I read that scientists say that masturbation can causebreast cancer. Is it true?

No. It also won't make you go blind or grow hair on your palms! there has been the suggestion that in women, masturbation may reduce the risk of breast cancer and that masturbation in men may reduce prostate cancer risk.
It's just not true. The world is full of people (including "health gurus" and "very spiritual people") who will tell you a bunch of lies to frighten and manipulate you. This is one of them. I'm sorry you were deceived.