Chest and mediastinal masses form in the front, middle, or back of the chest, between the sternum and the spinal column. Any such tumors are very rare. They are caused by several types of cysts and tumors and can be benign or malignant. In infants and younger children, they often form in the back of the mediastinum and are more often benign than malignant. But they may more often cause symptoms such as respiratory difficulties than in adults.