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 find out how the thymus cancer is classified, 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 (called grade).

The system used most often is the 2015 World Health Organization (WHO) Classification. It describes 6 different types of thymus cancer.

Type A thymoma is made up of cells with a spindle or oval shape with normal nuclei. There are few, if any, normal lymphocytes in the tumour. It is also called spindle cell or medullary thymoma.

Type AB thymoma is made up of cells like type A, but there are a large number of lymphocytes. It is also called mixed thymoma.

Type B1 thymoma is made up of normal cells with lots of lymphocytes. It is also called lymphocyte-rich thymoma, predominantly cortical thymoma or organoid thymoma.

Type B2 thymoma is made up of enlarged and abnormal cells with lots of lymphocytes. It is often associated with myasthenia gravis. It is also called cortical or polygonal cell thymoma.

Type B3 thymoma is made up of abnormal thymus cells with a few lymphocytes. It is also called epithelial thymoma, atypical thymoma, squamoid thymoma or well-differentiated thymic carcinoma.

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 thymus epithelial cells. These tumours often invade other structures and organs around the thymus. They may or may not spread (metastasize) to other parts of the body. Thymic carcinoma has a poor prognosis compared to thymoma.