PURPOSE: To experimentally investigate the behavior of a clinically used microcatheter during transarterial radioembolization (TARE) microsphere injection in a successively bifurcating MATERIALS AND METHODS: A symmetrical phantom was developed which bifurcated 3 times into 8 outlets. A blood-mimicking fluid was pumped through the phantom using a physiological representative waveform. Holmium-165 microspheres were injected in a pulsed manner at 3 different locations using a standard microcatheter and a rigid counterpart with same dimensions as a control. Motion of the catheter was studied with a top- and side-view camera on the phantom. Microspheres were collected at each outlet and their distribution over the 8 outlets was analyzed. RESULTS: Due to the pulsatile flow in the phantom, strengthened by the pulsatile microsphere injection, the clinical catheter showed maximum displacements of 0.87 mm within a vessel with a diameter of 3.6 mm. This motion resulted in a different microsphere distribution for the clinical catheter compared with the rigid counterpart (75.9% vs 49.4% of the microspheres went to outlet 1-4, respectively). CONCLUSION: In this CLINICAL IMPACT: Our study demonstrated that microsphere distribution during transarterial radioembolization (TARE) is affected by catheter motion. Furthermore, increased catheter motion was observed as a result of the injection profile. Predictive tools such as the contrast CBCT and scout dose use different injection profiles compared to therapeutic TARE injections, potentially altering catheter tip behaviour and microsphere distribution, which could compromise their predictive values. Additionally, current TARE microsphere injection guidelines provide limited details, which may lead to variability across institutes and interventional radiologists. Standardizing injection techniques could reduce catheter motion variability and may facilitate more consistent and predictable microsphere distribution patterns.