Abnormal pelvic uptake on post-therapeutic radioiodine (131I) whole-body scan

Abstract

A 36-year-old woman was referred after total thyroidectomy with central neck dissection to the Institut Gustave Roussy for radioactive iodine (I) ablation after recombinant human thyrotropin (rhTSH) stimulation for a pT2 N0 minimally invasive follicular thyroid carcinoma. The whole-body scan (WBS) performed 3 days after the administration of radioiodine (3.7 GBq) showed thyroid bed uptake due to normal thyroid remnant and an uptake focus in the left pelvic region (a, arrow). Stimulated thyroglobulin (TG) was 10 ng/ml in the absence of TG antibodies. The pelvic I uptake was correlated to an ovarian cystic mass of 37 mm, as shown on ultrasonography (US) (b). The cyst was removed and histology showed a mature teratoma with a normal thyroid tissue component (c, arrow). Ectopic uptake of I not due to thyroid cancer metastases has been described in several sites, the most frequent being breast, thymus and gastrointestinal tract [1, 2]. This might cause misleading interpretations of images, even if actually the use of SPECT/CT should help for the interpretation of such images. I uptake in extrathyroidal tissue can be due to the expression of NaI (sodium-iodide) symporter or to tracer excretion artefact, but in some cases the cause is still not clear [1, 3]. In the pelvic region, I uptake has been reported in cases of dermoid cysts containing thyroid tissue, but also in benign pathological conditions without thyroid tissue component such as cystadenofibromas or leiomyoma [1, 4, 5]. The presence of pelvic I uptake on post-ablation WBS should raise the hypothesis of a gynaecological pathological condition and be further explored. Conflicts of interest None.