Caren van Roekel

166 Chapter 6 INTRODUCTION Radioembolization with yttrium-90 ( 90 Y) or holmium-166 ( 166 Ho) microspheres is increasingly used in the treatment of primary and secondary liver cancers (1). It is an intra-arterial therapeutic procedure in which radioactive microspheres are delivered to hepatic tumors via their nutrient arteries (2). The goal of radioembolization is to deliver a tumoricidal absorbed dose to tumors while sparing the healthy liver tissue. Although it has been shown in multiple studies that the likelihood for tumor response critically depends on tumor absorbed dose, the dosing methods that are predominantly used in clinical practice do not incorporate the patient-specific biodistribution (i.e. locally absorbed doses) (1,3). Treatment with 166 Ho-radioembolization can be preceded by a scout dose consisting of a small batch (i.e. 250 MBq) of rheologically identical 166 Ho- microspheres. Official approval (CE-mark) was recently obtained in the EU (QuiremScout ® and QuiremSpheres ® ; Quirem Medical B.V., Deventer, The Netherlands). It was demonstrated that this scout dose predicts the absorbed dose to the lungs more accurately than technetium-99m-macroaggregated albumin ( 99m Tc-MAA) (4). And more recently, the scout dose was shown to have a superior predictive value for the intrahepatic therapy absorbed dose distribution (5). These findings support the use of a scout dose to better personalize dose planning (i.e. dosimetry) and patient selection. However, the relationship between tumor absorbed dose and response likelihood, needed for such a treatment personalization, has not yet been established. The aim of this exploratory study was to analyze the relationship between tumor absorbed dose, treatment response and survival in patients treated with 166 Ho-radioembolization. MATERIALS AND METHODS Patient Selection Candidates for this study were patients who were treated in the Holmium Embolization Particles for Arterial Radiotherapy I and II (HEPAR I and II; NCT01031784 (6) and NCT01612325 (7)) studies, which were conducted between 2009 and 2015. These studies were conducted in accordance with