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Surgical Pathology Criteria
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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

Original posting/updates: 2/22/12

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
Micronodular Thymoma with Lymphoid Stroma Microscopic Thymoma / Nodular Hyperplasia of Thymic Epithelium
Gross tumor mass Incidental microscopic finding
Epithelial nodules surrounded by lymphocytes Small epithelial nodule(s) surrounded by normal thymus or fat

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
  • 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.
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