Prognostic role of metabolic parameters of ¹⁸F-FDG PET-CT scan performed during radiation therapy in locally advanced head and neck squamous cell carcinoma

Purpose: To evaluate the prognostic value of ¹⁸F-FDG PET-CT performed in the third week (iPET) of definitive radiation therapy (RT) in patients with newly diagnosed locally advanced mucosal primary head and neck squamous-cell-carcinoma (MPHNSCC). Methodology: Seventy-two patients with MPHNSCC treated with radical RT underwent staging PET-CT and iPET. The maximum standardised uptake value (SUV_max), metabolic tumour volume (MTV) and total lesional glycolysis (TLG) of primary tumour (PT) and index node (IN) [defined as lymph node(s) with highest TLG] were analysed, and results were correlated with loco-regional recurrence-free survival (LRFS), disease-free survival (DFS), metastatic failure-free survival(MFFS) and overall survival (OS), using Kaplan-Meier analysis. Results: Optimal cutoffs (OC) were derived from receiver operating characteristic curves: SUV_max-PT=4.25 g/mL, MTV_PT=3.3 cm³, TLG_PT=9.4 g, for PT, and SUV_max-IN=4.05 g/mL, MTV_IN=1.85 cm³ and TLG_IN=7.95 g for IN. Low metabolic values in iPET for PT below OC were associated with statistically significant better LRFS and DFS. TLG was the best predictor of outcome with 2-year LRFS of 92.7 % vs. 71.1% [p=0.005, compared with SUV_max (p=0.03) and MTV (p=0.022)], DFS of 85.9% vs. 60.8% [p=0.005, compared with SUV_max (p=0.025) and MTV (p=0.018)], MFFS of 85.9% vs. 83.7% [p=0.488, compared with SUV_max (p=0.52) and MTV (p=0.436)], and OS of 81.1% vs. 75.0% [p=0.279, compared with SUV_max (p=0.345) and MTV (p=0.512)]. There were no significant associations between the percentage reduction of primary tumour metabolic parameters and outcomes. In patients with nodal disease, metabolic parameters below OC (for both PT and IN) were significantly associated with all oncological outcomes, while TLG was again the best predictor: LRFS of 84.0% vs. 55.3% (p=0.017), DFS of 79.4% vs. 38.6% (p=0.001), MFFS 86.4% vs. 68.2% (p=0.034) and OS 80.4% vs. 55.7% (p=0.045). Conclusion: The metabolic parameters of iPET can be useful predictors of patient outcome and potentially have a role in adaptive therapy for MPHNSCC. Among the three parameters, TLG was found to be the best prognostic indicator of oncological outcomes.