Caren van Roekel

184 Chapter 7 INTRODUCTION Colorectal cancer (CRC) is one of the most common types of cancer worldwide (1). The liver is the first site of hematogenous spread and 70-80% of patients with hepatic metastases are deemed unresectable because of tumor size, location, multifocality, or inadequate hepatic reserve (2). Hence, the majority of patients with metastatic CRC cannot be cured. Palliative treatment generally consists of several lines of systemic chemotherapy. If the available chemotherapeutic options fail, treatment with radioembolization should be considered for patients with liver-only or liver-dominant disease (3). During radioembolization, radioactive microspheres are delivered intra- arterially to hepatic tumors. The rationale of this treatment is to administer a high local radiation dose to the tumors, while relatively sparing the healthy liver parenchyma by using the predominant arterial blood flow to tumors. Currently, three types of microspheres are available: yttrium-90 ( 90 Y) resin (SIRspheres®, Sirtex), 90 Y glass (TheraSphere®, BTG/Boston Scientific) and holmium-166 ( 166 Ho) microspheres (Quiremspheres®, Quirem Medical). One advantage of 166 Ho-radioembolization is that treatment can be preceded by a scout dose of the same microspheres, using only limited activity (250 MBq). This 166 Ho-scout has proven to be a more accurate predictor of the distribution of the treatment dose (4). Another advantage is that 166 Ho-microspheres can be visualized by both MRI and SPECT/CT (5). The safety and efficacy of 166 Ho- radioembolization was determined in the HEPAR and SIM studies (6-9). In these studies, activity calculation was based on a whole-liver absorbed dose of 60 Gy. To allow for personalized, or optimized treatment, reference levels for efficacy and toxicity are needed (10). Hence, the aims of this study were to determine the relationship between dose and toxicity and to determine the relation between dose and metabolic response, in CRC patients who were treated with 166 Ho-radioembolization.