Thymus cancer. Light micrograph of a section through a thymus gland with thymoma cancer. The thymus is a gland in the upper chest that plays an important role in the immune system, especially before puberty. The cancer severely weakens the immune system, making the patient susceptible to infections and making normally harmless infections life threatening. A thymoma may also be responsible for some cases of myasthenia gravis, a condition which causes muscle weakness. Removal of the thymus can cure the disease in these cases. If the cancer spreads to other parts of the body (usually the lymphatic system), the prognosis is poor.