If my wife had breast reduction surgery 6 years ago and now she has cancer in both breasts, can I sue the doctor/ plastic surgeon for medical malpractice?
On what basis? You need to be able to show two things for a malpractice suit--
1) Causation: that something the doctor did, or a diagnosis he missed, caused the medical condition or harm. In this case, you'd need to show that either the breast reduction surgery caused the cancer, and/or that she had early stage cancer at the time which the doctor should have detected (so it could have been nipped in the bud or treated years ago), but did not. You need medical evidence to show this.
2) Fault, or that the doctor was negligent (unreasonably careless) or otherwise provided medical care that fell below the then-accepted standard or level of care. Malpractice is based on fault--on the doctor doing something wrong. Sometimes, though, bad things happen to patients even when the doctor does everything right, and when that happens, there is no liability. So you'd also have to have medical evidence or a medical expert's opinon that the care was faulty in order to have a claim.
Unless you can show these things, there is no claim.