You may have a case if--1) An ordinarily skilled and careful doctor would normally have been expected to find that polyp.2) You can establish that the missed  polyp led to the cancer.If, however, it would not be unreasonable for a doctor to have missed that polyp (doctors are not required to be perfect), such as because of how small it was or where exactly it was located, or if you can't establish the causal link between polyp and cancer, you would not have a case. If you believe you may have a case, you should consult with a medical malpractice attorney in greater detail.