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