Type B2 thymoma

Definition

A thymoma with scattered plump epithelial cells with vesicular nuclei within a heavy background of lymphocytes.

Synonyms

(Large) polygonal cell thymoma, mixed lymphocytic and epithelial thymoma, cortical thymoma2.

Epidemiology

Type B2 accounts for 20-35% of all thymomas1. Cases in children have been reported1.

Clinical features

Myasthenia gravis is common. Rarely, there is red cell aplasia or hypogammaglobulinaemia. There are reported cases with widespread pleural involvement.

Radiology

Macroscopic appearances

The tumours are encapsulated or circumscribed. There are white fibrous septa. Cystic change or haemorrhage may occur. The Masaoka stage varies: stage I; 21%, stage II; 28%, stage III; 40%, stage IV; 11%2.

Histopathology

There is a "mixed" pattern of epithelial cells and lymphocytes. Lobules are usually large, with thin septa.

Epithelial cells are more numerous than in type B1 thymomas and are larger, with vesicular nuclei and conspicuous nucleoli. These cells are round or polygonal. They may form tight clusters2 but large confluent sheets are uncommon. Medullary foci are less prominent than in type B1, but occur and may rarely contain Hassall corpuscles.

Perivascular spaces are common. Epithelial cells often form a palisade around these spaces.

The lymphocytes may appear "immature" with enlarged nuclei, increased cytoplasm and mitotic activity.

Up to 30% of cases include a B3 component, in which case classification should be as a combined B2/B3 thymoma.

The presence of lymphoid follicles is associated with myasthenia gravis. Immunosuppressive treatment causes necrosis, lymphocyte depletion and infiltration by histiocytes, which may be lipidised.

Immunohistochemistry

 

Epithelial cells

 

AE1/AE3

positive0

   

Cam5.2

positive0

   

CK5/6

90% positive0

   

CK7

80% positive0

   

CK19

100% positive0

   

CK20

negative0

   

EMA

negative0

   

CD5

negative0

   

CD20

negative4

   

CD57

positive0

   

CD70

negative0

   

PE-35

largely negative3

   

ERa

H score = 65±135

   

PR-B

H score = 45±105

   
       
       

 

Intraepithelial lymphocytes

 

CD1a

positive0

CD3

 

CD4

positive0

CD5

positive0

CD8

positive0

CD99

positive0

TdT

positive0

Ki-67

>80%2

   

Ultrastructure

Cytogenetics

Most cases are aneuploid.

Differential diagnosis

Management

5-15% of cases are not resectable.

Prognosis

After resection, up to 10% of cases recur and 10% of cases show metastases. The ten year survival is intermediate between that of B1 and B3 thymomas, at between 50 and 100%2.

References

0 Tumours of the Lung, Pleura, Thymus and Heart. WHO Classification of Tumours. IARC Press 2004.

1 J Rosai et al. Histological typing of tumours of the thymus. WHO International histological classification of tumours. Springer-Verlag, second edition, 1999.

2 Muller-Hermelink, H. K. and A. Marx (1999). "Pathological aspects of malignant and benign thymic disorders." Ann Med 31 Suppl 2: 5-14.

3 Hattori, H., H. Tateyama, et al. (2000). "PE-35-related antigen expression and CD1a-positive lymphocytes in thymoma subtypes based on Muller-Hermelink classification.An immunohistochemical study using catalyzed signal amplification." Virchows Arch 436(1): 20-7.

4 Pan, C. C., W. Y. Chen, et al. (2001). "Spindle cell and mixed spindle/lymphocytic thymomas: an integrated clinicopathologic and immunohistochemical study of 81 cases." Am J Surg Pathol 25(1): 111-20.

5 Ishibashi, H., T. Suzuki, et al. (2003). "Sex steroid hormone receptors in human thymoma." J Clin Endocrinol Metab 88(5): 2309-17.

 

This page last revised 4.1.2006.

©SMUHT/PW Bishop