Caren van Roekel

31 Radioembolization There are several factors that may have an influence on LSF calculation. LSF estimation without attenuation correction gives a large overestimation with respect to attenuation-corrected evaluations. There is reduced photon attenuation in the lungs compared to the photon attenuation in the abdomen. Another factor is scatter correction, which should be applied in order to avoid overestimations of the LSF. Moreover, 99m Tc-MAA degrades after several hours, which increases the LSF. Therefore, it is important to perform the image acquisition as soon as possible after administration of 99m Tc-MAA (1, 46). The studies of Yu et al. and Kao et al. have proven that SPECT/CT leads to more accurate calculation of lung shunt absorbed dose than planar imaging (55, 56). However, lung shunting differs significantly between different tumor types. Gaba et al. have published a study of 141 patients with primary and secondary hepatic malignancies who underwent radioembolization. The LSF was calculated for every patient and LSF>20% occurred much more often in HCC patients than in patients with other tumor types (14% versus 3%). In HCC patients, a high LSF was associated with infiltrative morphologic structure, tumor burden >50%, portal vein invasion and arterioportal shunting. In other tumor types, such as colorectal carcinoma metastases, neuroendocrine tumors and cholangiocarcinomas, a larger tumor size and greater tumor burden were associated with lung shunting (57). In patients with these factors, care should be taken to avoid deposition of radioactive microspheres in the lungs. 4.4. Extrahepatic activity and nontumor dose in the liver Pretreatment simulation with 99m Tc-MAA is not only useful to establish the LSF, but also to give an impression of the total absorbed radiation dose in the healthy liver parenchyma: the non-tumor dose. It is widely known that with an increasing non-tumor dose the risk of complications such as radioembolization- induced liver disease (REILD) is higher, but the maximum tolerable non-tumor dose is not well established. It is dependent on the type of tumor, the condition of the liver, prior treatments (e.g. bevacizumab) and the distribution of radiation within the non-tumor volume. With radioembolization, the absorbed dose in the liver is never uniform, which makes higher average absorbed doses tolerable. A non-tumor dose limit of <70 Gy in normal liver tissue and a non-tumor dose limit of <50 Gy in cirrhotic livers is proposed, but this is highly dependent on microspheres used and the number of microspheres administered (2, 46). 2