Endocrine Abstracts

Background: Early detection of residual or recurrent cancer in patients with differentiated thyroid cancer (DTC) is important, especially, when they do not uptake ¹³¹I uptake because these tumors do not likely benefit from radioiodine therapy (RAIT). We aimed to evaluate the usefulness of FDG-PET/CT as an early diagnostic work up in DTC patients with negative radioiodine whole body scan (I-WBS) and elevated stimulated Tg (sTg) levels.

Methods: This was a retrospective study. There were 48 consecutive patients with negative I-WBS and elevated sTg level (>5 ng/ml) or positive Tg antibodies (TgAb). FDG-PET/CT was performed within 12 months after first remnant ablation. True positive rate and positive predictive value was calculated according to different sTg levels (ng/ml). [5 ≤ sTg < 10 (n=11), 10 ≤ sTg < 20 (n=14), ≥20 (n=19)] and positive anti-thyroglobulin antibodies (n=4).

Results: FDG-PET/CT showed 12 (25%) true positive, 6 (%) false positive, 22 (38%) true negative and 12 (21%) false negative results (sensitivity 60% specificity 79%). True positive FDG-PET/CT leaded to additional surgery in 10 (21%) DTC patients. True positive rate increased as sTg (ng/ml) level increases [9% in sTg between 5–10, 21% in sTg between 10–20, 37% in sTg>20 and 25% in positive TgAb]. Positive predictive value tended to increase with sTg increment [50% in sTg between 5–10, 50% in sTg between 10 and 20, 88% in sTg >20 ng/ml and 50% in positive TgAb].

Conclusions: Early FDG-PET/CT is useful tool for tumor detection in DTC patients with negative I-WBS and increased sTg levels and may change a treatment plan, especially when sTg during I-WBS was greater than 20 ng/ml.