Classification of thymoma and thymic carcinoma

Thymoma and thymic carcinoma can be divided into groups based on what the cancer cells look like under a microscope. Along with the stage, this classification helps your healthcare team plan your treatment and predict future outcomes (your prognosis).

To classify thymus cancer, a pathologist looks at a tissue sample from the tumour under a microscope. They look at how different the cells look from normal cells. They also look at other features of the tumour, such as the size and shape of the cells, and how the cells are arranged. They can tell how fast a tumour is growing by looking at how many cells are dividing.

The World Health Organization (WHO) Classification is the system of classification that is most often used for thymus tumours. It assigns letters to the different types of thymus tumours.

Type A thymoma is made up of cells with a spindle or oval shape with normal nuclei. Type A is the rarest type of thymoma.

Type AB thymoma is made up of cells like type A, but there are a large number of lymphocytes found in the tumour. It is also called mixed thymoma. Type AB is one of the more common types of thymoma.

Type B1 thymoma is made up of normal cells with lots of lymphocytes.

Type B2 thymoma is made up of abnormal enlarged cells with lots of lymphocytes. This type of thymoma is often found in people with myasthenia gravis. Type B2 is one of the more common types of thymoma, along with type AB.

Type B3 thymoma is made up of abnormal thymus cells with a few lymphocytes.

Thymic carcinoma is sometimes called type C. It is made up of very abnormal thymus epithelial cells. The cells of the tumour do not look like normal thymus epithelial cells. These tumours often grow into other structures and organs around the thymus. They may spread (metastasize) to other parts of the body. Thymic carcinoma has a worse prognosis compared to thymoma.