TYPE: Case Report TOPIC: Pulmonary Vascular Disease INTRODUCTION: In-situ pulmonary thrombosis refers to de novo thrombus formation in the pulmonary arteries without any distant source. In-situ thrombosis has been associated with radiation therapy and lung lobectomy. However, little is known about thrombosis resolution after anticoagulation therapy. We present a case of a patient with a persistent pulmonary thrombus despite adequate anticoagulation therapy. CASE PRESENTATION: A 70-year-old male with a past medical history of chronic obstructive lung disease, right-sided lobectomy secondary to non-small cell lung cancer, and subsequent restrictive lung disease due to lobectomy and radiation therapy presented to the pulmonary clinic with shortness of breath. Further evaluation that included a CT scan of the chest with contrast revealed an incidental in-situ thrombosis in the right main pulmonary artery extending into the right upper lobar branch. There was no evidence of cancer recurrence. Previous imaging did not reveal a similar thrombosis. Venous duplex and echocardiography were normal. The patient was anticoagulated with rivaroxaban for over nine months and remained on anticoagulation. Despite this, follow-up imaging has not demonstrated resolution, propagation, or improvement of the thrombus. After optimization of his comorbidities, the patient has remained symptom-free. DISCUSSION: In-situ pulmonary thrombosis may be associated with prior lung radiation therapy and lobectomy. While studies have shown that most pulmonary embolisms resolve over three months with anticoagulation, little is known about the resolution of in-situ thrombosis. CONCLUSIONS: Pulmonary thrombosis may become chronic despite anticoagulation. Therefore, it may be clinically relevant to identify the speed of clot resolution for evaluating patients with suspected recurrent thrombosis. DISCLOSURE: Nothing to declare. KEYWORD: IN-SITU PULMONARY THROMBOSIS, RADIATION THERAPY, LOBECTOMY, ANTICOAGULATION