Primary Mediastinal (Thymic) Large B Cell Lymphoma

Definition

Primary site of involvement must be mediastinum
- Other nodal and extranodal sites may be involved
Diffuse infiltration and sclerosis frequent
- Frequent diffuse infiltration of mediastinal soft tissue and surrounding structures
  - Overrun thymus may be identifiable
  - Thymic epithelium may be hyperplastic or cystic
- Sclerosis, necrosis and large vessel vascular invasion frequent
  - Primarily when extrathymic or extranodal
Wide range of nuclear features
- May be even more variable than usual diffuse large B cell lymphoma
- Cytoplasm variable but abundant and clear (retraction artifact) in about 40% of cases
- Cytology does not correlate with behavior
B lineage required
- Immunoglobulin frequently negative, even in frozen tissue or by flow, although immunoglobulin genes are rearranged

John P Higgins MD
Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342