Time Sensitivity: A Parameter Reflecting Tumor Metabolic Kinetics by Variable Dual-Time F-18 FDG PET Imaging

The aim of this study was to define and investigate the time sensitivity of tumors by variable dual-time fluorodeoxyglucose positron emission tomography (FDG PET). Variable dual-time (t) protocol (P) FDG PET-computed tomography (CT) scans from 40 patients with pathologically proven head and neck tumors without brain metastasis were analyzed. The first protocol (P.I) consisted of 26 patients with early (E) and delayed (D) PET-CT obtained at 106 +/- 15 and 135 +/- 16 min after injection of 16.3 +/- 1.9 mCi FDG. The second protocol (P.II) recruited 14 patients with E- and D-PET performed at 54 +/- 13 and 151 +/- 28 min after injection of 9.6 +/- 1.7 mCi FDG. The maximum standardized uptake values (SUVs) were measured in the primary tumor (CA1) and the cerebellum (CBL). The time sensitivity (S) was defined as d{ln(SUV)}/d{ln(t)} and its value was obtained by linear regression of ln(D-SUV/E-SUV) vs ln(t (D)/t (E)). Patients with cerebellar variations greater than 30% in SUV between E- and D-PET was excluded from the analysis. Two patients from P.I were excluded due to wide cerebellar SUV variations. D-SUV were significantly higher than E-SUV in CA1 for both P.I (18.9 +/- 6.9 vs 14.8 +/- 5.6, p < 0.0005) and P.II (11.5 +/- 7.9 vs 9.7 +/- 6.9, p = 0.013). The S values for CA1 in P.I and P.II were 0.67 and 0.17, respectively. The D-SUV were also higher than E-SUV in CBL for both P.I (12.5 +/- 1.6 vs 11.6 +/- 1.6, p < 0.0005) and P.II (7.6 +/- 1.6 vs 7.0 +/- 1.6, p = 0.008). The S values for CBL in P.I and P.II were 0.47 and 0.04, respectively, which were over 1.4-fold smaller than that of CA1, suggesting fundamental kinetic differences between CA1 and CBL. The time sensitivity factor reflects another kinetic parameter of tumor metabolism besides SUV when using variable dual-time FDG PET. It offers another useful diagnostic tool in optimizing choices of dual-time protocols for oncologic PET-CT and in reducing SUV variations due to time interval differences with corrections using S.