Type B1 Thymoma

Predominant cortical areas with interspersed smaller medullary foci
- Lacks lobularity of normal thymus
- Usually thick fibrous capsule and septa
Cortical areas closely resemble normal cortex: lymphocytes with inconspicuous epithelial cells
- Overwhelmingly lymphocyte rich ("impossible to count")
  - Densely packed
  - Small lymphocytes
  - Immature T phenotype
- Inconspicuous epithelial cells
  - Nuclei range from medium sized to large
  - Nucleoli range from inconspicuous to prominent
  - Long dendritic processes
    - May be visible only on keratin stains
  - Epithelial cell aggregates infrequent
- Tingible body macrophages may be scattered
Medullary foci stand out as indistinctly circumscribed round pale zones
- On close inspection, they do not resemble normal medulla
- Pale appearance due to loose packing of lymphocytes
  - Largely mature T phenotype
- Epithelial cells range from infrequent to occasional
- Hassall corpuscles range from absent to occasional

Epithelial cells
- Keratin positive
  - Usually more prominent and dendritic in cortical areas
- EMA negative
- CD20 negative
- CD5 negative
Lymphocytes
- Cortical areas: immature T phenotype (CD1a, CD99, TdT)
- Medullary foci: usually mature T phenotype
- Both types of T cells express most pan-T cell markers (CD3, CD5)
- Few B cells

Associations
- Myasthenia gravis
- Pure red cell aplasia
- Hypogammaglobulinemia
Usually presents at low stage (75% of cases)
- May be aggressive
Close to 90% long term survival

Type B1 Thymoma	Type B2 Thymoma
Predominantly lymphocytes with inconspicuous epithelial cells	Both lymphocytes and epithelial cell prominent
Numerous scattered pale medullary foci	Only occasional medullary foci

Type B1 Thymoma	Normal Thymus
Lacks regular thymic lobular structure	Lobular with regular pattern of superficial cortex and underlying medulla
Thick fibrous capsule and septa produce large lobules	Thin normal thymic capsule
Medullary foci typically have few epithelial cells other than Hassall corpuscles	Medulla may have many epithelial cells
Hassall corpuscles variably present in medullary foci	Hassall corpuscles present in medulla of almost all cases

Lymphocytes have same phenotype in both: predominantly immature T cells with mature T cells restricted to medullary foci and medulla

Cortical areas may have the same density of epithelial cells

May be a significant problem on small biopsies and in cases analyzed by flow

Type B1 Thymoma	Precursor T Lymphoblastic Leukemia / Lymphoma
Scattered epithelial cells (may require anti-keratin stain)	Only keratin positive cells should be overrun thymus
Scattered pale medullary foci	Lacks medullary foci
Thick fibrous capsule and septa produce large lobules	Lacks thick fibrous capsule and septa and lobular growth pattern
Typically older adult but may occur in children	Typically adolescent or pediatric
No circulating blasts	Circulating blasts
Variable expression of surface CD3, CD4, CD8 in a given case	Uniform phenotype in a given case

Lymphocytes have same basic phenotype in both: predominantly immature T cells with mature T cells restricted to medullary foci

May be a significant problem on small biopsies and in cases analyzed by flow

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

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