In the last few weeks my wife has developed what appears to be a fluid-filled cyst on her sturnum between her breasts. It is soft and seems to shift in shape and size, which is generally about four inches laterally, two or three vertically. She has had two heart surgeries for aortic aneurysms, one in 1990, replacing the aortic valve, and the second, to resect the aortic arch, in 2011. The cyst is located beneath the scars from those surgeries. What could it be and how should it be treated?