Visual and Quantitative ¹⁸F-FDG PET Tumor-liver Ratio in Radioiodine Refractory Differentiated Thyroid Cancer:...

The progression rate and course of radioiodine refractory differentiated thyroid cancer (RAIR-DTC) vary significantly, yet there lacks a precise method for predicting its progression. We hypothesized that using the liver as a reference organ can enable selective patient stratification. We aimed to establish fluorine-18 fluorodeoxyglucose positron emission tomography ([¹⁸F] FDG-PET) tumor to-liver ratio (TLR score) to predict outcome for RAIR-DTC.

This study included 64 patients with RAIR-DTC undergoing baseline ¹⁸F-FDG PET/CT. Patients were categorized by visual TLR (vTLR) into high (most lesions show higher uptake than the liver) or low (most lesions show lower uptake than the liver) groups using 3D maximum intensity projection (MIP) images. Quantitative TLR (qTLR) scores, including qTLR max (tumor SUVmax/liver SUVmax) and qTLRmean (tumor SUVmean/liver SUVmean), were semiautomatically derived from baseline PET, with high (≥ 1.5) and low (< 1.5) groups defined. Outcome data were progression-free survival (PFS).

Among 64 patients, the distribution of high-TLR versus low-TLR groups varied across scoring methods: vTLR score allocated 36 (56.3%) high vs 28 (43.7%) low, qTLRmax score identified 29 (45.3%) high vs 35 (54.7%) low, and qTLRmean score demonstrated the most divergent pattern with 21 (32.8%) high vs 43 (67.2%) low. Agreement among qTLRmax, vTLR and qTLRmean score was moderate (Fleiss weighted k, 0.579). The median PFS of the high and low groups by vTLR score was 16.0, 29.0 months (P = 0.010) respectively, by qTLRmax score was 14.0, 27.0 months (P = 0.041), respectively, by qTLRmean score was 14.0, 28.0 months (P = 0.004), respectively.

The TLR score was prognostic for PFS of RAIR-DTC. The vTLR score assessed on 3D MIP PET images yielded substantial reproducibility and combining qTLR score provided reliable prognostic value.