Evaluating the prognostic impact of multiple ¹⁸FDG-PET imaging parameters in small-cell lung cancer: Insights from the long-term analysis of the CONVERT trial.

Abstract

BACKGROUND: The prognostic value of baseline 18F-FDG PET/CT-derived metrics in limited-stage small-cell lung cancer (LS-SCLC) remains uncertain. We evaluated whether baseline PET-derived parameters provide prognostic information beyond tumour volume and clinical factors in patients treated with curative-intent chemoradiotherapy within the phase III CONVERT trial. METHODS: CONVERT was an international, multi-centre randomised controlled trial. This study is a post-hoc analysis of baseline 18F-FDG PET/CT from the multiple PET imaging parameter (MPI) cohort across 8 UK sites. PET metrics, including whole-body metabolic tumour volume (MTV), total lesion glycolysis (TLG), SUV-based measures, and radiomic features, were derived under EARL-accredited conditions. CT-derived gross tumour volume (GTV) and a pre-specified clinical prognostic model (CPM) were assessed alongside PET metrics. Overall survival (OS) and progression-free survival (PFS) were analysed using Cox regression. Incremental prognostic value beyond CPM was evaluated using Harrell’s concordance index (C-index). Correlations between imaging, clinical, and circulating tumour cell (CTC) variables were explored. RESULTS: Ninety-four patients comprised the MPI cohort. In univariable analyses, CPM, CT-GTV, and PET-derived MTV and TLG, were associated with shorter OS and PFS. After multivariable adjustment, only CT-GTV remained independently prognostic for OS (HR 1.36, 95% CI 1.06–1.74) and PFS (HR 1.29, 95% CI 1.03–1.63). CPM demonstrated moderate prognostic discrimination (C-index 0.63 for OS; 0.59 for PFS). Addition of SUV-based PET metrics did not improve discrimination beyond CPM (ΔC-index ≤ 0.002). PET-derived volumetric parameters were strongly correlated with CT-GTV and CPM, indicating redundancy. Circulating tumour cell counts, available in a subset, showed no significant correlations. CONCLUSIONS: In LS-SCLC treated with curative-intent chemoradiotherapy, baseline tumour volume is the dominant prognostic determinant. Baseline 18F-FDG PET-derived metabolic and radiomic parameters do not provide meaningful incremental prognostic value beyond tumour burden and clinical factors. While PET/CT remains essential for staging and treatment selection, its role in baseline prognostic stratification appears limited.