Alex Aller, manager of Cell Biology and Immunology research in the Cancer Therapeutics Department at the Southern Research Institute, offers this explanation:

Any cell in the body has the potential to become malignant, thus cancer can, in fact, affect the heart. Cancer arises from mutations in the DNA of a cell. Usually a cancerous cell undergoes several mutations before it becomes a deadly, invasive cancer. Most of these mutations occur when the cell is dividing and replicating its DNA. The only way for a cell to propagate a mutation is to divide and pass those mutations on to its daughter cells. With regard to the heart cells, however, they just go right on pumping and doing their job and dont replicate to make new heart cells unless there has been some injury. With so little cell division going on in the heart there is very little chance for mutations to occur and get passed on to daughter cells.

Now think about the types of cancer that are most common-- breast, colon, and skin, among others. Most of the cells in these tissues are replacing themselves all the time. Breast tissue is constantly affected by hormones and is always growing and shrinking. The lining of the colon is continually sloughing off and being replaced. The same is true of the skin. In addition, skin and colon cells are constantly being exposed to things that induce mutations--ultraviolet rays for the skin and carcinogens in food for the colon. As a result, the likelihood of mutations is higher, and there is ample opportunity to pass these mutations on to daughter cells during cell division. This is why these types of cancer are common. The heart, in contrast, doesnt get exposed to many carcinogens, just those in the blood. That, combined with the fact that the heart cells do not often replicate, is why you dont see much cancer of the heart muscle. Indeed, according to cancer statistics, it does not appear to occur at any measurable rate.