The significance of 1-131 scan dose in patients with thyroid cancer: determination of ablation: concise communication.

Twenty-four patients with differentiated thyroid cancer were studied with diagnostic I-131 neck chest scans after having undergone bilateral subtotal thyroidectomy and initial I-131 therapy with either 30- or 100-mCi doses. With an endogenous stimulation protocol, follow-up studies were performed with neck and chest scans using 2 and 10 mCi I-131. A 400% increase in sensitivity was found with a 10-mCi dose relative to a 2-mCi dose. Comparison with therapeutic doses of 30 and 100 mCi resulted in further increases in the detection of residual iodine-avid tissue. We conclude that a 2-mCi or lower dose of I-131 is inadequate in evaluating residual iodine-avid tissue visually in patients with thyroid cancer. The study does not answer the critical question of whether it is necessary to treat a patient presenting a negative 2-mCi but a positive 10-mCi scan. It may be appropriate to define ablation visually as well as clinically, with further studies directed toward determining a treatment rationale in this patient population.