About 15 percent of people with myasthenia gravis have a tumor in their thymus, a gland under the breastbone that is involved with the immune system. Most of these tumors, called thymomas, aren't cancerous (malignant). Removal of the thymus gland is called a thymectomy. A variety of operative approaches, often dictated by the size and location of the tumor, are used to remove thymomas and thymic cancers.