Blood glucose level normalization and accurate timing improves the accuracy of PET-based treatment response predictions in rectal cancer

Purpose: To quantify the influence of fluctuating blood glucose level (BGLs) and the timing of PET acquisition on PET-based predictions of the pathological treatment response in rectal cancer. Material and methods: Thirty patients, diagnosed with locally advanced-rectal-cancer (LARC), were included in this prospective study. Sequential FDG-PET-CT investigations were performed at four time points during and after pre-operative radiochemotherapy (RCT). All PET-data were normalized for the BGL measured shortly before FDG injection. The metabolic treatment response of the tumor was correlated with the pathological treatment response. Results: During RCT, strong intra-patient BGL-fluctuations were observed, ranging from -38.7 to 95.6%. BGL-normalization of the SUVs revealed differences ranging from -54.7 to 34.7% (p < 0.001). Also, a SUVmax time-dependency of 1.30 ± 0.66 every 10 min (range: 0.39-2.58) was found during the first 60 min of acquisition. When correlating the percent reduction of SUVmax after 2 weeks of RCT with the pathological treatment response, a significant increase (p = 0.027) in the area under the curve of ROC-curve analysis was found when normalizing the PET-data for the measured BGLs, indicating an increase of the predictive strength. Conclusions: This study strongly underlines the necessity of BGL-normalization of PET-data and a precise time-management between FDG injection and the start of PET acquisition when using sequential FDG-PET-CT imaging for the prediction of pathological treatment response.