc‐MAA and 90 Y‐microsphere distributions could be caused by their differences in shape and size and by free circulating pertechnetate . A better representation of the microsphere distribution may be achieved by using identical particles for pretreatment and treatment, as is feasible in 166 Ho microsphere radioembolization …”

“…A similar objective may be pursued for 90 Y‐loaded microspheres. However, the total energy absorbed per Bq is higher for 90 Y than for 166 Ho, which limits the pretreatment activity to the estimated safety threshold of 100 MBq . In addition, SPECT of 90 Y is based on Bremsstrahlung imaging, which prevents the use of a photopeak energy window and energy window based scatter correction.…”

Surefire infusion system versus standard microcatheter use during holmium-166 radioembolization: study protocol for a randomized controlled trial

“…Besides β-radiation, these microspheres also emit low-energy γ-radiation and have paramagnetic characteristics, allowing for visualization and quantitative assessment of the microsphere distribution on single-photon emission computed tomography (SPECT) and magnetic resonance imaging (MRI) [4]. Furthermore, identical 166 Ho microspheres can be administered as a scout dose to predict the distribution of the therapeutic microspheres [3, 5]. …”