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Background Outcomes after middle cerebral artery occlusion can vary widely. Recent literature has identified a parameter of Distance to Thrombus (DT) which measures the distance of the occlusion along the M1 segment. DT has mixed findings in the literature, with some studies showing an association with outcomes, finding that more proximal occlusion along the M1 results in larger ischemic volume and worsened outcomes. This project aims to measure DT on endovascular treatment (EVT) angiography and quantify the association with clot migration after thrombolytic and potential impact on functional outcome.Methods A retrospective review was conducted to screen consecutive patients undergoing mechanical thrombectomy for M1 segment occlusion beginning in January 2023. Patients were excluded if pre-procedural imaging was not available, if there was tandem extracranial occlusion, or if motion degradation of imaging precluded measurements. The measurement of distance to thrombus was performed on coronal view from the terminus of the internal carotid artery to the point of occlusion. EVT images were calibrated from CTA images using carotid artery measurements. Clinical data was collected including stroke severity scores, discharge disposition, revascularization scores, thrombolytic administration, and functional outcomes measured as modified Rankin Scale (mRS) scores. Tabulations of means with standard deviation are reported along with student’s t-test with p value level of significance <0.05.Results An interim analysis was performed after data collection for 76 patients. The average DT on initial CTA was 7.85 ± 4.7mm and average distance to thrombus on thrombectomy digital subtraction angiography was 8.99 ± 4.6mm. The average change in DT between CTA to EVT was 1.16mm, indicating increased distance along the M1 segment between initial study and EVT images. Patients receiving thrombolytics (n=32) had a mean change in DT of 1.63 ± 3.0mm and patients not receiving thrombolytics (n=42) had a mean change in DT of 0.76 ± 1.4mm (p<0.05). Patients with DT less than 5mm at the time of EVT had a mean 90-day mRS of 3.81 and those with DT greater than 5mm had a mean 90-day mRS of 2.96 (p=0.07).Conclusion Distance to thrombus is a novel parameter in middle cerebral artery occlusion. Patients receiving IV thrombolytic show increased DT at the time of EVT compared to those without.Patients with more proximal M1 occlusion, i.e. less than 5mm from ICA terminus, had a worsened disability on mRS scores at follow-up.Disclosures D. Defta: None. M. Kruk: None. I. Sicilia: None. Y. Hu: None. K. Duncan: None.