Microscopic Thymoma / Nodular Hyperplasia of Thymic Epithelium

Definition

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

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.

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