An aggressive (fast-growing) type of B-cell non-Hodgkin lymphoma (cancer of the immune system). Primary mediastinal large B-cell lymphoma develops from B cells in the mediastinum (the area behind the breastbone). It may spread to organs and tissues such as the lungs, pericardium (sac around the heart), liver, gastrointestinal tract, ovaries, adrenal glands, and central nervous system. Most patients with primary mediastinal large B-cell lymphoma are women who are age 30 to 40 years but it may also occur in older children. Also known as primary mediastinal B-cell lymphoma.