You'll go nuts if you look at every theory about what causes, or triggers, breast cancer. I've had well meaning, and not so benign, suggestions about what I could have done, or not done, to avoid the predicament I am in. Most of it is bullshit and some is plain malicious.
I'd talk to your Onc about soy, the jury is still out regarding its effects on hormone receptive cancers, but, really, how much of the stuff are you going to consume? Most of these studies look at what happens to mice if you feed them a steady diet, or sit them in a pool of, or make them breath nothing but ***insert suspected carcinogens***
Common sense says that eating well and looking after yourself should help. Practical experience says that there is no guarantee you will be safe. Do what feels right for you.