BackgroundA recent trial failed to show any benefit of stenting plus medical therapy over medical therapy alone in patients with symptomatic intracranial stenosis. We aimed to examine whether the symptomatic qualifying artery modifies the effect of stenting plus medical therapy. MethodsThis is a post-hoc analysis of the CASSISS trial that included patients with symptomatic intracranial stenosis, randomly assigned to undergo stenting plus medical therapy or medical therapy alone; 358/380 patients were included. Multivariable logistic regression analysis was used with an interaction term to estimate the altered treatment effect by the qualifying artery. The primary outcome was a composite of stroke or death within 30 days or stroke in the qualifying artery territory beyond 30 days through 1 year. The five secondary outcomes included stroke or death related to the qualifying artery territory at 2 and 3 years. ResultsNo significant treatment allocation-by-stenosis site interaction was observed (P-interaction=0.435). Compared with medical therapy alone, the adjusted ORs for stenting plus medical therapy were 2.73 (95% CI 0.42 to 17.65) for internal carotid artery stenosis, 1.20 (95% CI 0.29 to 4.99) for M1 stenosis, 0.23 (95% CI 0.02 to 2.31) for vertebral artery stenosis, and 1.33 (95% CI 0.34 to 5.28) for basilar artery stenosis. Of the five secondary outcomes, none showed a significant treatment allocation-by-stenosis site interaction including stroke in the qualifying artery territory at 2 years (P-interaction=0.659) and 3 years (P-interaction=0.493). ConclusionsAmong patients with transient ischemic attacks or ischemic stroke due to severe intracranial atherosclerotic stenosis, there was no evidence that the symptomatic qualifying artery could determine the addition of stenting to medical therapy.