In a patient being evaluated for ocular myasthenia gravis, is a chest X‑ray sufficient for thymic assessment, or should a contrast‑enhanced chest CT be performed?

Thymic Evaluation in Myasthenia Gravis: Imaging Recommendation

A chest X-ray is insufficient for thymic assessment in patients with ocular myasthenia gravis; contrast-enhanced chest CT is the mandatory initial imaging modality, with MRI reserved for equivocal cases. 1

Primary Imaging Recommendation

Contrast-enhanced chest CT is the standard of care for initial thymic evaluation in all patients with myasthenia gravis, regardless of age or symptom severity. 1, 2 This recommendation is based on the 2025 NCCN guidelines, which explicitly designate chest CT with contrast as the required first-line imaging test for mediastinal masses. 1

Why Chest X-Ray is Inadequate

• Plain chest radiography has a sensitivity of only 58% for detecting thymomas in myasthenia gravis patients, missing 42% of tumors. 3
• Chest X-ray findings are often subtle and cannot reliably distinguish thymic hyperplasia from thymoma, thymic cysts, or normal thymic tissue. 4, 3
• In patients under 21 years old, the densely cellular normal thymus can completely obscure small thymomas on plain films. 3
• In patients aged 21-45 years, partial fatty involution creates parenchymal islands that mimic thymomas or hide small tumors on radiography. 3

CT Performance Characteristics

Contrast-enhanced CT achieves 85% sensitivity, 98.7% specificity, and 95.8% accuracy for thymoma detection in myasthenia gravis patients. 3 The enhanced CT value (120s post-contrast) demonstrates an AUC of 0.94 for distinguishing thymomas from thymic cysts. 5

When to Add MRI

Chest MRI with and without contrast should be obtained when CT findings are equivocal to differentiate thymic hyperplasia from thymic tumors or cysts. 1, 2, 6, 7 The NCCN guidelines specifically note that MRI provides superior discrimination compared to CT, potentially avoiding unnecessary thymectomy. 1

MRI Diagnostic Advantages

• Chemical-shift MRI is the key diagnostic tool for uncertain cases: thymic hyperplasia and normal thymus demonstrate signal loss on out-of-phase imaging due to microscopic fat, while thymic malignancies and lymphoma do not suppress. 6, 8
• MRI achieves 100% sensitivity and 80% specificity for thymic mass characterization, with an AUC of 0.880. 9
• Enhanced CT combined with MRI increases diagnostic accuracy to 70.3% for thymomas and 71.9% for thymic cysts (compared to 40.5% and 37.5% for CT alone). 5

Age-Specific Imaging Algorithm

Patients ≤20 Years Old

• Obtain posteroanterior, lateral, and 20-degree oblique chest radiographs initially. 3
• Proceed directly to contrast-enhanced chest CT only if local symptoms, signs, or radiographic findings suggest thymic abnormality. 3
• However, the 2025 NCCN guidelines supersede this older recommendation and mandate CT for all mediastinal mass evaluations regardless of age. 1

Patients ≥21 Years Old

• Routine contrast-enhanced chest CT is mandatory as the initial imaging study, bypassing chest X-ray entirely. 3, 1
• Fatty involution in this age group enhances recognition of even small thymic tumors on CT. 3

Additional Workup Components

Beyond imaging, the following laboratory tests are essential:

• Serum beta-hCG and AFP to exclude germ cell tumors in the differential diagnosis. 1, 2, 7
• Systematic immunological assessment: complete blood count with reticulocytes, serum protein electrophoresis, anti-acetylcholine receptor antibodies, and anti-nuclear antibodies. 2, 6, 7
• FDG-PET/CT from skull base to mid-thigh as clinically indicated, particularly for staging if thymic carcinoma is suspected. 1, 6

Critical Pitfalls to Avoid

• Never rely on chest X-ray alone for thymic assessment in myasthenia gravis—this misses nearly half of thymomas. 3
• Do not perform fine-needle aspiration for tissue diagnosis; core-needle biopsy or surgical biopsy is required if preoperative histology is needed. 2, 6, 7
• Do not assume benignity based on imaging alone, as distinguishing thymic hyperplasia from thymoma can be challenging even with advanced imaging. 6, 7
• Avoid ordering PET/CT as a routine screening tool for thymic masses, as normal and hyperplastic thymus can be FDG-avid, creating false positives. 1, 8

Size-Based Management After Imaging

• Lesions <30mm: observation with radiological follow-up is appropriate given low malignancy risk. 2, 6, 7
• Lesions ≥30mm: complete surgical resection (total thymectomy) is the standard of care due to increased malignancy risk and diagnostic uncertainty. 2, 6, 7