is it scientifically proven that cancer is hereditary?

Some people are predisposed to certain cancers. In other cases, there are environmental/behavioral factors. Sometimes it just happens by random chance...

Yes. It can run in families and some bloodlines are resistant as well. So yes there is a genetic component.

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?

Some kinds yes, and you can be genetic screened for some of them.

No... that's why genetic screening exists.

It can be. Mine was not. That creates a bit of a mystery, because guys don't generally get breast cancer.