What is the most likely diagnosis and recommended work‑up for a round, well‑circumscribed focus along the right lateral border of the left atrium seen on CT?

Most Likely Diagnosis: Pericardial Cyst

The most likely diagnosis for a round, well-circumscribed focus along the right lateral border of the left atrium on CT is a congenital pericardial cyst, which typically presents as a well-defined, fluid-dense structure along the cardiac borders. 1

Diagnostic Characteristics

The imaging features described are classic for a pericardial cyst:

• Well-circumscribed, round morphology is the hallmark presentation 1
• Fluid-dense appearance on CT distinguishes these benign lesions 1
• Location along cardiac borders (left or right heart border) is typical, though the right cardiophrenic angle is most common 1

The 2015 ESC Guidelines explicitly state that "the diagnosis of (congenital) pericardial cysts—presenting as well-defined, fluid-dense structures along the left or right heart border—as well as the differential diagnosis with other cystic structures, such as bronchogenic or duplication cysts, is usually straightforward" on CT imaging. 1

Recommended Work-Up

Initial Imaging Assessment

Obtain contrast-enhanced CT if not already performed to better characterize the lesion and confirm fluid density. 1

• Intravenous contrast administration helps differentiate fluid-filled cysts from solid masses or vascular structures 1
• CT provides excellent anatomical detail with high spatial resolution for precise characterization 1

Advanced Imaging for Tissue Characterization

Cardiac MRI (CMR) is the preferred next step for definitive characterization if the diagnosis remains uncertain after CT. 1

• CMR provides superior tissue characterization through multiple sequences 1
• T1-weighted and T2-weighted imaging can confirm fluid content 1
• CMR distinguishes pericardial cysts from other cystic structures (bronchogenic cysts, duplication cysts) 1

Echocardiography

Transoesophageal echocardiography should be considered if the lesion's relationship to cardiac structures needs clarification or if hemodynamic assessment is required. 1

• Transthoracic echocardiography is the first-line test for pericardial disease but may have limited visualization of certain locations 1
• TEE provides superior visualization of structures adjacent to the left atrium 1

Differential Diagnosis Considerations

While pericardial cyst is most likely, consider:

• Bronchogenic or duplication cysts - similar appearance but different tissue characteristics on MRI 1
• Cardiac varix - may present as a well-circumscribed cystic mass with calcification, though typically located at the fossa ovalis in the right atrium 2
• Loculated pericardial effusion - less well-defined borders, may have septations 1
• Neoplastic involvement - would typically show contrast enhancement, irregular borders, or associated findings 1

Clinical Pitfalls to Avoid

• Do not assume malignancy based solely on location; benign pericardial cysts are far more common than cardiac tumors in this presentation 1
• Avoid unnecessary biopsy if imaging characteristics are definitively consistent with a simple pericardial cyst 1
• Do not overlook the need for follow-up imaging even if asymptomatic, as documentation of stability confirms the benign nature 1

Management Algorithm

1. Review existing CT images for fluid density and well-defined borders 1
2. If fluid-dense and classic appearance → Likely pericardial cyst; consider surveillance imaging at 6-12 months 1
3. If atypical features present (solid components, enhancement, irregular borders) → Proceed to CMR for tissue characterization 1
4. If symptomatic or growing → Surgical consultation for potential resection 1
5. If asymptomatic and stable → Conservative management with periodic imaging surveillance 1