A risk factor is anything that increases your chance of getting a disease such as cancer. Different cancers have different risk factors. For example, unprotected exposure to strong sunlight is a risk factor for skin cancer. Smoking is a risk factor for lung, oral, laryngeal, bladder, and several other cancers.

There are rare instances of thymomas occurring in families. Doctors have found specific gene changes in the thymomas in these families that indicate a tendency to develop the tumors was probably inherited.

Men and women have about the same likelihood of developing tumors of the thymus. These tumors are more common in middle-aged or elderly people than in children and young adults. The majority of people are between the ages of 50 and 80.