My wife said she read last week that heart cells don't replace themselves and the number of heart cells you have is set at birth. Therefore, cancer, which is runaway cell replication, doesn't happen in the heart.
I did some limited research and found this primarily to be true... researchers think that only 1% of heart cells are replaced in a year.
http://newsroom.ucla.edu/releases/heart-muscle-can-regenerate-itself-in-very-limited-amounts-scientists-find
I don't know about the cancer aspect, but it kinda makes sense on face value.