When do you employ post-op external beam radiotherapy in papillary thyroid carcinoma?  

Is RAI sufficient in cases of +margin but negative post-op iodine scan? In cases where there was no pre-op radioactive iodine scan to ensure iodine uptake, if the resection margin is positive, and the post-op iodine scan is negative, was it because the disease doesn't take-up iodine, or was it negative because there is too little disease present to show up?  NCCN guidelines aren't clear on this situation.  Do you explain the challenge and let the patient choose between post-op external beam and RAI therapy, or is this a more clear-cut issue?