 Tumours are generally detected when they cause symptoms, such as a neck mass or affecting nearby structures such as the superior vena cava; detected because of screening in patients with myasthenia gravis, which has a strong association with thymomas and hyperplasia; and detected as an incidental finding on imaging such as chest x-rays.Hyperplasia and tumours originating form the thymus are associated with other autoimmune diseases - such as hypogammaglobulinemia, Graves disease, pure red cell aplasia, pernicious anaemia and dermatomyositis, likely because of defects in negative selection in proliferating T cells.Thymomas can be benign; benign but by virtue of expansion, invading beyond the capsule of the thymus ("invasive thyoma"), or malignant (a carcinoma).