thymoma type B1

Definition: Type B1 thymoma closely resembles normal thymus cytologically and architecturally. It is usually an encapsulated mass with fibrous bands (seen here) separating it into nodules or lobules. It has areas resembling normal thymic cortex which are composed of scattered epithelial cells in a dense background of immature T-cells. Foci of medullary differentiation (seen here as lighter areas) with or without Hassall’s corpuscles are always present.

Given the abundance of lymphocytes, it is also known as lymphocyte-rich thymoma or lymphocytic thymoma and the differential diagnosis includes T lymphoblastic lymphoma.

Images

 http://www.webpathology.com/image.asp?case=404&n;=1

Digital cases

 JRC:3190 : Mixed thymoma Thymoma, B1 (?B2)

Type B1 thymoma accounts for about 18% of all thymomas. It is seen most commonly in the 5th and 6th decades of life with a slight female predominance.

Two-thirds of patients are symptomatic and present with symptoms related to compression of anterior mediastinal structures (cough, chest pain, dyspnea) or autoimmune diseases (myasthenia gravis, pure red cell aplasia, hypogammaglobulinemia). Between 20% to 50% have myasthenia gravis.

Almost 90% of cases are either completely encapsulated grossly and microscopically (Stage I) or show only microscopic infiltration of the capsule (Stage II). Extension to adjacent mediastinal structures is rare.

The is a low power view of Type B1 thymoma showing a predominance of dark blue cortical areas and a few pale islands of medullary differentiation. The cortical areas are composed of scattered neoplastic epithelial cells in a background of a dense infiltrate of immature T-cells.

The paler medullary areas are less cellular and contain few or no immature T-cells but have increased numbers of B cells and mature T-cells, and may have myoid cells and Hassall corpuscles.

Type B1 thymomas usually show a lobular architecture with fibrous bands arising from the capsule dividing the tumor into lobules or nodules. The lobulation may sometimes be absent.

The tumor resembles normal thymus and shows a predominance of dark blue cortical areas and a few pale islands of medullary differentiation. The cortical areas are composed of scattered neoplastic epithelial cells in a background of a dense infiltrate of non-neoplastic immature T-cells. The presence of tingible-body macrophages surrounded by densely packed lymphocytes has created a starry-sky appearance in this image. The epithelial component is inconspicuous at this magnification.

Type B1 thymomas resemble normal non-involuted thymus and show a predominance of cortical areas composed mostly of densely-packed non-neoplastic immature T-cells and scattered pale areas of medullary differentiation. One such medullary focus is shown here.

The paler medullary areas are less cellular and contain few or no immature T-cells but have increased numbers of B cells and mature T-cells, and may have myoid cells and Hassall corpuscles.

The neoplastic epithelial cells in type B1 thymoma are scattered singly in a background of densely-packed non-neoplastic immature T-cells. They are difficult to appreciate at low magnification. If the epithelial cells are increased in number or form clusters (3 or more cells), the diagnosis of type B2 thymoma should be considered.

The epithelial cells have pale eosinophilic cytoplasm with ill-defined cytoplasmic borders, uniform round to oval vesicular nuclei, and variably-prominent nucleoli.

Type B1 thymoma is considered to a tumor of low-grade malignant potential since it is completely encapsulated in more than 50% of cases. Almost 90% of cases are considered completely resectable. The 5-yr and 10-yr survival rates are 95% and 90% respectively. Recurrences may occur in 10% to 15% of cases, but distant metastases are rare.

This high power view of a type B1 thymoma shows scattered neoplastic epithelial cells surrounded by a dense infiltrate of non-neoplastic lymphocytes most of which are TdT+ immature T-cells. The epithelial cells have pale eosinophilic cytoplasm with ill-defined cytoplasmic borders, uniform round to oval vesicular nuclei, and punctate nucleoli.

Immunochemistry

The epithelial component in type B1 thymomas is diffusely positive for CK19, p63, and PAX8. They are focally positive for CK7, CK8, CK14, and CK18. They are negative for CK20.

The cortical areas are composed predominantly of a dense infiltrate of immature T-cells which are TdT+, CD1a+, CD3+, CD4+, CD8+, and CD34-ve.

The medullary islands contain mature T-cells which are CD3+, TdT-ve, CD1a-ve, and either CD4+ or CD8+. There are also mature B-cells which express CD20 and CD79a. The epithelial cells in medullary islands are diffusely positive for CK19.

The image shows scattered neoplastic thymic epithelial cells surrounded by non-neoplastic lymphocytes. The epithelial cells have pale oval or round nuclei with inconspicuous nucleoli.

Differential diagnosis

 Type B1 thymoma vs Hyperplastic thymus

• Type B1 thymoma resembles normal non-involuted thymus but has larger lobules, thicker fibrous capsule, shows fibrous septa, and has a predominance of cortical over medullary areas. There are no or very few Hassall corpuscles in type B1 thymoma in comparison to the normal thymus.

 Type B1 Thymoma vs Type B2 Thymoma

• Type B1 thymoma has fewer neoplastic epithelial cells than type B2 thymoma and they are scattered singly. If their numbers appear increased or if they form clusters (3 or more contiguous cells), it supports the diagnosis of type B2 thymoma. Medullary islands with or without Hassall corpuscles are always seen in type B1 thymoma but are rare in type B2 thymoma.

 Type B1 Thymoma vs Type AB Thymoma

• Rare examples of type AB thymoma may show medullary foci in the lymphocyte-rich areas with TdT+ cells. However, type AB thymomas will always show (at least focally) spindled tumor cells, a greater proportion of keratin-positive tumor cells, and epithelial expression of CD20 in about 50% of cases.

 precursor T-cell acute lymphoblastic leukemia/lymphoblastic lymphoma

• Type B1 Thymoma vs T-Lymphoblastic Lymphoma (T-LBL): In T-LBL, the cortico-medullary architecture is effaced. The blasts of T-LBL are monomorphic, atypical, and frequently infiltrate into mediastinal fat. Necrosis is frequently present. Epithelial stains such as CK19 and p63 fail to reveal epithelial cell network in T-LBL. In contrast, type B1 thymomas will show a population of CK19+ and p63+ epithelial cells and the T-cells are not monomorphic and lack atypia.

The image shows scattered neoplastic thymic epithelial cells surrounded by non-neoplastic lymphocytes. The epithelial cells have pale oval or round nuclei with inconspicuous nucleoli.

Open References

 Flow cytometry in the differential diagnosis of lymphocyte-rich thymoma from precursor T-cell acute lymphoblastic leukemia/lymphoblastic lymphoma. Li S, Juco J, Mann KP, Holden JT. Am J Clin Pathol. 2004 Feb;121(2):268-74. PMID: 14983942 [Free]

See also

 thymomas

References

 T-cell lymphocytosis associated with lymphocyte-rich thymoma. Barton AD. Cancer. 1997 Oct 15;80(8):1409-17. Review.PMID: 9338464 [Free]

 Thymoma mimicking lymphoblastic lymphoma: a pitfall in fine-needle aspiration biopsy interpretation. Friedman HD, Hutchison RE, Kohman LJ, Powers CN. Diagn Cytopathol. 1996 Mar;14(2):165-9; discussion 169-71. PMID: 8964175