Seemingly, there is a cancer for everything: brain cancer, liver cancer, breast cancer, pancreatic cancer, bone cancer, blood cancer, skin cancer. Heart cancer? Wait ...
You've almost certainly never heard of it, but heart cancer—known properly as rhabdomyosarcoma—does exist. And by "exists" I just mean that it has occurred at least one time. That is, it doesn't not exist. The Mayo Clinic, for example, estimates that it sees on average a single case of primary heart cancer annually. These extremely, infinitesimally isolated occurrences are almost invariably the result of cancer that has spread from elsewhere in the body. Cancer originating in the lungs may spread to the heart or its protective pericardial sac.
But still, it almost certainly won't.
Cancer and heart disease are the twin deathstyles of our time, but it's almost as if they've negotiated a truce. What is it that makes the heart so uniquely impervious to cancer?
The answer lies in the cardiac myocyte, by far the most abundant heart cell type. Myocytes are, broadly, muscle cells, and this is one thing that makes the heart very different from any other organ—it's just a fist of muscle.
Cardiac myocytes have the property of being "terminally differentiated." The American Cancer Institute explains: "These cells reach a point very early on in a person's life where they permanently exit the cell cycle and stop dividing. After that, further growth occurs by expansion in cell size, not through cell division. This differs, for instance, from the epithelial cells that line other organs, which, in response to certain stimuli, actively divide and, when necessary, grow in number."
Cancer's whole thing is unregulated cell proliferation. Genetic mutations accumulate in cells over time, and, eventually, this may lead to chaotic, out-of-control cell division. This may in turn lead to the formation of a tumor, which may in turn become malignant and metastasize, spreading to other tissues and other organs. Eventually, cancer may wind up rooted in many places in the body, introducing its reproductive chaos into each one.
But heart cells aren't interested in division in the very first place. Or, rather, after a certain point very early on in human development they completely lose their ability to synthesize DNA and, thus, to create new copies of themselves. Not being able to make copies of their own genetic material means not being able to make copies of any genetic material that may have gone haywire.
The result of this winds up being a cruel catch-22. Cancer is the second-leading cause of death in the United States and falls short of heart disease by only a few tens of thousands of fatalities. Some very large part of what makes heart disease so lethal is what I just described. Heart cells don't reproduce.
The typical heart disease downward spiral starts with a heart attack in which blood is blocked to part of the heart for long enough for that part to die. Instead of replacing that tissue with new heart muscle, all we get is crappy scar tissue. And so the heart has to work harder to make up for less muscle, and soon enough another heart attack happens. More scar tissue. Eventually, the heart is whittled down to a flimsy wad of scar tissue and that's basically the end.
So, what keeps us from getting heart cancer is one of the main mechanisms behind heart disease. Poetry. (By the by, for this reason you will find a great deal of research into tricking heart cells into once again dividing, which is an interesting twist.)
It's worth noting that brain cancer is vastly more common than heart cancer, even though neurons share this same reproductive disinclination. The reason for this is that the brain is really only about half neurons, with the other half consisting of glial cells, which serve various support roles within the brain but don't do any of the information processing. Glial cells very much so continue dividing throughout human development, making them a prime ground zero for cancer. Heart cells, however, are mostly on their own.