Caren van Roekel

247 Use of an anti-reflux catheter in 166 Ho-radioembolization leading to imperfections in segmentation. Furthermore, in this study, the perfusion volumes of the left and right hepatic arteries were estimated on pretreatment CT. The more accurate method of using perprocedural C-arm CT with contrast injection via a microcatheter in the left and right hepatic arteries was logistically not possible since patients underwent the work-up angiography on the same day as the treatment angiography and 166 Ho-microspheres need to be ordered 7 days in advance. This study had a within-subjects design, which has several advantages. First, patients serve as their own control, limiting possible confounding by extraneous patient variables (24) and requiring less subjects to detect meaningful effects. However, a within-patient design is only applicable, when the treatment of one body part (in our case functional liver half) is unlikely to affect the other body part for the outcome under study. While designing this study, we judged that the technical nature of the relationship between catheter design and particle distribution was suitable for this study design, because we assumed that this interplay is limited to local fluid-dynamics and that systemic carry- across effects are unlikely (25). If, however, systemic effects (e.g. the activation of vasogenic factors during the occurrence of near-stasis) do play a role, they may have negated potential differences in preferential tumor targeting between the anti-reflux and standard microcatheter. In our patient population, some tumors were located close to the so-called watershed areas and may actually have received blood supply from both perfusion territories (although this was not observed on cone-beam CT). Another disadvantage of our design was that although patient-level characteristics are accounted for, there are still within-patient characteristics that may cause random error. The anti-reflux catheter was, for example, much easier deployed in the right hepatic artery, as this often was a much larger, less tortuous vessel. The new version of the anti-reflux catheter was (due to randomization, not deliberately) only used in right hepatic arteries, which may explain the difference in occurrence of vasospasm between the two anti-reflux catheter versions. In our experience, the standard microcatheter used in this study had a much more flexible shaft and was therefore superior in tracking the guidewire and navigating the liver vasculature, when compared to both versions of the anti-reflux catheter. Also, although accounted for in the randomization, the tumor burden was not always equal between perfusion territories. 9