Caren van Roekel

269 Discussion TABLE 1. Characteristic 90 Y resin 90 Y glass 166 Ho Half-life (h) 64.1 64.1 26.8 Type of radiation β β β, γ Decay product Zirconium-90 Zirconium-90 Erbium-166 Diameter (µm) 32.5±2.5 25±10 30±15 Density (g/mL) 1.6 3.3 1.4 Activity/microsphere (Bq) 40-70 2400-2700 300-330 Number per dose 50,000,000 4,000,000 33,000,000 The most apparent differences between the three types of microspheres are the type of radiation they emit and their specific activity. 166 Ho emits γ-radiation, which allows it to be visualized by SPECT/CT. It is also paramagnetic, so it can be visualized by MRI as well. On the contrary, imaging of 90 Y is possible by using either bremsstrahlung-SPECT/CT (73, 74) or PET/CT (75). The differences in specific activity are large: the amount of energy released per microsphere is smallest for 90 Y resin and greatest for 90 Y glass, with 166 Ho having a specific activity somewhere in between. This has many consequences: first, the difference in specific activity is translated into the number of particles injected, leading to a much larger embolic effect (and risk of stasis) for 90 Y resin and 166 Ho than for 90 Y glass. Also, the tolerability of the liver is different for the three types of microspheres, leading to a difference in safety thresholds. A higher number of microspheres leads to a higher number of targeted liver clusters (i.e. healthy liver parenchyma) and a more homogeneous distribution in the liver (76). With 90 Y glass microspheres, there generally is a more heterogeneous distribution of the microspheres, leading to a higher tolerability. This explains why the thresholds for safety are different for the three types of microspheres. Likewise, a more homogeneous distribution, as obtained with 90 Y resin and 166 Ho, most likely also requires lower tumor-absorbed doses to be effective. This is indeed reflected in the thresholds found in the literature, further described below. However, one should take into account that this also depends on the tumor type. Hepatocellular carcinomas are generally hypervascular, and will receive a high tumor-absorbed dose even with a small number of 90 Y glass microspheres, whereas colorectal carcinoma metastases are generally hypovascular, possibly 10