Caren van Roekel

49 Radioembolization restricted, so malignancies appear hyperintense on DWI. DWI can be quantified by apparent diffusion coefficient (ADC). Changes of DWI under therapy are applied for response evaluation and can be used as a surrogate for the degree of tumor necrosis: necrotic tissue reveals an increased ADC because the free water mobility is increased. However, drawbacks of DWI sequences are a lack of reproducibility and no uniform DWI protocol (96). DWI can also be used as a prognostic factor: Schmeel et al. have investigated the prognostic value of the pretreatment mean ADC in predicting treatment response, progression-free survival and overall survival in 46 patients who underwent radioembolization for unresectable colorectal cancer liver metastases. The mean post-treatment ADC values were increased by 23% on average. Patients with progressive disease had significantly lower tumor ADC values than patients with tumor response (p=0.04). A cut-off value for distinguishing long- and short-term survivors was estimated to be a tumor ADC value of 935. The difference in survival between patients below and above this threshold was significant (3 months versus 5 months) (97). These findings seem to be of great potential, but they have to be confirmed in larger prospective series. Due to the absence of uniform DWI protocols, these results cannot be translated to DWI assessments in other centers. Recently, the use of perfusion imaging has emerged. This is based on the assumption that tumors with a higher arterial flow would receive a greater number of microspheres and respond better to treatment. CT perfusion parameters are well correlated with histopathologically determined tumor vascularity. Morsbach et al. have analyzed 40 patients who underwent radioembolization for hepatic metastases of various tumors. They found a significant difference between responders and non-responders in arterial perfusion, with the latter being much higher in responders (98). Using dynamic contrast-enhanced (DCE) sequences, tumor perfusion can also be quantified on MRI images (96). In a multivariate analysis of prognostic factors in 45 patients with neuroendocrine neoplasms, Sommer et al. found that the vascularization of hepatic metastases is of great prognostic significance, with hypervascularized metastases showing a significantly longer progression-free survival (99). 2