Thymic carcinoma is a cancer that starts in the thymus gland, an H-shaped organ that sits behind the breastbone and right in front of the heart. The thymus plays an important role in the development of the immune system. For the first few years of life it produces t-cells that help the body  fight against infection and cancer. In puberty t-cell production declines as the thymus slowly shrinks to almost nothing and is eventually absorbed by fatty tissue. That’s when other organs such as the spleen and bone marrow take over that role.