Microscopic Thymoma / Nodular Hyperplasia of Thymic Epithelium

Definition

• Microscopic nodular aggregates of bland thymic epithelial cells within an otherwise non-neoplastic thymus

Alternate/Historical Names

• Originally named microscopic thymoma
  • Nodular hyperplasia of thymic epithelial cells is probably a more accurate term
    • Thymic tumorlet has also been proposed
  • Microthymoma refers to a small tumor with features of any of the usual types of thymoma (Cheuk 2005)
    • Fibrous capsule, perivascular spaces, pattern of B1 etc.

Diagnostic Criteria

• This is a microscopic finding
  • No tumor mass
  • Thymus is grossly normal for age or may be enlarged by lymphoid hyperplasia especially if associated with myasthenia gravis
  • Size <1 mm has been stated in the literature
    • No clear justification for this cutoff
    • We have seen groups of confluent nodules that exceed this limit
• Nodules of pure epithelial cells
  • May be cortical type (large, round, vesicular with prominent nucleolus)
  • Frequently appear to be medullary type (small, oval to spindled, small nucleolus)
  • May be smoothly circumscribed or appear as an irregular aggregate of smaller nests
    • Nests may have the appearance of small solid acini or cords
      • Rosette-like structures may be present
  • Few, if any, lymphocytes within nodules
• May be solitary or multiple
  • Occasionally confluent groups
• May be confined to the thymus or may extend out into the fat
• High molecular weight keratin and p63 positive, negative for chromogranin and synaptophysin
• Lacks classic features of thymomas
  • No capsule
  • No admixed lymphocyte population
  • No perivascular spaces, medullary foci
• Benign, no evidence that they are related to or a precursor of thymoma

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

Differential Diagnosis

• Thymic (Lymphoid) Hyperplasia and True Thymic Hyperplasia bear no resemblance to Nodular Hyperplasia of Thymic Epithelium / Microscopic Thymoma
  • The first two are characterized by an increase in lymphocytes while the last is purely epithelial
• Neuroendocrine carcinoma may need to be ruled out with IPOX

Classification / Lists

Miscellaneous Thymic Lesions and Conditions

• Atrophy
• Cysts
  • Most are non-specific
  • Neoplasms most commonly associated with cysts
    • Thymoma
    • Seminoma
    • Hodgkin lymphoma
    • Extranodal marginal zone lymphoma
    • Basaloid carcinoma
• Castleman disease
• Dysplasia
• Hyperplasia
  • B cell/lymphoid/germinal center
  • Rebound
  • True thymic
• Microscopic thymoma / nodular hyperplasia of thymic epithelium
• (Ectopic hamartomatous thymoma)

Bibliography

• Shimosato Y, Mukai K, Matsuno Y. Tumors of the Mediastinum, Atlas of Tumor Pathology, AFIP Fourth Series, Fascicle 11, 2010
  
• Vaideeswar P. Microscopic thymoma: a report of four cases with review of literature. Indian J Pathol Microbiol. 2011 Jul-Sep;54(3):539-41.
  
• Cornea R, Lazăr E, Dema A, Herman D. A nodular hyperplasia of the thymic epithelium (so-called microscopic thymoma). Rom J Morphol Embryol. 2009;50(4):729-31.
  
• Chalabreysse L, Orsini A, Vial C, Tronc F. Microscopic thymoma. Interact Cardiovasc Thorac Surg. 2007 Feb;6(1):133-5.
  
• Puglisi F, Finato N, Mariuzzi L, Marchini C, Floretti G, Beltrami CA. Microscopic thymoma and myasthenia gravis. J Clin Pathol. 1995 Jul;48(7):682-3. Erratum in: J Clin Pathol 1995 Sep;48(9):884..
  
• Pescarmona E, Rosati S, Pisacane A, Rendina EA, Venuta F, Baroni CD. Microscopic thymoma: histological evidence of multifocal cortical and medullary origin. Histopathology. 1992 Mar;20(3):263-6.
• Cheuk W, Tsang WY, Chan JK. Microthymoma: definition of the entity and distinction from nodular hyperplasia of the thymic epithelium (so-called microscopic thymoma). Am J Surg Pathol. 2005 Mar;29(3):415-9.