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

• Type B1 thymoma: the epithelial cells are less prominent but there are prominent medullary foci.

• Type B3 thymoma: confluent sheets of epithelial cells are common with lesser numbers of lymphocytes. Epithelial nuclei are smaller and irregular with grooved nuclei.

• T-lymphoblastic lymphoma: lacks an epithelial component. There is strong CDK6 expression.

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