Surgical Pathology Criteria

Type B1 Thymoma

Definition

Alternate / Historical Names

Diagnostic Criteria

Supplemental Studies

Clinical

Differential Diagnosis

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

    Bibliography

     

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