Type B1 thymoma

Definition

A thymoma predominantly resembling the normal thymic cortex, with areas of thymic medulla.

Synonyms

Lymphocytic thymoma, lymphocyte-rich thymoma, organoid thymoma, predominantly cortical thymoma.

Epidemiology

This type is relatively uncommon, accounting for up to 5-10% of all thymomas¹. Cases in children have been reported¹.

Clinical features

Myasthenia gravis is common⁰. Less often, there is red cell aplasia or hypogammaglobulinaemia⁰. Rarely, this type may occur in the pleura, neck or lung⁰.

Macroscopic appearances

The tumour is usually well demarcated or encapsulated. The capsule is often thick, with fibrous septa dividing the tumour. There may be cystic spaces, haemorrhage or necrosis. The Masaoka stage varies: stage I; 54%, stage II; 32%, stage III; 11%, stage IV; <3%².

Histopathology

At high power, the histology may be indistinguishable from that of the normal thymic cortex². The features that distinguish type B1 thymoma from normal thymus are the excess of cortical areas, a lesser degree of lobulation, medullary areas in immediate proximity to septa and a fibrous capsule². These thymomas are the most lymphocyte-rich. The scant epithelial cells are dispersed and small: they are surrounded by numerous T-cells. Tingible body macrophages may give a starry sky appearance.

The foci of medullary differentiation are a consistent feature: they are round and appear lighter. They may contain central aggregates of squamoid cells or well-formed Hassall corpuscles.

Perivascular spaces may be present but are not as well-formed as in types B2 and B3 thymomas.

Immunohistochemistry

	Epithelial cells
CD19	diffuse0
CK7	focal positivity0
CK14	focal positivity0
CK18	focal positivity0
CK20	negative0
CD5	negative0
CD20	negative0
CD70	negative0
PE-35	positive3
ERa	H score = 96±144
PR-B	H score = 51±124

		Admixed cortical lymphocytes	Lymphocytes in medullary islands
	CD1a	positive0	negative0
	CD3		positive0
	CD4	positive0
	CD5	positive0	positive0
	CD8	positive0
	CD99	positive0	negative0
	TdT	positive0	negative0

Differential diagnosis

• Type B2 thymoma: has a higher proportion of epithelial cells, which are larger with vesicular nuclei.

• Lymphoma

Management

Complete resection is possible in over 90% of cases, with a 10% recurrence rate.

Prognosis

The prognosis is good, with survival of 80-100% to ten years².

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