Table 1 Comparison of 18F-FDG and 68Ga-PSMA SUVR of reference lesions
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Fig. 1
figure 1

A 47-year-old female with papillary thyroid cancer with metastases to the lungs, spine, and right proximal femur as demonstrated on her baseline 123I whole-body anterior and posterior images (a). Her treatment history included a total thyroidectomy in 2015, decompression and fusion T8–L2 in 2015, and 3 previous radio ablations with a cumulative dose of 950 mCi 131I. On follow-up, she presented with rising thyroglobulin levels, from 3789.0 to 4240.0 ug/L. 123I whole-body anterior and posterior images (b) demonstrated minimal 123I avid disease in keeping with radioactive iodine (RAI)–refractory disease [1]. Whilst patients with differentiated thyroid cancer can expect to have a good prognosis, those with RAI-refractory disease have poorer outcomes and succumb to their condition early [2].

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Fig. 2
figure 2

18F-FDG PET/CT MIP (a) and fused axial soft tissue and lung window images (b, c) demonstrate residual thyroid tissue and lung metastases. Axial (d, e) 18F-FDG PET/CT fused bone window images demonstrated thoracic spinal and right femur metastatic lesions. The related standard uptake value ratio (SUVR) readings of lesions demonstrated in this figure are presented in Table 1. 18F-FDG PET/CT remains the recommended imaging modality in RAI-refractory disease, with an increasing sensitivity with rising thyroglobulin levels [3, 4]. Compared with the 123I whole-body scans (Fig. 1) the PET/CT images demonstrated a flip-flop phenomenon [5]. 68Ga-PSMA PET/CT, done 9 days after 18F-FDG PET/CT, MIP (f) and fused axial soft tissue and lung window (g, h) and axial skeletal window (i, j) demonstrated 68Ga-PSMA avid thyroid residual tissue, lung, and skeletal metastases. 68Ga-PSMA PET/CT images demonstrated far greater intense tracer uptake in the soft tissue and skeletal lesions compared with the 18F-FDG PET/CT scan; this was also confirmed by respective SUVR reference value comparison as noted in Table 1. Due to the fact that the injected activity for 18F-FDG is weight based whilst that for 68Ga-PSMA is fixed, SUVR rather than SUV max was the preferred semiquantitative method to compare lesion uptake between the two scans. The SUVR was calculated as a lesion to quadriceps muscle ratio using SUV mean. PSMA is a type II membrane antigen that is not only overexpressed in prostate cancer cells but has been demonstrated in other malignancies including thyroid and breast cancer as it overexpressed in tumor-associated neovasculature [6,7,8]. Tyrosine kinase inhibitors have taken a leading role in the management of patients with RAI-refractory thyroid cancer but suffer from side effects [9, 10]. To date, targeted radioligand therapy with 177Lu-PSMA has demonstrated good outcomes with limited side effects in the management of patients with metastatic castrate–resistant prostate cancer [11]. High uptake of the PSMA ligand on imaging is part of the requirements for consideration for treatment with 177Lu-PSMA radioligand therapy. To date, however, there is still limited evidence on the internalization and retention of therapeutic PSMA-labelled radioligands in de-differentiated thyroid cancer which is essential in the successful application of this radioligand treatment option for these patients. Imaging with 68Ga-PSMA in our case not only demonstrated the lesions visualized on 18F-FDG PET/CT but did so with a greater intensity suggesting possible suitability for theranostics with 177Lu-PSMA. 68Ga-PSMA PET/CT imaging in patients with RAI-refractory disease may prove to be a one-stop shop and should be considered as part of workup in these patients (Figure 2).

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