Proving your case
Failure to diagnose cases are hard to prove and hard to win. In general, to succeed you have to prove that you saw the doctor, and that the reason you saw the doctor was reasonably related to the cancer you were eventually diagnosed with (such as pain in the same area of the body). Then you need to prove that the failure to diagnose led to a change in your condition. In other words, you need to prove that the cancer progressed between the missed diagnosis and the actual diagnosis.
More than a missed diagnosis
The fact that a diagnosis was missed isn't enough on its own to make a case for malpractice. It's not about what could have happened, but rather what did happen. It can be very upsetting to find out your doctor failed to diagnose such a serious disease, however there is generally not legal recourse unless you can prove that the missed diagnosis allowed your condition to worsen before it was eventually diagnosed.
Finding the right attorney
Because these cases are complex and difficult to prove, it's necessary to find an experienced attorney. Look for someone with specific experience in failure to diagnose. You should be able to get a free consultation. Ask lots of questions, including how the attorney plans to proceed with your case and how they will keep you informed along the way. If you aren't comfortable with their advice, get a second opinion.