Thymic Hyperplasia
Definition
- Includes lesions characterized by either:
- Increase in thymic size to a degree not expected for the patient's age and clinical condition
- Influx of reactive B cells into an otherwise normal thymus
Alternate/Historical Names
- Thymic hyperplasia
- As defined in the literature, refers only to thymic lymphoid hyperplasia
- The term is potentially vague and the more precise terminology below should be used
Diagnostic Criteria
- Thymic lymphoid hyperplasia
- An more descriptive alternate term is thymic germinal center hyperplasia
- Increased numbers of germinal centers
- Predominantly in the interstitium and at corticomedullary junction
- Occasional germinal centers may be seen in the normal thymus
- No clear cutoff is defined
- Residual thymus may be essentially normal in appearance or be distorted into ribbons and nests of epithelial cells
- Usually does not lead to an abnormal thymic weight or size
- Most frequently associated with myasthenia gravis
- May be idiopathic or seen in other autoimmune disorders
- Including sytemic lupus erythematosis, scleroderma and rheumatoid arthritis
- Removal may improve myasthenia gravis symptoms
- May be idiopathic or seen in other autoimmune disorders
- Germinal centers may also be associated with involvment of the thymus by neoplasms
- True thymic hyperplasia
- Thymus too big for patient's age
- Under age 30, this means over 50 gm, see table
- Histologically normal
- Normal cortex, medulla, Hassal corpuscles and lobularity
- No neoplasm
- No reactive influx of cells
- Cured by excision
- Thymus too big for patient's age
- Thymic rebound hyperplasia
- Rapidly growing anterior mediastinal mass in a post chemotherapy patient
- Some but not all cases exceed normal weight limits for age
- Histologically normal
- May be PET positive
- Most often noted in lymphoma or germ cell tumor patients
- Probably because the mediastinum is watched closely for recurrence of these neoplasms
- Rapid growth and PET positivity may lead to surgical excision
- Rebound hyperplasia itself is of no clinical significance
- Rapidly growing anterior mediastinal mass in a post chemotherapy patient
Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342
