Comparison of I-124 and I-131 Radiopharmacokinetics in DTC Patients With Thyroid Hormone Withdrawal

This research study is to compare the radiopharmacokinetics of I-124 to the radiopharmacokinetics of I-131 in patients who have well-differentiated thyroid cancer after thyroid hormone withdrawal. I-131 is routinely used for imaging and dosimetry for patients with well-differentiated thyroid cancer. In this study, I-124 is administered orally in capsular form, and the radiopharmacokinetics of I-124 is compared with I-131. I-124 is another isotope of iodine, which is cyclotron-produced. I-124 has multiple advantages: * Ideal Half-Life (4.2 days) for delayed imaging. * High resolution tomographic imaging. * Feasibility of quantitating lesion uptake. * Potential of dosimetry for the planning of radioiodine therapy. Voluntary patients will have I-124 dosimetry performed in addition to the I-131 dosimetry, which is planned as part of routine clinical care. I-124 dosimetry is composed of three parts: (1) the administration of I-124, (2) imaging, and (3) drawing blood samples. Patients will start 3-5 weeks of thyroid hormone withdrawal. This is similar to the procedure for I-131 dosimetry. Second, they will receive I-124. I-124 is similar to I-131 except I-124 decays in a different way to emit a positron so that the PET scanner can be used for imaging. I-124 is given in the form of one or several capsules, which are taken by mouth. This is also similar to I-131. Third, PET/CT imaging is done for approximately 30 minutes to one hour on five consecutive days. Radiation from PET/CT scan is far less than what they receive from a diagnostic CT scan. For the fourth part, a technologist will draw about 5 cc from the forearm on each of the five consecutive days. This is also similar to I-131. Initially, all patients will be randomized to one of two study groups. The first group will have the I-131 dosimetry performed first followed by the I-124 dosimetry, and the second group will have the I-124 dosimetry performed first followed by the I-131 dosimetry. The risk of this study is considered very low, and the potential benefits to the patient are considered very high.