Assessment of the efficacy of iodine-131 for thyroid ablation

It is customary to ablate residual tissue after near-total thyroidectomy for thyroid carcinoma by administering 131I. A recent trend has been to use lower 131I doses. This study was designed to assess the efficacy of thyroid ablation by 1110 MBq of 131I (30 mCi) in patients who had near-total thyroidectomy for papillary, mixed or follicular thyroid carcinoma. Four months after surgery, a whole-body scan was done using 185 MBq (5 mCi) of 131I after withdrawal of L-thyroxine for 5-6 wk. Residual thyroid area was then measured by planimetry of the thyroid scan. Patients received ablation therapy within 5 days after scanning and one or more subsequent scans were performed 6 mo later. Forty-four patients were treated to ablate residual functional thyroid tissue. Of these, 12 (27%) had successful ablation. Total body areas (1.63 +/- 0.16 versus 1.83 +/- 0.30, p < 0.03) and residual thyroid tissue (1.4 +/- 1.4 versus 2.0 +/- 1.2 cm2, p < 0.05) were less in patients with total thyroid ablation while there was a trend for a smaller incidence of associated goiter in those patients (1/12 versus 13/32, p < 0.07). Nine of the 17 (53%) patients with a total body area less than 1.9 m2 and/or with a residual thyroid tissue less than 2.1 cm2 and/or without associated previous associated diffuse or multinodular goiter had a total thyroid ablation, while 3 of the 27 (11%) patients who did not have these characteristics had a successful therapy (p < 0.005). Our data suggest that 1110 MBq (30 mCi) of 131I can achieve total ablation of residual thyroid tissue after near-total thyroidectomy particularly in patients with lower total body area and smaller residual thyroid tissue without associated previous diffuse or multinodular goiter.