Superior Sagittal Sinus Filling Defect in Metastatic Lung Cancer: Tumour Thrombus, Bland Thrombus or Both?

Background: Cerebral venous sinus thrombosis is an uncommon but potentially life-threatening cause of stroke. In patients with active malignancy, dural venous sinus filling defects present a diagnostic challenge, as they may represent bland thrombosis, tumour thrombus or benign anatomic variants such as arachnoid granulations. Although anticoagulation is recommended for confirmed cerebral venous sinus thrombosis, guidance is limited when imaging findings are equivocal, or tumour involvement is suspected. Diagnostic misclassification may result in unnecessary anticoagulation or failure to treat thrombosis. Case description: A 66-year-old woman with a history of subtotal colectomy complicated by chronic malabsorption and newly diagnosed metastatic lung cancer underwent brain magnetic resonance imaging for staging. Imaging revealed multiple intracranial metastases and a heterogeneous filling defect within the posterior superior sagittal sinus, with mild sinus expansion and internal flow voids, raising concern for cerebral venous sinus thrombosis or tumour thrombus. Anticoagulation was initiated despite the patient remaining neurologically intact. Subsequent computed tomography angiography and venography demonstrated preserved venous flow and findings consistent with arachnoid granulations rather than thrombosis or tumour invasion, allowing anticoagulation to be discontinued. The patient's altered gastrointestinal anatomy was clinically relevant, as it would have limited the reliability of direct oral anticoagulant therapy. Conclusions: This case demonstrates that magnetic resonance imaging alone may be insufficient to reliably differentiate cerebral venous sinus thrombosis, tumour thrombus and benign anatomic variants. In patients with active malignancy, confirmatory vascular imaging should be obtained before committing to prolonged anticoagulation. A structured, multimodal diagnostic approach can reduce misdirected therapy and unnecessary risk. LEARNING POINTS: In patients with active malignancy, dural venous sinus filling defects have a broad differential that includes bland cerebral venous sinus thrombosis, tumour thrombus and benign anatomic variants, each with distinct management implications.Magnetic resonance imaging findings may be indeterminate; confirmatory vascular imaging with computed tomography angiography or computed tomography venography is often necessary before initiating or continuing systemic anticoagulation.Anticoagulation decisions should be individualised, particularly in patients with altered gastrointestinal anatomy or malabsorption, where direct oral anticoagulant efficacy may be unreliable.