Within-Patient Comparison of [¹⁸F]FC303 ([¹⁸F]Florastamin) and [⁶⁸Ga]Ga-PSMA-11 PET/CT in Prostate Cancer: Protocol-Conditional Biodistribution and Quantitative Non-Interchangeability
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Background
Prostate-specific membrane antigen (PSMA) PET/CT is central to prostate cancer staging and theranostic workflows. To our knowledge, no direct within-patient comparison of [¹⁸F]FC303 ([¹⁸F]Florastamin) and [⁶⁸Ga]Ga-PSMA-11 has been reported. We performed a preliminary paired method-comparison study under non-harmonized acquisition protocols.
Patients and Methods
Twenty patients with histologically confirmed prostate cancer underwent [⁶⁸Ga]Ga-PSMA-11 PET/CT (185 ± 37 MBq, 60 ± 10 min) followed by [¹⁸F]FC303 PET/CT (370 ± 37 MBq, 105 ± 15 min) on the same PET/CT system within each patient (median interval, 29.5 days). Index targets were anatomically matched to the biopsied or surgically sampled lesion or target region. The primary malignant set included 18 histologically malignant targets; two histology-negative or indeterminate targets were included only in sensitivity analysis. Fixed [⁶⁸Ga]Ga-PSMA-11-first scan order and the 45-min uptake-time difference were central interpretive constraints.
Results
Across five predefined reference organs, [¹⁸F]FC303 showed lower SUVmean than [⁶⁸Ga]Ga-PSMA-11 (all Benjamini–Hochberg-adjusted p < 0.001; [⁶⁸Ga]/[¹⁸F]FC303 geometric mean ratio [GMR], 1.29–3.89). In the primary malignant set, [¹⁸F]FC303 lesion SUVmax was lower than [⁶⁸Ga]Ga-PSMA-11 (median, 11.3 vs 18.1; paired median difference, −5.50; 95% CI, −6.85 to −2.90; Wilcoxon p = 8.4 × 10⁻⁴), with strong rank correlation (Spearman ρ = 0.90). Passing–Bablok regression yielded β = 1.13 (95% CI, 1.04–1.45), and log-Bland–Altman GMR (FC303/[⁶⁸Ga]) was 0.75, consistent with proportional non-interchangeability. Tumor-to-liver and tumor-to-mediastinum ratios did not differ significantly (GMR, 1.17 [95% CI, 0.94–1.45] and 0.96 [0.80–1.15], respectively); the study was not powered for equivalence. The n = 20 sensitivity analysis showed consistent directionality.
Conclusions
Under non-harmonized acquisition conditions, [¹⁸F]FC303 showed lower physiologic reference-organ SUVmean and malignant target-region SUVmax than [⁶⁸Ga]Ga-PSMA-11, whereas tumor-to-liver and tumor-to-mediastinum ratios were not significantly different. Absolute SUVs were not interchangeable; [⁶⁸Ga]Ga-PSMA-11-derived SUV thresholds should not be directly transferred to [¹⁸F]FC303 without tracer-specific calibration.
