Abstract
In response to the letter by Giovanella and Verburg (1), regarding our proposal that ablation with 131I may not be necessary in some patients with well-differentiated thyroid carcinoma (2), we again emphasize that this only applies to those patients who are at low risk of recurrence, who have a stimulated serum thyroglobulin (Tg) of <1 ng/mL and no anti-Tg antibodies (TgAb), and whose neck ultrasound shows no anomalies at approximately 3 months after thyroidectomy.
In the studies cited in the letter by Giovanella and Verburg (1), regarding discrepancies between negative postoperative serum Tg values and positive postoperative and post-131-I ablative therapy scanning (RxWBS), ultrasound was not performed. This method could detect some of the foci visualized by RxWBS. In addition, some of the patients with metastases on RxWBS were at high risk and the authors failed to demonstrate that the ectopic cervical uptakes corresponded to metastases. False-positive results are possible in patients who are at low risk and have stimulated serum Tg values of <1 ng/mL in the absence of positive tests for TgAbs and ultrasounds that show no lesions that correspond to the foci of 131I uptake. Data regarding the addition of SPECT/CT to planar imaging, specifically in the patients discussed here (low risk, stimulated Tg <1 ng/mL, and negative ultrasound), are not available or are scarce. Thus, these concerns do not seem to apply to the patients considered in our proposal (2). Particularly in low-risk patients with negative ultrasound, stimulated Tg, and TgAb after surgery, the negative predictive value of ectopic uptake on RxWBS, although not 100%, is certainly very high, and exposure of all these patients to radioiodine to eventually detect one patient with residual disease is not justified.
With respect to uptake in the thyroid bed as detected in the presence of stimulated Tg <1 ng/mL (without TgAb) and negative ultrasound, in our understanding this has no clinical relevance even if the uptake was >2%. This is uncommon, however, in patients with stimulated Tg <1 ng/mL. If this is not true, there would be no consensus [see Recommendations 45 and 46 in Ref. (3)] that diagnostic whole-body scanning (DxWBS) is unnecessary in the situation of low risk, stimulated Tg <1 ng/mL, and negative ultrasound. Regarding this, the literature is quite clear [see Recommendations 45 and 46 in Ref. (3)] that DxWBS is no longer recommended and does not add any value in these cases.
Regarding the technical limitations of serum Tg measurement (1), these are well known. Nevertheless, studies have repeatedly shown the excellent negative predictive value of serum Tg in clinical practice. These limitations are not restricted to Tg ablation but are inherent to serum Tg measurements in any situation. Thus, if Tg cannot be used to spare low-risk patients from ablation when stimulated serum Tg is <1 ng/mL in the absence of TgAb, it also cannot be used after initial therapy as the main parameter for the definition of complete remission [see Recommendation 45 in Ref. (3)], which contradicts all current consensuses and a vast body of literature [see Recommendation 45 in Ref. (3)].
We, therefore, conclude that our proposal (2) does not need to be rewritten. Moreover, we think that many elements of our proposal (2) conform to the recent study by Nascimento et al. (4).