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.