Extra-thoracic tumor burden but not thoracic tumor burden on ¹⁸F-FDG PET/CT is an independent prognostic biomarker for extensive-disease small cell lung cancer

Purpose: The aim of this study was to evaluate the relationship and difference in prognostic significance between whole-body tumor burden, thoracic tumor burden, and extra-thoracic tumor burden on ¹⁸F-FDG PET/CT for patients with extensive-disease small cell lung cancer (ED-SCLC). Materials and methods: We performed a retrospective, two-center analysis for patients with ED-SCLC who underwent pretreatment ¹⁸F-FDG PET/CT. Metabolic tumor burden was estimated using whole-body metabolic tumor volume (MTV_WB), thoracic metabolic tumor volume (MTV_TRX), extra-thoracic metabolic tumor volume (MTV_EXT), and the number of extra-thoracic tumor foci. Uni- and multivariate analyses were performed using various clinical factors and the metabolic indices. Results: A total of 91 patients were eligible for this study. MTV_WB showed stronger correlation with MTV_EXT than MTV_TRX (r²=0.804 vs. 0.132, p<0.001, both), whereas no correlation was observed between MTV_EXT and MTV_TRX (r²=0.007, p=0.428). Patients with smaller MTV_WB, MTV_EXT, and extra-thoracic tumor foci showed longer survival than patients with larger MTV_WB, MTV_EXT, and extra-thoracic tumor foci, respectively, whereas the survival difference between patients with smaller MTV_TRX and those with larger MTV_TRX was not significant. Results of uni- and multivariate analyses showed that ECOG performance status (HR=2.31, p=0.015), initial chemotherapy cycles (HR=0.24, p<0.001), and the number of extra-thoracic tumor foci (HR=2.75, p<0.001) were independent prognostic factors for overall survival, and initial chemotherapy cycles (HR=0.25, p<0.001), and MTV_EXT (HR=2.04, p=0.013) were independent prognostic factors for progression-free survival. Conclusion: These data provide evidence indicating that extra-thoracic tumor burden but not thoracic tumor burden is an independent prognostic biomarker for ED-SCLC, and support further exploration of novel treatment strategies targeting extra-thoracic tumor burden in order to improve the clinical outcomes of patients with ED-SCLC.