Caren van Roekel

29 Radioembolization should be exactly the same, with the same distance to bifurcations and the same orientation in the vascular lumen (48). Another important factor is the vasoactive status of the hepatic arteries at the time of injection, since vasospasm may occur during radioembolization procedures and they may impact the distribution of microspheres (46). Because of the discrepancies between 99m Tc-MAA and 90 Y-microsphere distribution, several alternatives to 99m Tc-MAA are currently under investigation (52). 4.2. 166 Ho scout dose When planning radioembolization with 166 Ho, a small batch of 166 Ho-microspheres with limited radioactivity (200-250 MBq) can be used as a scout dose, instead of 99m Tc-MAA. This 166 Ho scout dose is sufficient to be visualized and quantified on SPECT imaging, but limited enough not to cause tissue damage in case of shunting to the gastrointestinal organs or the lungs. To prevent embolization of the arteries before treatment, only 60 mg is administered, whereas a treatment dose consists of 540-600 mg of microspheres. Because the same particles are used, lung shunting can be estimated more accurately (53). The safety of the 166 Ho scout dose has been established recently by Braat et al (54). Figure 4 shows a microscopic image of 99m Tc-MAA particles and 166 Ho-microspheres. FIGURE 4. Figure 4a shows a microscopic image of 99m Tc-MAA particles. Figure 4b shows a microscopic image of 166 Ho-microspheres. 2