Primary Mediastinal (Thymic) Large B Cell Lymphoma

Differential Diagnosis

Nodular sclerosis Hodgkin lymphoma and mediastinal B cell lymphoma share features of age, location and sclerotic background and frequently have to be distinguished on the basis of small specimens
If the biopsy is large enough, the following features may be definitive for the diagnosis of NSHL
- Nodular architecture
- Polymorphous background
- Cytologic features of RS cells
Most specimens will need immunohistochemical study as below
- Note that in mediastinal B cell lymphoma, immunoglobulin staining is frequently negative, even in frozen tissue
- Note that Reed Sternberg cells are frequently positive for both kappa and lambda, apparently due to nonspecific uptake of serum immunoglobulin.

Sclerosis usually confined to thick bands and capsule
Thymic epithelial cells should be numerous
- Keratin stain may be useful to highlight epithelial cells
  - However, we have occasionally found thymoma epithelial cells to stain only variably with common anti-keratin antibodies
- Overrun or even hyperplastic thymic epithelial cells may be seen in lymphoma
- Even distribution is more typical of thymoma
Immature T cells are numerous in most thymomas
- CD1a, CD99 and TdT positive
CD20 may stain epithelial cells in Type A and Type AB thymomas
- Positive cells are spindled
- Keratin stains and other B lineage markers should resolve the diagnosis
Necrosis may be widespread in both thymoma and lymphoma

Keratin and CD20 should resolve the differential if lesional cells are present in specimen

Sclerosing mediastinitis lacks the population of large atypical B cells of mediastinal B cell lymphoma

Embryonal carcinoma and yolk sac tumor should be clearly keratin positive
- AFP and HCG may be positive, but frequently negative
Seminoma is frequently negative for keratin
- Look closely for punctate keratin staining
- PLAP is variably positive in seminoma, uniformly negative in lymphoma
- HCG may be positive if trophoblastic giant cells are present
Serum HCG or AFP may be positive in the absence of tissue section reactivity in germ cell tumors
B lineage markers and LCA should be negative in all germ cell tumors
Both embryonal carcinoma and mediastinal B cell lymphoma may be CD30 positive
One case of mediastinal lymphoma with HCG expression has been reported
Necrosis may be widespread in both germ cell tumors and lymphoma