Evaluation and Management of Thymic Hyperplasia
For thymic hyperplasia, observation is appropriate for lesions <30mm, while lesions ≥30mm require surgical resection due to increased malignancy risk and the inability to reliably distinguish hyperplasia from thymoma on imaging alone. 1
Initial Diagnostic Evaluation
Imaging Studies
- Contrast-enhanced chest CT is the standard initial imaging modality for evaluating suspected thymic masses 1
- MRI with and without contrast should be obtained when CT findings are equivocal to differentiate thymic hyperplasia from thymic tumors or cysts 1
- Chemical-shift MRI can detect microscopic fatty infiltration (present in hyperplasia but not thymoma), though this finding alone is insufficient for definitive diagnosis 2
- PET/CT is generally not recommended for routine assessment of thymic masses 1
Laboratory Assessment
- Systematic immunological workup is mandatory, including complete blood count with reticulocytes, serum protein electrophoresis, anti-acetylcholine receptor antibodies, and anti-nuclear antibodies 1
- Serum beta-hCG and AFP should be measured to exclude germ cell tumors 1, 2
- Clinical evaluation for myasthenia gravis and other paraneoplastic syndromes is essential 1
Size-Based Management Algorithm
Lesions <30mm
- Observation without therapeutic intervention is recommended given the low risk of progression or malignancy 1
- Serial imaging surveillance is appropriate 1
Lesions ≥30mm
- Surgical resection is indicated due to increased malignancy risk and diagnostic uncertainty 1, 3
- The 30mm threshold is critical because thymic hyperplasia cannot be reliably distinguished from thymoma radiologically, even with advanced imaging 3, 2
- Preoperative biopsy is not required if upfront surgical resection is achievable 1
Surgical Approach
Operative Technique
- Median sternotomy is the standard surgical approach providing optimal exposure 1
- Complete thymectomy (removal of entire thymus gland and perithymic fat) is preferred over partial resection 1, 3
- Surgical planning should be discussed in a multidisciplinary tumor board setting 1
- Clips should be placed to mark areas of concern if complete resection is challenging 1, 4
Rationale for Complete Resection
- Definitive histological diagnosis is required to exclude thymic epithelial tumors, which account for 35% of anterior mediastinal masses 4
- True thymic hyperplasia is extremely rare and cannot be distinguished from lymphoid hyperplasia or thymoma preoperatively 5, 6
- Research shows that thymic hyperplasia can reach sizes of 3.5-24 cm (mean 10.46 cm), often initially misdiagnosed as thymoma 6
Histopathological Differentiation
Key Diagnostic Features
- True thymic hyperplasia shows preserved corticomedullary architecture with scattered Hassall corpuscles, no lymphoid follicles, and no cytologic atypia 6, 7
- Lymphoid (reactive) hyperplasia demonstrates lymphoid follicles and is associated with autoimmune diseases 5, 6
- On contrast-enhanced CT, lymphoid hyperplasia shows significantly higher attenuation than true hyperplasia (>41.2 HU threshold with 83% sensitivity and 89% specificity), though this cannot replace histological diagnosis 8
Clinical Pitfalls and Caveats
- Up to 66% of thymic hyperplasia cases are initially misdiagnosed as thymoma clinically or pathologically 6
- Thymic hyperplasia can cause symptoms (chest discomfort, shortness of breath) or cardiac displacement when large, mimicking malignancy 4, 3, 6
- Association with hyperthyroidism and other autoimmune conditions should be considered, as treatment of the underlying condition may result in thymic regression 9
- Fine-needle aspiration is not recommended; core-needle biopsy or surgical biopsy is required if preoperative tissue diagnosis is needed 1