Type B1 thymoma

by Jason Wasserman MD PhD FRCPC
September 18, 2024

Type B1 thymoma is a rare tumour that starts in the thymus, a small organ located in the chest. The thymus plays an important role in developing the immune system, especially during childhood. Type B1 thymomas contain many lymphocytes, a type of white blood cell, and the tumour cells resemble normal thymic epithelial cells. This type of thymoma is considered low-grade, meaning it grows slowly and is less likely to spread to other body parts than higher-grade tumours.

Why is it called type B1?

The classification of thymomas into types A, AB, B1, B2, and B3 is based on the number and type of epithelial cells and lymphocytes in the tumour. Type B1 thymoma is so named because it contains many lymphocytes that resemble normal T cells found in the thymus, with only a few epithelial cells that look like normal thymic epithelial cells. In contrast, type A and AB thymomas have many more epithelial cells.

What are the symptoms of type B1 thymoma?

Most people with type B1 thymoma do not experience any symptoms, and the tumour is often discovered by accident during imaging for another reason. When symptoms do occur, they are usually caused by the tumour pressing on nearby structures in the chest. Common symptoms include:

• Chest pain
• Shortness of breath
• Cough
• Difficulty swallowing

In some cases, patients may develop autoimmune diseases like myasthenia gravis, which causes muscle weakness and fatigue.

What causes type B1 thymoma?

The exact cause of type B1 thymoma is unknown, but it is thought to develop from abnormal growth of epithelial cells in the thymus. These cells normally support the development of lymphocytes, which are important for the immune system. In this type of tumour, these lymphocytes are abundant, and the tumour cells resemble normal thymic epithelial cells. Although genetic mutations have been identified in some thymomas, there is no known single genetic cause for the development of this tumour.

What medical conditions are associated with type B1 thymoma?

Type B1 thymoma is commonly associated with autoimmune diseases, particularly myasthenia gravis. Myasthenia gravis is an autoimmune disorder affecting communication between nerves and muscles, leading to muscle weakness and fatigue. Other autoimmune conditions, such as pure red cell aplasia and hypogammaglobulinemia, can also occur in patients with this tumour, although these are less common.

How is the diagnosis of type B1 thymoma made?

Type B1 thymoma is diagnosed by examining part or all of the tumour under a microscope. Pathologists look for specific microscopic features, including a large number of lymphocytes and a smaller number of epithelial cells, to make the diagnosis. Imaging studies, such as CT scans or MRIs, may be used to assess the tumour’s size and location and guide the biopsy.

What are the microscopic features of type B1 thymoma?

Microscopically, type B1 thymoma is characterized by a dense population of T lymphocytes with fewer thymic epithelial cells. The lymphocytes in type B1 thymoma resemble normal immature T cells found in the thymus. These lymphocytes express markers such as CD3, CD1a, and TdT (terminal deoxynucleotidyl transferase), which are used in immunohistochemical staining to identify their immature nature. Though less numerous, the epithelial cells express cytokeratin and p63, typical markers of thymic epithelial cells. The epithelial cells appear bland, without signs of significant atypia or malignancy.

Is a type B1 thymoma benign or malignant?

Type B1 thymoma is generally considered to be benign, meaning it has a low risk of spreading or becoming life-threatening. However, like all thymomas, it can still grow and cause symptoms by pressing on nearby structures. While these tumours have a slightly higher risk of recurrence or local spread compared to type A thymomas, they are usually slow-growing and are effectively treated with surgery.

What is the Masaoka–Koga system and how is it applied to a type B1 thymoma?

The Masaoka–Koga system is used to stage thymomas based on how far the tumour has spread at the time of diagnosis. This system divides thymomas into four stages:

• Stage I: The tumour is confined to the thymus and has not spread beyond its capsule.
• Stage II: The tumour has grown beyond the thymus but is still contained within nearby tissues.
• Stage III: The tumour has invaded nearby organs, such as the lungs or heart.
• Stage IV: The tumour has spread to distant body parts, such as the lungs or bones.

Type B1 thymomas are often diagnosed at an early stage (Stage I or II) and have an excellent prognosis with appropriate treatment. The stage of the tumour is important for guiding treatment decisions, and surgery is often the first step for early-stage thymomas.

What is the prognosis for people with type B1 thymoma?

The prognosis for people with type B1 thymoma is generally good. This tumour tends to grow slowly, and surgical removal is often curative, especially when the tumour is confined to the thymus. Even in cases where the tumour has spread outside the thymus, treatment options such as radiation therapy and chemotherapy can be effective. Long-term follow-up is important to monitor for recurrence and to manage any associated autoimmune conditions, such as myasthenia gravis.