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 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
Original posting/updates: 9/24/10
Normal Thymic 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
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
