Can I sue an emergency room doctor for misdiagnosing my medical condition?

Yes, you can sue, as long as this happened less than 2 1/2 years ago, you have a permanent injury as a result of the failure to diagnose, and you can get a doctor to testify that but for the failure to diagnose the cancer, he/she is reasonably certain you would have had a good outcome.  If, on the other hand, you were diagnosed correctly by another doctor in time to get successful treatment, you have no case.