Caren van Roekel

222 Chapter 8 Other studies evaluated the efficacy of redistribution by assessment of the treatment response, and found favorable response rates in the dependent tumors (4,6–8). Spreafico et al. found an overall response rate of 100% (3 CR, 8 PR, and 6 SD, according to mRECIST) in dependent tumors at three months after treatment (7). Abdelmaksoud et al. compared tumor response in dependent tumors to their non-dependent counterparts and found inferior response in only one case out of twenty two (4.5%) (6). While it does support the efficacy of radioembolization treatment in tumors with redistributed blood flow, the endpoint of tumor response does not provide insight into the differences in activity distribution. Subgroup analysis showed that middle hepatic artery / segment IV artery redistribution was most successful, which may be attributable to the central location in the liver and the potential intrahepatic collaterals that can reroute blood flow from both the right and the left hepatic artery (Figure 5). Success rates for obtaining redistribution were lowest when parasitized arteries were embolized (Figure 6). This is possibly explained by the fact that these arteries were newly recruited by the tumorous process and did not (yet) have adequate collateral connections with the adjacent hepatic arteries. Furthermore, the distance between parasitized arteries and hepatic arteries may be an exacerbating factor, as parasitized arteries often vascularize peripheral parts of the liver. In our comparison between primary tumor types we found substantially better redistribution rates in CRC compared to NET metastases. This difference was most pronounced in the tumor analysis, in which all NET patients had an absorbed dose difference of ≥ 30%. This was expected to some extent, as hypervascular tumors are more likely to recruit parasitized arteries. However, this could not account for the entire difference, as only one NET case involved a parasitized artery. Perhaps also the hypervascular nature of the tumors make these tumors more prone for under-dosing after redistribution. Other primary tumor types could not be compared due to the small sample size.