Caren van Roekel

182 Chapter 7 ABSTRACT Introduction Radioembolization is a treatment option for colorectal cancer (CRC) patients with inoperable, chemorefractory hepatic metastases. Personalized treatment requires established dose thresholds. Hence, the aim of this study was to explore the relation between dose and effect (i.e. response and toxicity) in CRC patients treated with holmium-166 ( 166 Ho) radioembolization. Materials and methods CRC patients treated in the HEPAR II and SIM studies were analyzed. Absorbed doses were estimated using the activity distribution on post-treatment 166 Ho- SPECT/CT. Metabolic response was assessed using the change in total lesion glycolysis on 18 FDG-PET/CT between baseline and three-months follow-up. Toxicity between treatment and three months was evaluated according to the Common Terminology Criteria for Adverse Events (CTCAE) version 5, and its relation with parenchymal-absorbed dose was assessed using linear models. The relation between tumor-absorbed dose and patient- and tumor-level response was analyzed using linear mixed-models. Using a threshold of 100% sensitivity for response, the threshold for a minimal mean tumor-absorbed dose was determined and its impact on survival was assessed. Results Forty patients were included. The median parenchymal-absorbed dose was 37 Gy (range 12-55 Gy). New CTCAE grade ≥3 clinical and laboratory toxicity were present in eight and seven patients, respectively. For any clinical toxicity (highest grade per patient), the mean difference in parenchymal dose (Gy) per step increase in CTCAE grade category was 5.75 (95% confidence interval (CI) 1.18-10.32). On a patient level, metabolic response was: complete response (CR) n=1, partial response (PR) n=11, stable disease (StD) n=17 and progressive disease (PD) n=8. The mean tumor-absorbed dose was 84% higher in patients with CR/PR than in patients with PD (95%CI: 20-180%). Survival for patients with a mean tumor-absorbed dose >90 Gy was significantly better than for patients with a mean tumor-absorbed dose <90 Gy (hazard ratio=0.16, 95%CI 0.06-0.511).