Dear IKAP: 1) Neither test distinguishes benign from malignant. They may indicate more or less suspicion but the only way to distinguish with certainty is biopsy. Each test may be ordered for different reasons.
2) If one is having a prophylactic mastectomy, there is no reason to remove any lymph nodes. By definition, prophylactic means it is being done for prevention - there is no cancer. If by some weird coincidence, cancer were discovered in the specimen, a discussion of whether or not lymph nodes should be removed and type of procedure would be discussed at that time. If there is a strong suspicion that there is cancer in the breast now, then this is not a prophylactic procedure and should be treated as such regarding addressing the lymph nodes.