Caren van Roekel

9 Introduction and outline assess the distribution of the treatment particles by means of the injection of a scout dose. Yttrium-90 ( 90 Y)-based microspheres are the most widely used particles in radioembolization, but in the UMC Utrecht, holmium-166 ( 166 Ho)- based microspheres were developed as an alternative. A detailed overview of radioembolization is provided in the next chapter of this thesis (Chapter 2). Although radioembolization is a promising treatment strategy for colorectal cancer patients with liver metastases and does lead to improved survival rates (4), many patients still experience early progressive disease (5, 6). With a personalized treatment strategy, patient outcomes could be improved (7). The aim of this thesis was to investigate different aspects of 166 Ho-radioembolization that can be used to develop a tailored treatment approach for individual patients. An individualized treatment approach is based on two major principles: selection and planning. Selection comes down to using patient characteristics determining who is a good candidate for treatment with radioembolization. An example of patient characteristics that may be used for selection is the location of the primary tumor: patients with a right-sided primary tumor generally have a much worse prognosis than patients with a left-sided primary tumor, and poorly respond to treatment with cetuximab (8). Similar patient characteristics are important for radioembolization as well. In patients with unfavorable prognostic characteristics , treatment with radioembolization may do more harm than good. As trivial as this may seem, these prognostic characteristics are not always accounted for in routine clinical practice. For example, the distribution of the scout dose can be used for patient selection: in case of an unfavorable activity distribution with a high parenchymal-absorbed dose and a low tumor-absorbed dose, it may be better to withhold patients from radioembolization. Besides selection, the distribution of the scout dose can also be used in a very important aspect of treatment planning. After the right patients are selected, treatment may be optimized for each individual patient, based on scout dose distribution as a simulation of the treatment itself. The most important factor in treatment planning is personalized activity calculation: this should be done based on known thresholds for safety and efficacy (i.e. the maximum 1