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

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.

5 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