Evaluation of third treatment week as temporal window for assessing responsiveness on repeated FDG-PET-CT scans in...

Early assessment of tumour response to treatment with repeated FDG-PET-CT imaging has potential for treatment adaptation but it is unclear what the optimal time window for this evaluation is. Previous studies indicate that changes in SUV_mean and the effective radiosensitivity (α_eff, accounting for uptake variations and accumulated dose until the second FDG-PET-CT scan) are predictive of 2-year overall survival (OS) when imaging is performed before radiotherapy and during the second week. This study aims to investigate if multiple FDG-PET-derived quantities determined during the third treatment week have stronger predictive power.

Twenty-eight lung cancer patients were imaged with FDG-PET-CT before radiotherapy (PET1) and during the third week (PET2). SUV_mean, SUV_max, SUV_peak, MTV41%-50% (Metabolic Tumour Volume), TLG41%-50% (Total Lesion Glycolysis) in PET1 and PET2 and their change (), as well as average α_eff (α¯_eff) and the negative fraction of α_eff values [Formula: see text] ) were determined. Correlations were sought between FDG-PET-derived quantities and OS with ROC analysis.

Neither SUV_mean, SUV_max, SUV_peak in PET1 and PET2 (AUC = 0.5-0.6), nor their changes (AUC = 0.5-0.6) were significant for outcome prediction purposes. Lack of correlation with OS was also found for α¯_eff (AUC = 0.5) and [Formula: see text] (AUC = 0.5). Threshold-based quantities (MTV41%-50%, TLG41%-50%) and their changes had AUC = 0.5-0.7. P-values were in all cases ≫0.05.

The poor OS predictive power of the quantities determined from repeated FDG-PET-CT images indicates that the third week of treatment might not be suitable for treatment response assessment. Comparatively, the second week during the treatment appears to be a better time window.