Thymoma, types A and B

Types A and B thymomas are epithelial neoplasms showing thymus-like (organotypic) differentiation. The number of non-neoplastic lymphocytes is variable.

They may occur at any age, with a peak incidence at 55-65 years. Cases in the first two decades of life are extremely rare. There is an association of thymomas with the MEN1 syndrome.

Classification

Various histological classifications of thymoma have been proposed:

 

WHO0

Muller-Hermelink

Kuo3

  

Type A

Medullary

Spindle cell

Type AB

Mixed

Small polygonal cell

Mixed

Type B1

Predominantly cortical

Organoid

Type B2

Cortical

Large polygonal cell

Type B3

Well-differentiated thymic carcinoma

Squamoid
     

 

Prevalences of subtypes0,4

 

Type A

5-10%

  

Type AB

20-35%

Type B1

5-10%

Type B2

20-35%

Type B3

10-25%

Type C

10-25%

Rare types

2.5%4
   

 

Correlation of type with stage7

Type

I

II

III

IVA

IVB

Invasive

subsequent recurrence

20 year survival

  

A

7

 

1

 

 

13%, 11%8

0% (0/8)

100%8

  

AB

27

15

1

 

1

39%, 42%8

2% (1/44)

87%

  

B1

15

5

4

1

 

40%, 47%8

9% (2/25)

91%

  

B2

11

10

12

2

1

69%, 69%8

19% (7/36)

59%8

  

B3

2

2

6

 

 

80%, 85%8

20% (2/10)

36%8

  

C

 

1

8

 

5

100%

-

 

  

Incidence of myasthenia gravis7

Type

Incidence of myasthenia gravis

  

A

24%0

  

AB

14%0

  

B1

18%-56%0

  

B2

30%-82%0

  

B3

30%-70%0

  

C

0%0

  

 

Cytogenetic abnormalities:

 

 

chromosomal gains

chromosomal losses

  
 

Type A

 

homogenous -6p6

  

Type AB

 

-5q21-22, -6q, -12p, -16q

Type B1

 

 

Type B2

 

aberrations of multiple chromosomes6

Type B3

+1q

aberrations of multiple chromosomes6, including -6, -13q

Thymic squamous cell carcinoma

+1q, +17q, +18

aberrations of multiple chromosomes6, including -3p, -6, -13q, -16q, -17p,
     

Immunohistochemistry

In type B thymomas, the T-lymphocytes are immature, as evidenced by their positivity for CD1a and CD99. Positivity for CD79a may also be common1.

The cytokeratin staining pattern is distinctively dendritic in thymomas but not in thymic carcinomas9.

CD205 and Foxn1 have been proposed as specific markers for thymoma (benign and malignant), and CD5 and CD117 as markers for thymic carcinomas9:

 

 

CD205

 

Foxn1

 

CD5

 

CD117

  

0

+1

+2

+3

+4

0

+1

+2

+3

+4

0

+1

+2

+3

+4

0

+1

+2

+3

+4

Thymoma type

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A

1/9

2/9

 

 

6/9

1/9

 

 

4/9

4/9

8/9

1/9

 

 

 

9/9

 

 

 

 

AB: component A

1/18

8/18

3/18

2/18

4/18

0/18

1/18

3/18

6/18

8/18

16/16

 

 

 

 

16/16

 

 

 

 

AB: component B

 

 

1/19

11/19

7/19

 

 

1/19

3/19

15/19

18/19

1/19

 

 

 

19/19

 

 

 

 

B1

 

 

 

1/7

6/7

 

 

 

 

7/7

7/7

 

 

 

 

7/7

 

 

 

 

B2

 

 

1/16

3/16

12/16

 

 

2/16

4/16

10/16

15/15

 

 

 

 

15/15

 

 

 

 

B3

 

1/7

1/7

1/7

4/7

 

 

 

1/7

6/7

5/7

2/7

 

 

 

7/7

 

 

 

 

C

7/17

4/17

3/17

2/17

1/17

4/17

7/17

2/17

2/17

2/17

5/16

2/16

 

4/16

5/16

6/17

1/17

1/17

4/17

5/17

Neuroendocrine carcinoma

3/3

 

 

 

 

3/3

 

 

 

 

3/3

 

 

 

 

3/3

 

 

 

 
                                         

Steroid receptor positivity for oestrogen receptor ERa and progesterone receptor PR-B has been demonstrated in the epithelial cells of thymomas.

 

 

cases positive

H score

  
 

ERa

87/1322

86±802

  
 

ERb

9/1322

7±92
 

PR-A

5/1322

3±52
 
 

PR-B

65/1322

56±682
 

Androgen receptor

20/1322

14±122
       
       

There is a progressive reduction in immunoreactivity for both ERa and PR-B from Type A, through types AB, B1 and B2 to Type B32.

 

Survival

Histological type correlates with stage, but is also an independent predictor of survival for stage I and II tumours4,8.

 

 

5 years

    
 

Type A

100%4

    

Type AB

100%4

Type B1

94%4

Type B2

75%4

Type B3

70%4

Type C

48%4
   

Smaller studies have perhaps lacked the power to show differences in survival other than between thymoma (types A and B) and thymic carcinoma (type C)5.

References

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

1 Hashimoto, M., Y. Yamashita, et al. (2002). "Immunohistochemical detection of CD79a expression in precursor T cell lymphoblastic lymphoma/leukaemias." J Pathol 197(3): 341-7.

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

3 Kuo, T. (2000). "Cytokeratin profiles of the thymus and thymomas: histogenetic correlations and proposal for a histological classification of thymomas." Histopathology 36(5): 403-14.

4 Chen G, Marx A, Wen-Hu C, et al. New WHO histologic classification predicts prognosis of thymic epithelial tumors: a clinicopathologic study of 200 thymoma cases from China. Cancer 2002; 95:420-9

5 Chalabreysse L, Roy P, Cordier JF, et al. Correlation of the WHO schema for the classification of thymic epithelial neoplasms with prognosis: a retrospective study of 90 tumors. Am J Surg Pathol 2002; 26:1605-11

6 Inoue M, Starostik P, Zettl A, et al. Correlating genetic aberrations with World Health Organization-defined histology and stage across the spectrum of thymomas. Cancer Res 2003; 63:3708-15 FULL TEXT

7 Okumura M, Miyoshi S, Fujii Y, et al. Clinical and functional significance of WHO classification on human thymic epithelial neoplasms: a study of 146 consecutive tumors. Am J Surg Pathol 2001; 25:103-10

8 Okumura M, Ohta M, Tateyama H, et al. The World Health Organization histologic classification system reflects the oncologic behavior of thymoma: a clinical study of 273 patients. Cancer 2002; 94:624-32

9 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 11.11.2007.

©SMUHT/PW Bishop