Holmium-166 Microspheres for Image-Guided Radioembolization: No Need for Patient Isolation after Treatment

Purpose Holmium-166 ( 166 Ho) microspheres have been developed for radioembolization (RE) of liver tumors. The radiation emission of 166 Ho-microspheres outside the body can raise concerns regarding radiation safety after RE. The purpose of this study was to perform an external dose assessment for patients treated with 166 Ho-RE. Materials and Methods In the phase 1 HEPAR study (Holmium Embolization Particles for Arterial Radiotherapy), the safety and toxicity profile of 166 Ho-RE was evaluated in fifteen patients with unresectable, chemorefractory liver metastases (1). Patients (mean liver volume 2.1 L, range 1.5 L - 3.6 L) were treated with a mean net infused activity of 5.1 GBq (SD 2.9). Radiation dose rates were measured at 1 m distance from a right lateral and frontal position using a portable dose rate meter (Radiagem 2000, CANBERRA, Meriden, Ct.). The measurements were done 0, 3, 6, 24, and 48 hours after infusion, and corrected to an aimed whole-liver absorbed dose of 60 Gy, which is used in subsequent phase 2 treatments. A total effective dose equivalent (TEDE) was calculated in accordance with published guidelines of the U.S. Nuclear Regulatory Commission (NRC) (2). Results The mean dose rate at discharge, 48 hours after infusion, measured from 1 m distance from a right lateral position was 6.4 μSv/h (SD 3.8). Assuming invariable presence, the mean TEDE was 0.25 mSv (SD 0.15). When adjusted to a whole-liver absorbed dose of 60 Gy, the mean dose rate in the air at 1 m immediately after treatment was 30.9 μSv/h (SD 13.2), corresponding to a mean TEDE of 1.19 mSv (SD 0.51). None of the corrected dose rates, at any time, measured from either frontal or lateral position, would have led to a TEDE above 5 mSv. Conclusion TEDE of patients treated with 166 Ho-RE aimed at a 60 Gy whole-liver absorbed dose, will not exceed the NRC limit (3) of 5 mSv to another individual: not even when discharged immediately after treatment. Patients are to be provided with written instructions as to keep the exposure to other individuals as low as reasonably achievable. References 1. Smits MLJ, et al., Lancet Oncol 2012;13:1025-34. 2. NRC, Regulatory Guide 8.39, April 1997. 3. NRC, NRC-10 CFR 35.75.