thymoma type B2

Definition: Type B2 thymoma is composed of clusters of neoplastic epithelial cells in a background of abundant immature T-cells. The epithelial cell density is higher than that seen in type B1 thymoma or the normal thymus. Foci of medullary differentiation may or may not be seen. It makes up 25% to 30% of all thymomas.

This image shows thick fibrous septa dividing the tumor into lobules.

Type B2 thymoma is usually seen in adults in their 5th or 6th decade of life. Almost 55% of patients present with myasthenia gravis (muscle weakness, fatigue, diplopia, drooping eyelids, and dysphagia). Others have symptoms related to compression of anterior mediastinal structures (dyspnea, cough, chest pain) and fever, night sweats, and weight loss. Some are asymptomatic. About 5% of patients have pure red cell aplasia, hypogammaglobulinemia or other autoimmune disorders.

Type B2 thymomas are more likely to invade beyond capsule into the surrounding fat or mediastinal structures as compared to thymomas type A, type AB, or type B1. Only about 32% of type B2 thymomas present in Stage I (completely encapsulated grossly and microscopically) as compared to 50% of cases for type B1 thymoma.

This low magnification view shows fibrous septa dividing the tumor into irregularly shaped lobules.

Digital cases

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

Images

 Thymoma type B2 (Webpathology)

Microscopy

At low power, type B2 thymoma has lobular architecture with delicate or thick collagenous bands dividing it into irregularly shaped lobules.

The tumor appears basophilic due to abundance of lymphocytes which are predominantly immature T-cells.

Within this rich lymphocytic infiltrate, there are polygonal epithelial cells scattered singly or in small clusters. The epithelial cell density is higher than that seen in type B1 thymoma or the normal thymus.

Scattered neoplastic epithelial cells are seen in a background of lymphocytes. The tumor cells have vesicular nuclei with prominent nucleoli. The tumor cells are arranged in loose network, or palisading around fibrous septa or vascular spaces. Large confluent solid sheets of tumor cells are not seen.

Type B2 thymomas show polygonal epithelial cells scattered singly or in small clusters in a lymphocyte-rich background. The epithelial density is higher than that seen in type B1 thymoma or normal thymus.

The epithelial cells have round or oval vesicular nuclei with punctate nucleoli. Anaplastic features may be focally present.

Type B2 thymomas contain perivascular spaces consisting of a central venule surrounded by a clear space which contains proteinaceous fluid and lymphocytes.

Hassall corpuscles are sometimes present. Medullary foci are uncommon.

In patients who present with myasthenia gravis, lymphoid follicles may be present in the fibrous septa.

Preoperative corticosteroid therapy may alter the appearance due to lymphocyte depletion, histiocytic infiltrates, and necrosis.

Predominantly epithelial (and lymphocyte-poor) areas resembling type B3 thymoma are seen in almost 40% of cases of type B2 thymoma.

Differential diagnosis

 Type B2 Thymoma vs Type B1 Thymoma

• Type B2 thymoma has greater epithelial cell density and epithelial cell clusters (as shown here) whereas the epithelial cells are fewer in number and scattered singly in type B1 thymoma.
• The presence of perivascular spaces is more typical of type B2 than type B1 thymoma; on the other hand, foci of medullary differentiation and Hassall corpuscles are more often seen in type B1 than type B2 thymomas.

 Type B2 Thymoma vs Type B3 Thymoma

• Type B2 thymoma appears basophilic at low magnification due to its lymphocyte-rich background containing small clusters of epithelial cells.
• Type B3 thymoma is epithelial-predominant and lymphocyte-poor and therefore appears pink at low magnification.

 Type B2 thymoma vs Thymic Carcinomas

• Type B2 thymomas may show focal anaplasia; however, they can be distinguished from thymic carcinomas by lobular architecture, perivascular spaces, TdT+ cells, and the absence of CD5/CD117 immunoreactivity.

 Type B2 thymoma vs T-Lymphoblastic Lymphoma (T-LBL)

• 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 B2 thymomas will show a population of CK19+ and p63+ epithelial cells and the T-cells are not monomorphic and lack atypia.

Immunochemistry

Type B2 thymomas show a prominent epithelial network with CK19, p63, and PAX8. There is focal positivity for CK7, CK8, CK14, and CK18.

The epithelial component is usually negative for CK20.

The background lymphocytic infiltrate is composed predominantly of immature T-cells which are TdT+, CD1a+, CD3+, CD4+, and CD8+.

This high power view shows cytologic features of tumor cells in a Type B2 thymoma.

The tumor cells are greatly out-numbered by non-neoplastic lymphocytes.

The biologic behavior of Type B2 thymoma is intermediate between B1 and B3 thymomas.

It is often invasive at the time of diagnosis and has higher rate of recurrences and metastases than B1 thymomas.

Synopsis

The tumor is usually located in the anterior mediastinum and myasthenia gravis is the most frequent manifestation. Type B2 thymoma accounts for between 20% to 40% of all thymomas.

Type B2 thymoma is composed of large polygonal tumor cells with vesicular nuclei in a background population of immature T lymphocytes.

There are scattered tumor cells in a background of lymphocytes. The tumor cells have vesicular nuclei with prominent nucleoli. The tumor cells are arranged in loose network, or palisading around fibrous septa or vascular spaces. Large confluent solid sheets of tumor cells are not seen.

High power view showing cytologic features of tumor cells in a Type B2 thymoma. The tumor cells are greatly out-numbered by non-neoplastic lymphocytes.

The biologic behavior of Type B2 thymoma is intermediate between B1 and B3 thymomas. It is often invasive at the time of diagnosis and has higher rate of recurrences and metastases than B1 thymomas.

Perivascular space in a type B2 thymoma. The loose connective tissue in the space contains lymphocytes and a few epithelial tumor cells.

Staging

Type B2 thymomas are more likely to invade beyond capsule into the surrounding fat (as shown here) or mediastinal structures as compared to thymomas type A, type AB, or type B1.

Only about 32% of type B2 thymomas present in Stage I as compared to 50% of cases for type B1 thymoma.

Thymomas are staged according to the Masaoka-Koga Classification System which combines histologic criteria with elements of the TNM classification.

 Stage I: Completely encapsulated grossly and microscopically.
Stage II: Microscopic capsular invasion (Stage IIa) OR macroscopic invasion into thymic or surrounding adipose tissue, or gross adhesions to but no invasion of pleura or pericardium (Stage IIb).
 Stage III: Macroscopic invasion into surrounding structures (lung, pericardium, great vessels).
 Stage IV: Pleural or pericardial metastases (Stage IVa) OR distant metastases via lymphoid or hematogenous route (Stage IVb).

Management

Type B2 thymomas are treated by complete resection.

The biologic behavior of Type B2 thymoma is intermediate between B1 and B3 thymomas.

It is often invasive at the time of diagnosis and has higher rate of recurrences and metastases than type A (spindle cell type), type AB (mixed type), and type B1 (lymphocyte-rich) thymomas.

Prognosis

The 10-yr and 20-yr survival rates are 70-90% and 60-80% respectively.

The recurrence rates for Stage I-II tumors and Stage III tumors at 10-yrs are 13% and 41% respectively.

See also

 thymomas

• thymoma type B1
• thymoma type B3