Percent I-131 uptake and post-therapy I-131 scans: Their role in the management of thyroid cancer

This short review focuses on two questions commonly asked about I-131 therapy of thyroid cancer. The first relates to radiation dosimetry. What percentage of I-131 uptake is needed for effective therapy? Studies have shown that I-131 uptake and therapeutic outcome may not be closely related. A more powerful relationship, however, occurs by using percent uptake per gram. A rearrangement of the basic dosimetry equation shows that for a specific level of radiation, percent uptake per gram and effective half-life are inversely related. Formulas developed from this relationship are useful in determining whether uptake is satisfactory but require estimates of effective half-life and tumor mass. The second question is whether post-therapy I-131 scans are useful. Because the sensitivity of I-131 scanning is related directly to the amount of radioactivity administered, imaging after therapy may show previously undetected lesions that not infrequently are distant metastases. Thus, information from post-therapy scans may 1) after therapy, 2) direct new diagnostic studies, 3) lead to formal dosimetry studies when maximal I-131 doses are needed to treat distant metastases, and 4) be necessary to obtain when I-131 therapy is administered to patients with elevated thyroglobulin and negative diagnostic scans. Should post-therapy scans always be performed? They are easy to perform and are clinically useful but may be inconvenient to patients and costly overall. At this time, however, there is not a simple paradigm for selecting patients who may forego these scans.