PET response assessment in apatinib-treated radioactive iodine-refractory thyroid cancer

This work evaluated the use of the positron emission tomography (PET)/computed tomography (CT) technique to assess the early therapeutic response and predict the prognosis of patients with radioactive iodine-refractory differentiated thyroid cancer (RAIR-DTC) who underwent apatinib therapy. Standardised uptake value (SUV), metabolic tumour volume (MTV) and total lesion glycolysis (TLG), derived from ¹⁸F-FDG PET/CT and SUV from ⁶⁸Ga-NOTA-PRGD2 PET/CT were evaluated. Tumour response was evaluated using the Response Evaluation Criteria in Solid Tumors (RECIST) 1.1. Sixteen of 20 patients achieved partial response (PR) and four of 20 had stable disease (SD) after apatinib therapy. Six progression-free survival (PFS) events occurred. A strong correlation was observed between the best change in the sum of the longest diameters of target lesions (ΔCT%) and ¹⁸F-FDG PET/CT indices after the completion of the first treatment cycle (ΔMTV% (P = 0.0019), ΔTLG% (P = 0.0021) and ΔSUVmax% (P = 0.0443)). A significant difference in PFS was observed between patients with ΔMTV% <-45% and ≥-45% (P = 0.0019) and between patients with ΔTLG% <-80% and ≥-80% (P = 0.0065). Ten of 11 patients presented a decrease in SUVmax on ⁶⁸Ga-NOTA-PRGD2 PET/CT after two cycles of apatinib therapy and showed PR, whereas one patient presenting an increase in SUVmax only showed SD as the best response. When a cut-off value of the target/background ratio at -20% was used, two PFS curves showed a significant difference (P = 0.0016). Hence, early assessment by ¹⁸F-FDG and ⁶⁸Ga-NOTA-PRGD2 PET/CT was effective in the prediction and evaluation of RAIR-DTC treated with apatinib.