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Not much is known about why thymus tumors (thymomas and thymic carcinomas) develop in some people but not in others. There are very few known risk factors for thymus cancer. These tumors don’t appear to have obvious inherited, environmental, or lifestyle-related causes.

Gene changes (mutations) and cancer

Cells are the basic building blocks of all living things, including our bodies. Genes  control how cells function. DNA is the chemical in our cells that makes up those genes. Our DNA, which comes from both our parents, affects more than just how we look.

Cancer is caused by changes in the DNA inside our cells.

Different genes have different functions in our bodies. If they are working properly, certain genes help control when our cells grow, divide to make new cells, or repair mistakes in DNA. They also cause cells to die when they’re supposed to.

If these genes aren’t working the way they should, it can lead to cells growing out of control. For example:

• Changes in genes that normally help cells grow, divide, or stay alive can lead to these genes being more active than they should be, causing them to become oncogenes. These genes can result in cells growing out of control.
• Genes that normally help keep cell division under control or cause cells to die at the right time are known as tumor suppressor genes. Changes that turn off these genes can result in cells growing out of control.
• Some genes normally help repair mistakes in a cell’s DNA. Changes that turn off these DNA repair genes can result in the buildup of DNA changes within a cell, which might lead to them growing out of control.

Any of these types of DNA changes might lead to cells growing out of control and forming a tumor. To learn more, see Oncogenes, Tumor Suppressor Genes, and DNA Repair Genes.

Inherited versus acquired gene mutations

People have 2 copies of each gene in their cells. One copy comes from each parent.

Some people inherit DNA changes (mutations) from a parent that increase their risk of some types of cancer. But inherited gene mutations do not seem to play a role in thymus cancers.

In many cancers, the gene changes inside the cancer cells are acquired during a person’s life. Thymus cancers (especially thymomas) tend to have very few gene mutations compared to many other types of cancer. Genes that are sometimes mutated in thymoma cells include:

• GTF2I
• TP53
• KIT
• CDKN2A

It’s not clear if there are any outside factors that cause these gene changes. Sometimes they may just be random events that happen inside a cell when it divides to make two cells, without having an outside cause.