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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
    • 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
  • 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

Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342

Original posting/updates: 9/24/10, 12/1/13

Normal Thymic Weights

Approximate mean weights
Birth 10 gm
12 months 20 gm
7-25 years 25-35 gm
40-60 years 15 gm
  • Because of wide variations in normal subjects, maximum weights are difficult to define
    • Occasional normal thymuses will weigh as much as 50 gm, between birth and age 30 years
    • Occasional normal thymuses will weigh as much as 30 gm, between ages 30-60 years

Differential Diagnosis

Type B1 Thymoma True Thymic Hyperplasia
Dendritic epithelial cells scattered throughout broad lymphocyte rich area with only scattered medullary like foci Normal alternation of cortical and medullary areas
Medullary-like areas rarely contain Hassal corpuscles Medullary areas contain Hassal corpuscles
Large lobules separated by fibrous bands Small lobules surrounded by capsule and interstitium

 

Micronodular Thymoma with Lymphoid Stroma Thymic Lymphoid (Germinal Center) Hyperplasia
Small almost purely epithelial nodules with appearance of Type A thymoma alternating with B cell rich areas that frequently contain germinal centers Residual normal thymus or ribbons and nests of dendritic epithelial cells containing Hassall corpuscles
Other than the presence of germinal centers, these two do not look alike

Classification / Lists

Miscellaneous Thymic Lesions and Conditions

Bibliography

  • Shimosato Y, Mukai K, Matsuno Y. Tumors of the Mediastinum, Atlas of Tumor Pathology, AFIP Fourth Series, Fascicle 11, 2010
  • den Bakker MA, Oosterhuis JW. Tumours and tumour-like conditions of the thymus other than thymoma; a practical approach. Histopathology. 2009 Jan;54(1):69-89
  • Bratton AB. The normal weight of the human thymus. J Pathol Bacteriol 1925;28:609
  • Wekerle H. The thymus in myasthenia gravis. Ann N Y Acad Sci. 1993 Jun 21;681:47-55
  • Mori T, Nomori H, Ikeda K, Kobayashi H, Iwatani K, Kobayashi T. The distribution of parenchyma, follicles, and lymphocyte subsets in thymus of patients with myasthenia gravis, with special reference to remission after thymectomy. J Thorac Cardiovasc Surg 2007;133:364-8
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