Case Report: Regaining radioiodine uptake following PRRT in radioiodine-refractory thyroid cancer: A new re-differentiation strategy?

Abstract

A 61-year-old woman with a history of metastatic follicular thyroid carcinoma became radioiodine-refractory following two doses of radioiodine (RAI) therapy (cumulative = 230 mCi). While no RAI-avid lesion was noticed in the last post-ablation whole-body radioiodine scan (WBIS), she reported sternal pain, which was accompanied by rapidly rising thyroglobulin levels. ¹⁸F-FDG and ⁶⁸Ga-DOTA-TATE PET/CT was performed, showing metastatic pulmonary nodules and a lytic sternal lesion with acceptable avidity (i.e. uptake ≥ liver). Following four cycles of peptide receptor radionuclide therapy (PRRT) with ¹⁷⁷Lu-DOTA-TATE, the thyroglobulin levels dropped significantly, and the sternal pain was partially alleviated. Despite only experiencing grade I thrombocytopenia, the treating physician decided to discontinue PRRT and repeat the diagnostic WBIS. Surprisingly, the scan revealed significantly increased tracer uptake in the sternum. The patient received 200 mCi ¹³¹I, and WBIS showed increased RAI uptake in all pulmonary nodules as well as bone metastases. We report a case of RAI-refractory thyroid carcinoma with a somatostatin-receptor expression that re-differentiated and gained significant RAI uptake capacity after PRRT.

Keywords: PRRT; RAI-refractory; lu177-dotatate; somatostatin receptor; thyroid cancer.

Figure 1

(A) First whole-body iodine scan after administration of 30 mCi of ¹³¹I showed post-surgical thyroid remnant. (B, C) follow-up CT scan from the thoracic region showed a lytic lesion in the sternum (blue arrow) along with multiple bilateral pulmonary nodules (green arrows). (D) The second post-treatment whole-body iodine scan after administration of 200 mCi ¹³¹I revealed no radioiodine-avidity in the tumoral lesion.

Figure 2

(A–D) ¹⁸F-FDG PET/CT MIP revealed increased tracer uptake in pulmonary nodules, sternal, and acetabular lesions (green, blue and white arrows, respectively). (E–H) A few days later, ⁶⁸Ga-DOTA-TATE PET/CT confirmed somatostatin-receptor (SSTR) uptake in the above-mentioned areas, albeit most of the pulmonary nodules were non-SSTR-avid.

Figure 3

(A) Post-treatment whole-body imaging following administration of 200 mCi of ¹⁷⁷Lu-DOTA-TATE (the second cycle is shown as an example). (B–D) Post-treatment SPECT/CT correlation showed moderate SSTR avidity in the lung metastases (E–G), while the uptake in the sternal lesion was unremarkable. Although interesting, the discordance of uptake in the pulmonary nodules and sternum between SSTR-targeted imaging (refer to figure 2) and therapy is confusing, and the underlying mechanism is unknown to us.

Figure 4

(A–D) Diagnostic WBIS following administration of 3 mCi of ¹³¹I showing regained sternal uptake. (E–H) Whole body WBIS after administration of 200 mCi of ¹³¹I depicted increased uptake in pulmonary metastases and a sternal lesion. Considering the appearance of new lesions as well as the increasing size of the main pulmonary nodules in the right middle lobe, disease progression is evident. Faint activity is also apparent in the left acetabulum (red arrowhead).