Thrombosed pulmonary artery aneurysm. A rare cause of a high-probability lung scan.

The high-probability ventilation-perfusion lung scan is accepted as supportive of pulmonary embolism and often negates further diagnostic evaluation; however, there are processes that mimic the clinical presentation and radiographic findings of pulmonary emboli, including a unilateral segmental or greater perfusion defect. We present the findings in a patient whose presentation and ventilation-perfusion scans over a three-month course were suggestive of pulmonary embolism, yet pulmonary angiography revealed a thrombosed pulmonary artery aneurysm. The interpretation of a unilateral segmental perfusion defect as high probability does not secure the diagnosis of pulmonary embolism and should not preclude further evaluation for alternative etiologies.