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
	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.

This page last revised 4.1.2006.

©SMUHT/PW Bishop