The stage of a cancer is how much and how far it has spread in your body. Your healthcare provider uses exams and tests to find out the size of the tumor and where it is. He or she can also see if the cancer has grown into nearby areas, and if it has spread to other parts of your body. The stage is one of the most important things to know when deciding on treatment.

The place where cancer starts is called the primary site. Thymus cancer can spread from the primary site to other parts of your body. Cancer that has spread is called metastatic cancer. When a cancer spreads, it’s said to have metastasized.

Thymus cancer starts in the cells that line the thymus gland. As thymus cancer grows, it can grow through the layer of tissue that surrounds the thymus (the capsule). Then, like all cancers, it can spread (metastasize) to other parts of the body.

The most commonly used system to stage thymus cancer is the TNM system from the American Joint Committee on Cancer (AJCC). Be sure to ask your healthcare provider to explain the stage of your cancer to you in a way you can understand.

The first step in staging is to find the value for each part of the TNM system. Here's what the letters stand for in the TNM system:

• T tells how far the main tumor has spread into nearby tissues.
• N tells if the lymph nodes in the area of the original tumor have cancer in them.
• M tells if the cancer has spread ( metastasized) to distant organs in the body, such as the lungs or heart.

Numbers or letters after T, N, and M provide more details about each of these factors. There are also 2 other values that can be assigned:

• X means the provider does not have enough information to tell the extent of the main tumor (TX), or if the lymph nodes have cancer cells in them (NX).
• 0 means no sign of cancer, such as no sign of lymph node spread (N0).

Stage groupings are determined by combining the T, N, and M values from the TNM system. These groupings give an overall description of your cancer. A stage grouping is listed as a Roman numeral and can have a value of I through IV (1 through 4). The higher the number, the more advanced the cancer is.

These are the stage groupings of thymus cancer and what they mean:

Stage I. The cancer has not spread from the thymus to nearby lymph nodes or to distant parts of the body, and one of these is true:

• The cancer has not grown into the capsule surrounding the thymus or it has grown into nearby fatty tissues, but not the lining of the space between the lungs (called the mediastinal pleura).
• The cancer has grown into nearby fatty tissues and into the lining of the space between the lungs.

Stage II. The cancer has grown into nearby fatty tissue and into the sac that covers the heart (called the pericardium). It hasn't spread to nearby lymph nodes or to distant parts of the body.

Stage III. The cancer hasn't spread to nearby lymph nodes or to distant parts of the body, and is divided into one of these groups:

• Stage IIIA. The cancer has grown into nearby tissues or organs, such as the lungs, the blood vessels to and from the lungs, the superior vena cava (the big vessel carrying blood away from the heart), or the nerve that controls breathing (the phrenic nerve).
• Stage IIIB. The cancer has grown into nearby tissues or organs, such as the windpipe (trachea), swallowing tube (esophagus), or the main vessels carrying blood away from the heart.

Stage IV. This stage is divided into these groups:

• Stage IVA. The cancer may or may not have grown into nearby tissues or organs, and one of these is true:
  • It has spread to lymph nodes in the front of the chest, but not to distant parts of the body.
  • It may or may not have spread to nearby lymph nodes in the front of the chest. But it has spread to the lining of the lungs (the pleura) or the pericardium (sac around the heart).
• Stage IVB. The cancer may or may not have grown into nearby tissues or organs, and one of these is true:
  • It has spread to lymph nodes deep in the chest or neck, and may or may not have spread to the pleura or the pericardium.
  • It may or may not have spread to lymph nodes in the chest or neck, but has spread to the inside of the lungs or to other distant parts of the body.

Once your cancer is staged, talk with your healthcare provider about what the stage means for you. Ask any questions and talk about your concerns.