What is the appropriate initial evaluation and management for a pediatric patient with an unexplained mass in the chest?

Initial Evaluation of Unexplained Pediatric Chest Mass

For a pediatric patient with an unexplained chest mass, obtain a chest radiograph first, followed immediately by CT chest with IV contrast as the primary cross-sectional imaging modality to define the mass location, characterize its nature, and guide further management.

Imaging Algorithm

First-Line Imaging

• Chest radiography (posteroanterior and lateral views) remains the essential initial study to localize the mass and define basic morphology 1, 2
• The chest radiograph establishes whether the mass is in the mediastinum, lung parenchyma, pleura, or chest wall 3, 4

Second-Line Cross-Sectional Imaging

CT chest with IV contrast is the primary next step for most pediatric chest masses 1, 5, 2:

• CT accurately characterizes mass location, size, tissue composition (solid vs. cystic, presence of fat or calcification), and relationship to adjacent structures 1, 2
• CT can demonstrate macroscopic fat and fluid attenuation, permitting noninvasive diagnosis of mature teratomas 6
• CT is superior for evaluating airway compression and extent of mediastinal involvement 5

MRI chest (with and without IV contrast) is preferred in specific scenarios 1, 7:

• Posterior mediastinal masses (neurogenic tumors are most common here) 6, 2
• Suspected vascular lesions 2
• Assessment of vascular involvement and tissue plane invasion, particularly relationship to ascending aorta and pulmonary artery 7
• Dynamic MRI during free-breathing can assess mass mobility and adherence to major vessels 7

Ultrasound has selective utility 4:

• Characterizing pleural fluid collections (simple vs. complicated vs. fibroadhesive) 4
• Evaluating chest wall lesions to distinguish soft tissue from extrapleural intrathoracic origin 4
• Differentiating pulmonary consolidation from masses using air/fluid bronchograms and color Doppler 4
• May be diagnostic for foregut cysts and certain mediastinal masses 2

Critical Differential Diagnosis by Location

Anterior (Prevascular) Mediastinum

The most common masses in this compartment include 6, 2:

• Thymomas (28% of prevascular lesions) - consider in patients >40 years, especially with myasthenia gravis 1, 6
• Lymphomas (16% of prevascular lesions) - look for additional lymphadenopathy in neck, axilla, or elsewhere 1, 6
• Benign cysts (20% of prevascular lesions) 6
• Teratomas - CT showing fat and fluid is diagnostic 6, 2
• Thymic hyperplasia - appears as soft tissue conforming to thymic shape, especially in young patients 1

Middle (Visceral) Mediastinum

• Benign cysts are most common here (foregut cysts, bronchogenic cysts, esophageal duplications) 6, 8
• Lymphadenopathy - short-axis measurement ≥15 mm is the threshold for concern 1

Posterior (Paravertebral) Mediastinum

• Neurogenic tumors predominate in this location 6, 2

Key Clinical Pitfalls to Avoid

Diagnostic Delays

• Perihilar and subcarinal cysts can be "hidden" behind the cardiac silhouette on chest radiograph - 35% of mediastinal cysts in one series were initially missed or never visualized on plain films 8
• Esophagram has poor sensitivity (diagnostic in only 26% of cases) and should not delay cross-sectional imaging 8
• In symptomatic children with respiratory distress or pneumonia, consider complications extending into the chest including pneumatocele and empyema necessitans 9

Life-Threatening Considerations

• Superior mediastinal obstruction from malignancy poses significant anesthetic risk - avoid large volume aspiration and general anesthesia/sedation if this is suspected 1
• If malignancy is suspected (absence of acute fever, evidence of mediastinal mass or lymphadenopathy), perform only small volume diagnostic aspiration (5 mL) without general anesthesia 1

Tissue Diagnosis Approach

When Biopsy is Indicated

If imaging remains indeterminate after CT or MRI 1, 7:

• CT-guided percutaneous core biopsy has 87% diagnostic yield for mediastinal masses and is safe 7
• Core biopsy is superior to fine-needle aspiration 7
• Image-guided transthoracic needle biopsy is appropriate for masses indeterminate on FDG-PET/CT or MRI 1

Special Considerations for Lymphoma

• FDG-PET/CT is recommended for staging and treatment response assessment of primary mediastinal large B-cell lymphoma 6
• Specimens should be sent to hematology laboratory for cytospin since most pediatric malignancies are hematological 1

Monitoring for Non-Responding Cases

If initial imaging suggests infection-related mass (parapneumonic effusion, abscess) but the patient fails to improve 1:

• Repeat chest imaging (radiography, ultrasound, or CT) if clinical evidence suggests increased respiratory effort or new areas of abnormal lung sounds 1
• Consider chest CT if pulmonary abscess, necrotizing pneumonia, or chest mass is suspected 1
• Obtain pleural fluid for culture, Gram stain, and PCR if moderate to large effusion is present 1