Mother had breast cancer in mid fifties now i'm worried i'll get it. nobody else in family had it. should i get prophylactic mastectomy?

No....: Your chance of getting breast cancer, with a positive maternal history, is still only about 15%. (50% higher than a negative history.) however, if a brca test was positive, your odds would jump to about 85-86%. I would suggest a brca test if you really want to consider preemptive surgery. Speak to your primary physician and see what can be done. Good luck.