The complementary role of PSMA expression and [¹⁸F]FDG PET/CT in predicting thyroid cancer outcome: from black and white to shades of gray, in the era of precision oncology

Background: The value of Prostate Specific Membrane Antigen (PSMA) in thyroid carcinoma (TC) is still unknown. We aimed to test the potential complementary role of PSMA expression and 2-[¹⁸F]fluoro-2-deoxy-D-glucose ([¹⁸F]FDG) uptake on PET/CT as biomarkers for TC outcome prediction.

Materials and methods: From a retrospective cohort of TC patients we selected those fulfilling the following inclusion/exclusion criteria: thyroidectomy in our Institution, available primary tumor tissue PSMA immunostaining, [¹⁸F]FDG PET/CT and follow-up data. PSMA staining was visually assessed. PET/CT was considered positive in case of [¹⁸F]FDG uptake higher than the background at the site of TC confirmed by cyto-/histology, and/or follow-up. Disease recurrence, radioiodine refractoriness (RAI-R) and status at last follow-up (LFU) were used as outcome endpoints.

Results: We included 23 subjects. Disease recurrence occurred in 18 patients (median time 11 months, range 1-40); among these 12/18 developed RAI-R (median time 28 months, range 2-221), and 13/18 had evidence of disease at LFU. PSMA expression was negative in 6/23 cases. PET/CT was negative in 11/23 patients (7/11 experienced recurrence). PET/CT was positive in 9/12 patients showing RAI-R and 10/13 cases with evidence of disease at LFU. All patients with positive PET/CT had a positive PSMA immunostaining. Six out of 11 patients with negative PET/CT were positive at immunostaining, showing lower PSMA expression (median score of 30%, range 0-80%) than patients with positive PET/CT. The TC samples without PSMA expression belonged to patients who resulted negative also at PET/CT (3 experienced recurrence, 2 were RAI-R, and 1 had disease at LFU). Four out of 11 patients who resulted negative at PET/CT exhibited very high PSMA expression (≥ 70%) and although 3 of them experienced recurrence, none resulted RAI-R, and only 1 had persistent disease at LFU.

Conclusions: Primary tumor PSMA expression and [¹⁸F]FDG uptake seem to play a complementary prognostic role in TC. The majority of patients who expressed PSMA recurred. In the intermediate ATA risk class, patients with negative PSMA immunostaining recurred less than patients expressing PSMA. Additionally, although patients with a negative [¹⁸F]FDG PET/CT had a favourable long-term outcome, PSMA assessment might be useful to timely identify subjects at higher risk of recurrence.