Type AB thymoma

Definition

There is a mixture of type A thymoma and type B-like thymoma.

Synonyms

Mixed thymoma (in the sense of mixed medullary and cortical2).

Epidemiology

Type AB accounts for 15% to 43% of all thymomas1. Patients have an average age of 55 years.

Clinical features

15% of type AB thymomas are associated with myasthenia gravis. Pure red cell aplasia may occur.

Macroscopic appearances

Most type AB thymomas are Masaoka stage I2. The tumours are usually encapsulated. White fibrous bands separate tumour nodules.

Histopathology

The two components vary in their proportions2 and the segregation may be sharp4 or ill-defined2. The type A areas may be very scanty2 and may be misinterpreted as hypercellular fibrous septa1. If intermixed, the type A areas may be composed of extremely elongated fibroblast-like cells1. The type A areas show all the features usual in type A thymoma2. The type B component consists of small polygonal cells with small pale round/oval/spindle nuclei lacking conspicuous nucleoli; in this it differs from other type B thymomas. The number of lymphocytes in the type B areas is variable, often less than in type B1 thymomas. Medullary differentiation is rare and Hassall's corpuscles are not seen.

Immunohistochemistry

Epithelial cells:

 

AE1/3

29/294,6

 
 

34bE12

29/294

 
 

Cam5.2

29/294

 
 

CK14

positive in type B areas1

 
 

KL-1

29/294

 
 

EMA

strongly positive in elongated fibroblast like cells of A areas1

 
 

vimentin

strongly positive in elongated fibroblast like cells of A areas1

 
 

CD5

negative1, 0/294

 
 

CD20

positive in both A and B areas1, 26/294

 
 

CD57

variable/weak1, 79% of cases4

 
 

PE-35

positive3

 
 

bcl-2

variable/weak1

 
 

laminin

strong in A areas, less in B areas1

 
 

Type IV collagen

strong in A areas, less in B areas1

 
 

ERa

H score = 105±165

 
 

PR-B

H score = 65±145

 
 

p53

very low expression1

 
 

Ki67

very low expression1

 
       
       
       
       
       

 

 

Lymphocytes in both A and B areas

 

CD3

positive1

CD5

positive1

CD1a

variable proportion of T-cells1

CD99

variable proportion of T-cells1

CD20

usually absent1

   
   

Ultrastructure

Cytogenetics

Deletions of chromosome 6 or formation of ring chromosome 61. Some show the loss of heterozygocity at 4q21-22 seen in type B thymomas0.

Differential diagnosis

Management

Radical surgery is usually curative.

Prognosis

The prognosis is good, with survival of 80-100% to ten years2. Radical surgery is usually curative. Metastases are very rare.

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.

6 Nonaka D, Henley JD, Chiriboga L, et al. Diagnostic utility of thymic epithelial markers CD205 (DEC205) and Foxn1 in thymic epithelial neoplasms. Am J Surg Pathol 2007; 31:1038-44

This page last revised 4.1.2006.

©SMUHT/PW Bishop