Chronic Pulmonary Embolism: Given the presence of an eccentric subocclusive filling defect in a right middle lobe segmental branch, which is suggestive of chronic or partially recanalized embolism, this diagnosis is highly plausible. The absence of right heart strain also supports a chronic rather than acute process.

Atherosclerotic Disease: The borderline ascending thoracic aortic ectasia could be related to atherosclerotic disease, which is a common cause of aortic dilation and could also contribute to embolic events due to plaque instability.
Pulmonary Artery Sarcoma: Although less common, a sarcoma could present as a filling defect within the pulmonary arteries and could potentially cause ectasia due to obstruction or invasion of the aortic wall.

Pulmonary Embolism with Acute Onset: Despite the description suggesting chronicity, missing an acute pulmonary embolism could be catastrophic. The absence of right heart strain does not rule out a significant and potentially life-threatening embolism.
Aortic Dissection: The presence of aortic ectasia increases the risk of dissection, which is a medical emergency. Even if the ectasia is borderline, any signs of dissection (e.g., intimal flap) would necessitate immediate intervention.
Infectious or Inflammatory Aortitis: Conditions like syphilis, tuberculosis, or giant cell arteritis could cause aortic ectasia and might also lead to embolic phenomena due to valvular or aortic wall involvement.

Behçet's Disease: A rare condition that can cause vascular inflammation, leading to aneurysms or ectasias, and potentially embolic events due to vascular wall damage.
Takayasu Arteritis: A large vessel vasculitis that could result in aortic ectasia and might also cause pulmonary artery involvement, leading to embolic phenomena.
Pulmonary Artery Aneurysm: A rare condition that could present similarly to an ectatic or aneurysmal segment of the pulmonary artery, potentially causing embolic events.