Type B3 thymoma

by Jason Wasserman MD PhD FRCPC
September 19, 2024

Type B3 thymoma is a rare tumour that starts in the thymus, a small organ located in the chest. The thymus is part of the immune system and helps produce T cells. This type of thymoma predominantly comprises epithelial cells, which look more abnormal than those in type B1 and B2 thymomas. It has fewer lymphocytes than type B1 and B2 thymomas.

Why is it called type B3?

Thymomas are classified into types A, AB, B1, B2, and B3 based on the appearance and ratio of epithelial cells to lymphocytes. In type B3 thymoma, the epithelial cells are dominant and appear more atypical (abnormal). This subtype has the least lymphocytes among the B types.

What are the symptoms of type B3 thymoma?

Many patients with type B3 thymoma are asymptomatic, and the tumour is often found incidentally. When present, symptoms may include chest pain, shortness of breath, and cough due to the tumour pressing on nearby organs. Autoimmune diseases, like myasthenia gravis, are associated with this tumour, although less frequently than in type B1 or B2 thymomas.

What causes type B3 thymoma?

The exact cause of type B3 thymoma is not known. It is thought to arise from abnormal growth of the epithelial cells in the thymus, which normally support the development of T cells. Genetic mutations have been found in some thymomas, but no single mutation is known to cause this tumour type.

What medical conditions are associated with type B3 thymoma?

Type B3 thymoma can be associated with autoimmune conditions, including myasthenia gravis, although these associations are less frequent compared to type B1 and B2 thymomas. Other autoimmune conditions, such as hypogammaglobulinemia, may also be seen in some patients.

How is this diagnosis made?

Type B3 thymoma is diagnosed by examining a tumour sample under a microscope. It is characterized by sheets of abnormal epithelial cells. Lymphocytes are less common than in type B1 or B2 thymomas. Imaging techniques, such as CT scans or MRIs, are used to determine the size and location of the tumour and guide a biopsy.

What are the microscopic features of type B3 thymoma?

Microscopically, type B3 thymoma shows a lobulated growth pattern, meaning the tumour is divided into rounded sections or lobules, separated by prominent fibrous bands. The epithelial cells are arranged in sheets or clusters and are positive for markers such as cytokeratins, p40, and p63, while the lymphocytes are positive for TdT. Markers typically expressed in thymic carcinoma, such as CD5, CD117, and EZH2, are negative in this type of thymoma.

Is a type B3 thymoma benign or malignant?

Type B3 thymoma is considered a low grade malignant tumour. It grows slowly but can invade nearby tissues and spread to other body parts. Surgery is the primary treatment, and when the tumour is localized, the prognosis is usually good.

What does invasion mean?

Invasion refers to the tumour spreading beyond the thymus into surrounding tissues, such as the lungs or the pericardium (the tissue covering the surface of the heart). This is important because invasion makes the tumour more challenging to remove completely and increases the risk of recurrence. Tumours that invade other structures may require additional treatments, such as 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 B3 thymoma?

The Masaoka–Koga system stages 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 B3 thymomas are diagnosed at stages I or II, and surgery is often curative in the early stages.

Prognosis

The prognosis for people with type B3 thymoma depends on the stage at diagnosis. Early-stage tumours that are completely removed by surgery have a good prognosis. In cases where the tumour has invaded nearby structures or spread to other parts of the body, additional treatments like radiation or chemotherapy may be needed. Long-term follow-up is important to monitor for recurrence.