Abstract
In-situ thrombosis of the pulmonary artery is a filling defect in a pulmonary artery and is often mislabeled as the most common cause of a pulmonary artery filling defect, an embolism. Thrombus in-situ has been associated with chronic pleural effusions, pulmonary resections, infections and other souces of chronic inflammation, it has not been described as occuring with rounded atelectasis. Herein, we present a rare case of a pulmonary artery thrombus in-situ adjacent to rounded atelectasis; this case underscores the necessity for careful assessment and advocates for heightened awareness of in-situ thrombus as a differential diagnosis when assessing for pulmonary embolisms.