The prognostic value of post thyroidectomy Tc-99m pertechnetate thyroid scan in patients with differentiated thyroid...

To evaluate the prognostic value of postoperative Tc-99m pertechnetate scanning in patients with differentiated thyroid cancer (DTC).

Patients with pathologically proven low or intermediate-risk DTC were enrolled in this study. Postoperative Tc-99m pertechnetate scintigraphy was performed 20 min after IV of 185 MBq of Tc-99m pertechnetate Positive thyroid residual uptake was characterized as higher Tc-99m pertechnetate uptake at the thyroid bed than the surrounding background's activity. A negative residual was considered if there was no definite abnormal radioactivity at the thyroid bed. Follow-up by thyroglobulin, thyroglobulin antibodies (Tg Abs), neck ultrasound (US) and diagnostic I-131 WBS (Dx WBS) were considered the reference standard. Successful ablation outcome was considered if there was free Dx WBS, stimulated serum thyroglobulin < 1 ng/ml with negative Tg Abs, and free US.

Two hundred and two patients, mean age; of 38.8 years were retrospectively recruited in this study. Positive residual uptake at the thyroid scan was detected in 131 patients wherea the remaining 71 patients had no detectable uptake. According to the reference standard we encountered 114 and 88 cases with successful ablation and unsuccessful ablation respectively, Tc-99m pertechnetate scanning successfully detected 63 true positive and 46 true negative cases giving 72% sensitivity, 40% specificity, 48% positive predictive value and 56% negative predictive value.

In low- and intermediate-risk DTC patients; despite the relatively high sensitivity of postoperative Tc-99m pertechnetate thyroid scan, it has low specificity and low negative predictive value so it cannot be used to predict the ablation outcome.