What is the approach to diagnosing and treating a patient with a mediastinal mass?

Approach to Mediastinal Masses

Begin with contrast-enhanced CT chest as the initial imaging study for any suspected mediastinal mass, as it definitively localizes the lesion to a specific compartment and provides superior tissue characterization compared to chest radiography. 1, 2

Initial Diagnostic Imaging Algorithm

Step 1: CT Chest with Contrast

• Obtain thin-section imaging (≤5 mm slices) with multiplanar reconstructions to assess relationships to adjacent structures and evaluate for invasion across tissue planes 2, 3
• Pre- and post-contrast phases are essential to distinguish vascular structures from lymph nodes and identify enhancing cellular components 2, 4
• CT demonstrates calcium, macroscopic fat, water attenuation fluid, and can noninvasively diagnose many mature teratomas 1
• CT is superior for detecting invasion into chest wall, diaphragm, and neurovascular structures—critical for surgical planning 2

Step 2: Compartment Localization

The 2014 International Thymic Malignancy Interest Group classification divides the mediastinum into three compartments 1, 5:

Prevascular (Anterior) Compartment:

• Most common lesions: thymomas (28%), benign cysts (20%), lymphomas (16%) 1
• Check serum tumor markers (beta-HCG, AFP) if germ cell tumor suspected 4
• Chemical shift MRI can distinguish thymic hyperplasia from malignancy 4

Visceral (Middle) Compartment:

• Benign cysts are most common 1
• Use 15 mm short-axis threshold for lymph node assessment 4
• Consider lymphoma, metastatic disease, or infection if multiple enlarged nodes present 4

Paravertebral (Posterior) Compartment:

• Neurogenic tumors are most common 1

Advanced Imaging for Indeterminate Masses

When to Proceed to MRI

Order MRI chest (with and without IV contrast) when CT findings are indeterminate, as MRI can prevent unnecessary biopsies and surgery. 1, 2

MRI provides superior characterization of:

• Hemorrhagic and proteinaceous fluid (distinguishes benign hyperattenuating thymic cysts from thymomas) 1, 2
• Microscopic or intravoxel fat 1
• Cartilage, smooth muscle, and fibrous material 1, 2
• Cystic versus solid nature of lesions 2

Key advantage: Diffusion-weighted imaging (DWI) assists in lesion characterization and directs biopsy toward areas of higher cellularity 2, 4

Role of PET/CT

• Obtain FDG-PET whole body imaging to distinguish benign from malignant disease, identify metabolically active areas to guide biopsy, and detect occult metastatic disease 4
• Particularly valuable when distinguishing thymic epithelial tumors from lymphoma 1
• PET/CT guidance for biopsy yields no diagnostic advantage over CT guidance 1

Tissue Diagnosis Strategy

First-Line Approach: Endoscopic Biopsy

Prioritize endoscopic/bronchoscopic mediastinal biopsy (EBUS or endoscopic transesophageal ultrasound with FNA) as the preferred first-line approach, as it may be safer and have higher yields than percutaneous biopsy. 4

Alternative: CT-Guided Percutaneous Biopsy

When endoscopic approach is not feasible:

• Core biopsy is more effective than fine-needle aspiration 1, 2, 4
• Diagnostic yield: 87% for masses with mean size 5.3 cm; 77% for masses with mean size 6.9 cm 1, 2
• DWI MRI and DCE MRI can direct biopsy toward sites of higher cellularity and away from hemorrhagic necrosis 1, 2
• Transthoracic US-guided biopsy is feasible when the lesion is visible within the sonographic window, with color Doppler providing additional value 1

When Biopsy Can Be Avoided

• If imaging definitively demonstrates mature teratoma (macroscopic fat, calcium, water attenuation fluid) 1
• If MRI proves cystic nature of thymic mass (prevents unnecessary thymectomy) 1
• If typical imaging combined with appropriate clinical context provides confident diagnosis 6

Surveillance for Indeterminate Masses Not Requiring Immediate Biopsy

• Perform surveillance at 3-, 6-, or 12-month intervals over 2 or more years, depending on clinical concern 2
• MRI is preferred over CT for surveillance due to greater sensitivity for detecting increased lesion complexity 2
• Alternating MRI and CT follow-up is acceptable 2

Critical Pitfalls to Avoid

• Do not rely on size criteria alone for lymph node assessment—nodes >1 cm in short axis have limited sensitivity and specificity 2
• Do not skip contrast administration unless absolute contraindications exist (severe renal insufficiency or documented severe contrast allergy) 2
• Do not proceed directly to biopsy without MRI evaluation of indeterminate CT findings, as MRI can prevent unnecessary procedures 1, 2, 4
• Do not perform conservative management or follow-up imaging only for symptomatic mediastinal masses (particularly those causing chest pain or compressive symptoms) 4
• Benign hyperattenuating thymic cysts on CT can be misinterpreted as thymomas—MRI prevents this error 1

Emergent Considerations

Immediately assess for life-threatening complications requiring urgent intervention: 4

• Superior vena cava syndrome
• Airway compression
• Cardiac tamponade

Surgical Planning

• Complete resection is the goal for mediastinal tumors, particularly thymic epithelial tumors 2
• Assessment of invasion across tissue planes is critical for determining resectability 2
• Lymphoma diagnosis carries significant therapeutic implications (chemotherapy/radiotherapy, not surgical resection) 7