Caren van Roekel

28 Chapter 2 hepatic segments analysed(48). Haste et al. have evaluated the value of 99m Tc- MAA in predicting subsequent 90 Y glass microspheres distribution in patients with hepatocellular carcinoma. 99m Tc-MAA was found to be a poor surrogate to quantitatively predict the tumor absorbed dose of 90 Y, but there was a correlation between 99m Tc-MAA and 90 Y in the distribution in normal liver tissue (49). Ilhan et al. have analyzed the predictive value of 99m Tc-MAA SPECT for radioembolization with 90 Y resin microspheres, by comparing uptake on pretherapeutic 99m Tc- MAA SPECT with uptake on posttherapeutic 90 Y bremsstrahlung SPECT. They analyzed 502 patients who underwent radioembolization for primary and secondary liver tumors. They found a significant but quite low correlation between the 99m Tc-MAA and 90 Y-microsphere tumor-to-background ratio (50). Another study, however, evaluated the agreement between 99m Tc-MAA SPECT/ CT-based predictive dosimetry and posttreatment 90 Y PET/CT-based dosimetry in patients with hepatocellular carcinoma, treated with both glass and resin spheres. They found that predictive dosimetry based on 99m Tc-MAA SPECT/CT provides good estimates of absorbed doses as calculated on posttreatment 90 Y PET/CT, for tumor and nontumor tissues (51). However, the majority of the treatments analysed (25/27) was selective, which may have led to more positive results and a better correlation between 99m Tc-MAA and 90 Y. Not only the size, density and number of the injected particles play a role in the different distributions of 99m Tc-MAA and 90 Y –microspheres. Other confounding factors are tumor type, tumor vascularization, tumor size, prior therapy, 99m Tc- MAA injection parameters and angiographic considerations such as catheter position and vasoactive arterial status. There are large differences in 99m Tc- MAA uptake between hepatocellular carcinomas that are generally large and hypervascular and liver metastases such as colorectal cancer liver metastases, which are often smaller and hypovascular. Prior therapy can also have an influence, because it can induce arterial disorders and weaknesses. There is no established protocol for the injection of 99m Tc-MAA, but guidelines indicate that the injection time should be 20-30 seconds. This is quite comparable to a bolus injection, which is significantly different from the pulsing method used during the actual administration of resin- and 166 Ho-microspheres. The most important factor is the injection position of the catheter during 99m Tc-MAA infusion and 90 Y-microsphere infusion. This catheter position