Micronodular Thymoma with Lymphoid Stroma

Thymoma composed of multiple epithelial nodules surrounded by prominent lymphoid stroma

Multiple small epithelial nodules resembling Type A thymoma
- Bland oval nuclei
  - Nucleoli inconspicuous or absent
- Few lymphocytes within epithelial nodules
- May form micro and macroscopic cysts
- Mitotic figures very rare
- No Hassall corpuscles or perivascular spaces
Abundant surrounding lymphocytic stroma
- Mostly B cells
  - Frequent prominent germinal centers
- Small numbers of immature T cells may be scattered in and narrowly surrounding the nodules
- No epithelial cells within lymphoid areas
Lacks dense fibrous septa and thick capsule
Monoclonal B cell populations have been reported in 33% of cases in one series
- Extranodal marginal zone B cell lymphoma reported in half of monoclonal cases
  - None of the patients developed systemic lymphoma
- Follicular lymphoma in one case

Epithelial cells
- Keratin positive
  - No epithelial cells in lymphoid areas
- CD20 negative
Lymphocytes
- Lymphoid stroma
  - Predominantly B cells
  - Scattered mature T cells
- Immature T cells variable
  - May be scattered in and surrounding the nodules
  - Absent in some cases

Type AB Thymoma	Micronodular Thymoma with Lymphoid Stroma
Lymphocyte rich areas contain scattered epithelial cells	Lymphocyte rich areas lack epithelial cells
Lymphocyte rich areas composed of T cells	Lymphocyte rich areas contain many B cells, frequently with germinal centers

Micronodular Thymoma with Lymphoid Stroma	Metastatic Carcinoma in a Lymph Node
Epithelial nodules usually scattered throughout	Carcinoma usually subcapsular, sinusoidal or focal
Cytologically very bland	Usually cytologically atypical
Mitotic figures very rare	Mitotic figures may be numerous
Lacks a subcapsular sinus	Subcapsular sinus present
Presence of adjacent nonneoplastic thymus supports a thymic neoplasm	Typically no adjacent thymus
Immature T cells may be present	Immature T cells absent

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Thomas De Montpréville V, Zemoura L, Dulmet E. [Thymoma with epithelial micronodules and lymphoid hyperplasia: six cases of a rare and equivocal subtype] Ann Pathol. 2002 Jun;22(3):177-82.