Type B3 thymoma

Definition

An organotypic thymoma composed of round or polygonal epithelial cells with limited atypia, admixed with relatively few lymphocytes.

Synonyms

Epithelial thymoma, atypical thymoma, well-differentiated thymic carcinoma2, squamoid thymoma.

Epidemiology

Type B3 accounts for 7-25% of thymomas.

Clinical features

Myasthenia gravis is common. Red cell aplasia, hypogammaglobulinaemia, other autoimmune conditions and SVC obstruction are rare presentations. Regression may result in cyst formation or calcification.

Radiology

Macroscopic appearances

These are invasive tumours and are usually not encapsulated. The tumour is divided by white fibrous septa. Masaoka stage varies: stage I; 0% stage II; 16%, stage III; 59%, stage IV; 25%2.

Histopathology

The epithelial cells are round or polygonal, smaller than those of type B2 thymoma with less prominent nucleoli, and form sheets. They vary from small with inconspicuous nucleoli to large with nuclei and nucleoli, similar to those of type B2 thymoma. The epithelial cells may form squamoid foci but intercellular bridges are not apparent. Focal keratinisation may mimic Hassall's corpuscles. Perivascular spaces, with palisading of epithelial cells, occur. Lymphocytes are relatively sparse. The presence of lymphoid follicles is associated with myasthenia gravis.

Steroid treatment may produce regression with the accumulation of foam cells.

Variants:

Immunohistochemistry

 

 

 

Epithelial cells

 
 

AE1/AE3

positive0

 
 

EMA

focally positive0

 
 

CK5/6

positive0

 
 

CK7

positive0

 
 

CK8

positive0

 
 

CK10

positive0

 
 

CK19

positive0

 
 

CK20

negative0

 
 

CD5

negative0

 
 

CD20

negative0

 
 

CD57

positive0

 
 

CD70

negative0

 
 

TTF-1

negative0

 
 

PE-35

4/6 focally positive3

 
 

ERa

H score = 37±74

 
 

PR-B

H score = 18±104

 
       

 

Lymphocytes

 

CD1a

positive0,2

CD3

positive

CD4

positive0

CD5

positive0

CD8

positive0

CD99

positive2

TdT

positive0

   

Ultrastructure

Cytogenetics

There are gains of chromosome 1q and losses of chromosomes 6 and 13q1. Almost all cases are aneuploid.

 

Differential diagnosis

Management

The tumour is often not resectable at presentation.

Prognosis

Local recurrence occurs in about 15% of cases and metastases in up to 20% of cases. The prognosis is the worst among the organotypic thymomas, with survival of 80% at 5 years and 40-70% at 10 years0,2. Metastases are commonly to lung, liver, bone and soft tissues.

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 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