Background <p>Fibrosing mediastinitis (FM) is a rare chronic fibrotic disease of the mediastinum, characterized by diffuse fibrous tissue proliferation in the mediastinal space that can compress adjacent pulmonary vessels bilaterally. This fibrous proliferation is the core pathological basis for simultaneous stenosis of pulmonary arteries (PAs) and pulmonary veins (PVs) in affected patients, with adhesion as a secondary pathological manifestation of FM rather than the primary cause of vascular stenosis. Fibrous tissue proliferation compresses the pulmonary veins, causing stenosis, obstructing pulmonary circulation reflux, leading to pulmonary congestion and pulmonary hypertension, with symptoms such as dyspnea and hemoptysis. Diagnosis relies on contrast-enhanced chest CT and pulmonary angiography, and multimodal endovascular imaging including intravascular ultrasound (IVUS) and optical coherence tomography (OCT) can provide refined morphological details of stenotic lesions.</p> Case presentation <p>IVUS and OCT revealed a rare triangle-like lumen in FM-induced PV stenosis, a specific imaging feature that supplements the morphological data of FM-related PV stenosis and provides a new intravascular evaluation reference for such rare lesions. The combined use of IVUS and OCT helps differentiate extrinsic compression from intrinsic vascular pathology, which is critical for treatment selection. Treatment is mainly based on drugs to inhibit fibrosis, interventional procedures or surgery to relieve obstruction.</p> Conclusions <p>This case suggests that a triangle-like lumen on intravascular imaging may represent a useful clue to FM-induced pulmonary vein stenosis. However, this is a single observation; the specificity of the triangle-like lumen for FM remains unknown. Combined IVUS and OCT is resource-intensive and not routinely recommended. We report this morphology solely as a potential diagnostic clue when clinical suspicion of FM exists.</p>