Type B2 thymoma

by Jason Wasserman MD PhD FRCPC
September 19, 2024

Type B2 thymoma is a rare tumour that starts in the thymus, a small organ located in the chest. The thymus plays an important role in the immune system by producing T cells. In this type of tumour, there is a mix of lymphocytes (a type of white blood cell) and epithelial cells, with more prominent and abnormal-looking epithelial cells compared to type B1.

Why is it called type B2?

Thymomas are classified into types A, AB, B1, B2, and B3 based on the ratio of lymphocytes to epithelial cells and the appearance of the epithelial cells. Type B2 thymoma has many lymphocytes, but the epithelial cells are more numerous and have a more abnormal appearance than in type B1 thymomas.

What are the symptoms of type B2 thymoma?

Many patients with type B2 thymoma have no symptoms, and the tumour is often found incidentally. Symptoms may include chest pain, shortness of breath, cough, or difficulty swallowing caused by the tumour pressing on nearby organs. Autoimmune diseases, such as myasthenia gravis, are also commonly associated with type B2 thymomas.

What causes type B2 thymoma?

The exact cause of type B2 thymoma is unknown, but it is thought to arise from the epithelial cells of the thymus, which support the development of lymphocytes. Genetic mutations have been identified in some thymomas, but no single mutation is known to cause this tumour.

What medical conditions are associated with type B2 thymoma?

Type B2 thymoma is commonly associated with autoimmune conditions, particularly myasthenia gravis. Myasthenia gravis affects the communication between nerves and muscles, leading to muscle weakness. Other autoimmune conditions, such as hypogammaglobulinemia and pure red cell aplasia, can also occur in patients with this tumour.

How is this diagnosis made?

Type B2 thymoma is diagnosed by examining a tumour sample under a microscope. Pathologists look for a mix of lymphocytes and epithelial cells. The epithelial cells in this tumour appear more atypical (abnormal) and are more prominent than in type B1 thymomas. Imaging studies, such as CT scans or MRIs, can help determine the size and location of the tumour and guide a biopsy.

What are the microscopic features of type B2 thymoma?

Microscopically, type B2 thymoma shows more abundant and atypical epithelial cells than type B1 thymoma. The epithelial cells are often present in groups or clusters. Although the tumour contains many lymphocytes, the prominent and abnormal appearance of the epithelial cells is a key feature. Immunohistochemical staining may be used to confirm the presence of markers such as CD3 (for lymphocytes) and cytokeratin (for epithelial cells).

Is a type B2 thymoma benign or malignant?

Type B2 thymoma is considered a low-grade malignant tumour. While it grows slowly and has a low risk of spreading to distant body parts, it can invade nearby tissues and cause symptoms. Surgery is the primary treatment, and the prognosis is generally good when the tumour is confined to the thymus.

What does invasion mean?

Invasion in the context of type B2 thymoma refers to the tumour spreading beyond the thymus into surrounding tissues, such as the lungs or the pericardium (the tissue covering the outside of the heart). This is important because invasion can make the tumour more challenging to remove and increase the risk of recurrence. Tumours that invade other structures often require additional treatments, like radiation therapy, to reduce the risk of the cancer returning after surgery.

What is the Masaoka–Koga system and how is it applied to a type B2 thymoma?

The Masaoka–Koga system is used to stage thymomas based on how far the tumour has spread.

• Stage I: The tumour is confined to the thymus.
• Stage II: The tumour has grown into nearby tissues.
• Stage III: The tumour invades nearby organs.
• Stage IV: The tumour has spread to distant organs.

Most type B2 thymomas are diagnosed at stages I or II, and surgery can be curative.

Prognosis

The prognosis for people with type B2 thymoma is generally good, especially when the tumour is detected at an early stage and surgically removed. Long-term follow-up is important to monitor for recurrence and manage any associated autoimmune conditions, such as myasthenia gravis.