Type A thymoma

by Jason Wasserman MD PhD FRCPC
September 17, 2024

A type A thymoma is a rare type of tumour that forms in the thymus, a small organ in the upper chest that plays a role in the immune system. Thymomas arise from the epithelial cells of the thymus and type A thymomas are considered the least aggressive form. They are usually slow-growing and have a low risk of spreading to other body parts.

Why is it called type A?

The term “Type A” is used to classify this tumour based on its microscopic appearance. Thymomas are categorized into different types (A, AB, B1, B2, and B3) according to the characteristics of the cells and the arrangement of the tumour tissue. Type A thymomas are made up of spindle-shaped epithelial cells and lack the immature lymphocytes seen in other types of thymomas. This classification helps doctors understand the tumour’s behaviour and potential treatment options.

What are the symptoms of type A thymoma?

Many people with type A thymoma do not have any symptoms, and the tumour may be discovered by accident during imaging studies for other reasons. When symptoms do occur, they can include:

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

These symptoms occur when the tumour grows large enough to press on surrounding structures in the chest.

What causes type A thymoma?

The exact cause of type A thymoma is not well understood. Like most tumours, it is believed to result from genetic changes in the cells of the thymus, which cause the cells to grow uncontrollably. These genetic mutations are usually not inherited but develop over time. Although there is no known strong environmental link, ongoing research is investigating factors that may contribute to the development of thymomas.

What medical conditions are associated with Type A thymoma?

Type A thymomas are often associated with autoimmune diseases, particularly myasthenia gravis, a condition in which the immune system attacks the muscles, causing weakness. Other autoimmune conditions, such as red cell aplasia (a disorder affecting red blood cells) and hypogammaglobulinemia (a condition causing low levels of antibodies in the blood), may also be seen in patients with thymomas.

How is this diagnosis made?

The diagnosis of type A thymoma is typically made by examining a tissue sample from the tumour, such as a biopsy, or when the entire tumour is surgically removed. A pathologist reviews the tissue under the microscope and looks for characteristic features of type A thymoma. Imaging tests such as a CT scan or MRI may also be used to assess the size and location of the tumour and to guide the biopsy.

What are the microscopic features of type A thymoma?

Under the microscope, type A thymomas consist of spindle-shaped or oval epithelial cells that grow in a uniform, orderly pattern. The cells generally look bland, meaning they do not show the irregular shapes or high levels of cell division seen in more aggressive tumours. Unlike other types of thymomas, type A thymomas do not contain immature lymphocytes. The overall appearance is typically described as bland and low-grade, meaning the tumour has a low potential for aggressive behaviour.

What tests may be used to confirm the diagnosis?

Immunohistochemistry (IHC) is commonly used to confirm the diagnosis of type A thymoma. This test uses special stains to highlight specific proteins in the cells. In type A thymoma, the epithelial cells may show positivity for markers such as cytokeratins, which help identify the tumour as being of epithelial origin.

Molecular testing is generally not required for diagnosing type A thymoma, but genetic studies may sometimes be performed to rule out other types of tumours.

What is the prognosis for a patient with Type A thymoma?

The prognosis for patients with type A thymoma is generally excellent. This tumour type is typically slow-growing and has a low risk of spreading beyond the thymus. Most patients can be successfully treated with surgery to remove the tumour. In some cases, additional treatments such as radiation therapy or chemotherapy may be recommended if the tumour is large or if there is concern that it has spread. The long-term survival rate for patients with type A thymoma is very high, especially when the tumour is caught early.

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

The Masaoka–Koga system is a staging system used to classify 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 remains within the surrounding 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 A thymomas are often diagnosed at an early stage (Stage I or II) and have an excellent prognosis when treated appropriately. The stage of the tumour helps guide treatment decisions, with surgery being the most common option for early-stage disease.

Is a type A thymoma benign or malignant?

Type A thymoma is generally considered to be a benign tumour, meaning it has a low risk of spreading or becoming life-threatening. However, the term “benign” can be a bit misleading because type A thymomas can still grow and cause symptoms by pressing on nearby structures in the chest. While they rarely spread to other parts of the body, they are treated with care due to their location and potential to grow. With early detection and proper treatment, the prognosis is typically excellent.