is it scientifically proven that cancer is hereditary?

There is a correlation of cancer with genetics. But genes don't cause cancer. There's a strong epigenetic component having to do with environment, attitude and lifestyle. 50% of women who have the same BRCA1 gene never get the cancer. Also, a child from a cancer-free family who is adopted into a family which shows a propensity for contracting a particular kind of cancer, will develop the same propensity for the same cancer.

Interesting isn't it?