Surgical Pathology Criteria

Thymic Hyperplasia

Differential Diagnosis

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


Micronodular Thymoma with Lymphoid Stroma Thymic Lymphoid (Germinal Center) Hyperplasia
Small almost purely epithelial nodules with appearance of Type A thymoma alternating with B cell rich areas that frequently contain germinal centers Residual normal thymus or ribbons and nests of dendritic epithelial cells containing Hassall corpuscles
Other than the presence of germinal centers, these two do not look alike

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