Caren van Roekel

199 Dose-effect relationships of holmium-166 radioembolization in colorectal cancer in accordance with other studies comparing these response assessments after radioembolization (25, 26). Metabolic response assessment is not hampered by the presence of necrosis, cystic changes and hemorrhage, as can be the case with size evaluation on transaxial images (27). Moreover, several studies found that changes in functional metrics, such as TLG, were related with overall survival and were more accurate predictors than anatomic changes (22, 27, 28). Although the majority of our patients underwent ≥2 prior lines of systemic treatment, the response rate seems suboptimal. Before treatment, patients with CRC are currently selected based on clinical criteria, such as WHO performance status and progressive disease after several lines of chemotherapy (29). In case patients are deemed eligible for treatment with radioembolization, a second selection criterion should be the activity distribution based on either 99m Tc-MAA or 166 Ho-scout. Based on the results of this study, we would argue that patients should only be selected for treatment if there is a favorable activity distribution with a sufficient mean tumor-absorbed dose >90 Gy and a parenchymal-absorbed dose of <55 Gy. Although a causal relationship cannot be claimed solely based on these observational data, the findings of this study suggest that below a mean tumor-absorbed dose of 90 Gy, metabolic response seems unlikely. However, since the discriminatory power of absorbed dose for response is limited, this number should be used with caution. The need for personalized dosimetry is widely accepted, with several studies showing a dose-response relationship in CRC patients treated with 90 Y-resin radioembolization (22-24, 28). There also is growing evidence for the possibility of improving treatment outcomes by using personalized treatment planning in radioembolization (10, 11). However, thus far, the DOSISPHERE study was the only study implementing personalized radioembolization planning in a prospective clinical study, investigating the tumor-absorbed dose and response rate in HCC patients using a standard versus a personalized dosimetric approach with 90 Y glass microspheres. Preliminary results showed that both the response rates and tumor-absorbed doses were significantly higher in the personalized dosimetry arm (30). 7