Lung cancer is a close second. The risk of developing breast cancer increases with age. An estimated 1 in 10 women over the age of 75 will develop breast cancer. The difference is six fold. There is a connection between fat and estrogen.
Breast tissues accumulate fat.
Body fat increases estrogen levels, and estrogen increases the tendency to accumulate body fat. It's a vicious cycle. In fact women in the Untied States produce more estrogen than third world women. Breast Cancer, as well as other cancers, are best treated preventatively. It is far easier to prevent them in the first place than to fight them later on.