Background and Purpose-The observed distribution of cerebral infarcts varies markedly from expectations based on blood-flow volume or Doppler embolus detection. In this study, we used an in vitro model of the cerebral arteries to test whether embolus microspheres encountering the circle of Willis are carried proportionally to volume flow or express a preferred trajectory related to arterial morphology or embolus size. Methods-Our model consisted of a patient-specific silicone replica of the cerebral macrocirculation featuring physiologically realistic pulsatile flow of a blood-mimicking fluid at approximately 1000 mL/min and an input pressure of approximately 150/70 mm Hg. Particles of 200, 500, and 1000 m diameter with equivalent density to thrombus were introduced to the carotid arteries and counted on exiting the model outlets. Results-The middle cerebral arteries (MCAs) of the replica attracted a disproportionate number of emboli compared with the anterior cerebral arteries; 98%Ϯ3% of 1000 m and 93%Ϯ2% of 500 m emboli entered the MCA compared with 82%Ϯ5% of the flow. The observed distribution of large emboli was consistent with the ratio of MCA:anterior cerebral artery infarcts, approximately 95% of which occur in territories supplied by the MCA. With decreasing embolus size, the distribution of emboli approaches that of the flow (approximately 89% of 200 m emboli took the MCA). Key Words: cerebral hemodynamics Ⅲ embolic stroke Ⅲ experimental Ⅲ transcranial Doppler E pidemiological evidence, 1,2 autopsy studies, 3,4 and the trajectories of "balloon emboli" in patients undergoing cerebral angiography 5 suggest that emboli incident from the common carotid artery (CCA) are more than 20 times as likely to come to rest in territories supplied by the middle cerebral artery (MCA) than the anterior cerebral artery (ACA). However, in vivo transcranial Doppler ultrasound insonation of the MCA-ACA junction performed by Wijman et al found emboli to be only 2 to 3 times more likely to enter the MCA 6 suggesting that emboli are carried roughly proportionally to flow volume. Wijman et al concluded that "cerebral embolism is unlikely to be the only cause of ACA and MCA infarcts." In this study, we clarify the relationship among blood flow, arterial topology, and embolization by implementing a physiologically realistic 3-dimensional phantom of the major cerebral arteries to investigate embolus trajectory. We hope that a better understanding of the relationship among arterial topology, blood flow, and embolus trajectory will assist in the development of patient-specific computer simulations for monitoring patients at high risk of stroke. 7 Although previous models of the circle of Willis have been developed to investigate blood flow, 8 -11 ours is the first to use a 3-dimensional in vitro model of the cerebral arteries to study embolization. Conclusions-Embolus Materials and Methods Replica AnatomyA physical model of the cerebral arteries was purchased from a company specializing in vascular replicas (Elastrat, Geneva, Switz...

Scanned ion beam delivery enables the highest degree of target dose conformation attainable in external beam radiotherapy. Nominal pencil beam widths (spot sizes) are recorded during treatment planning system commissioning. Due to changes in the beam-line optics, the actual spot sizes may differ from these commissioning values, leading to differences between planned and delivered dose. The purpose of this study was to analyse the dosimetric consequences of spot size variations in particle therapy treatment plans. For 12 patients with skull base tumours and 12 patients with prostate carcinoma, scanned-beam carbon ion and proton treatment plans were prepared and recomputed simulating spot size changes of (1) ±10% to simulate the typical magnitude of fluctuations, (2) ±25% representing the worst-case scenario and (3) ±50% as a part of a risk analysis in case of fault conditions. The primary effect of the spot size variation was a dose deterioration affecting the target edge: loss of target coverage and broadening of the lateral penumbra (increased spot size) or overdosage and contraction of the lateral penumbra (reduced spot size). For changes ≤25%, the resulting planning target volume mean 95%-isodose line coverage (CI-95%) deterioration was ranging from negligible to moderate. In some cases changes in the dose to adjoining critical structures were observed.