Caren van Roekel

153 Mode of progression after radioembolization in colorectal cancer patients (50). Importantly, these studies were performed in first line refractory disease. This limits comparison with our study in a more advanced-stage population. Objective response (CR or PR) at three months after treatment was obtained in only 6% of our patients. This is in line with other studies in salvage mCRC patients, with reported response ranges of 6%-24% (24, 56, 57). Median OS in our study was ten months, which is also in line with other studies in a comparable patient population (22, 24, 58). A reason for the modest treatment results in our study might be the dosimetric models that were used: the BSA and MIRD methods. These methods can lead to underdosing (59, 60). A personalized treatment approach, as was used in the DOSISPHERE study in HCC patients, could have led to a much higher response rate (61). The results of earlier studies on the dose-response relations in mCRC patients treated with 90 Y-resin or 166 Ho prove this point: a significant dose- response relationship was found in both studies (22, 62). Implementing the results of these studies in future patients, using an individualized treatment approach, likely will lead to a higher treatment accuracy. In our study, response was evaluated using the anatomic criteria as defined by the RECIST guidelines. However, this can be hampered by the presence of necrosis, hemorrhage and cystic changes (63). Response assessment based on changes in functional metrics as determined on [ 18 F]-FDG PET/CT would be a better evaluation method, especially since several studies found that these are related with overall survival (13, 22, 64, 65). Unfortunately, not all patients in our study underwent baseline and post-treatment imaging by [ 18 F]-FDG PET/CT. The added value of the present study to the existing knowledge on radioembolization in mCRC patients is the fact that the development of new metastases is the primary cause for progressive disease after treatment. Furthermore, the study shows that the development of new lesions, as well as progressive disease in general, is more common in patients with extrahepatic disease at baseline. The current study also has several limitations. First of all, the sample size was small. Secondly, the retrospective setting was prone to selection bias. Since 5