Heart cancer isn’t a thing you hear much about. That’s because it — almost — doesn’t exist. Most hospitals report fewer than one case of heart cancer per year. So why are tumors on this organ so rare?…Cancerous cells become a problem when they start to divide uncontrollably and take over everything. …But unlike cells in other organs, our heart cells do almost all of their dividing during fetal development. …Once we’re born, though, our heart cells stop dividing. …That’s because our hearts are mostly made of muscle cells, which can grow in proportion to our bodies, but they don’t multiply the same way that other cells in our bodies do.