Time Sensitivity Factor of Single Pulmonary Nodule: A New Cancer Characteristic Metabolic Parameter by (18) F-FDG PET

Objective. To calculate the time sensitivity factor (S) for discriminating the solitary pulmonary nodule (SPN) by FDG PET at different time points. Methods. The multiple time-point FDG PET images from 41 patients for evaluating SPN seen on chest X-ray or CT were prospectively analyzed to calculate and evaluate S against the gold standard of tissue histology (n = 38) or long term clinicoradiographic follow-up (n = 3). The maximal standardized uptake values (SUV) at the 3 hourly time points were measured. The S was calculated using S = d{ln⁡(SUV)}/d{ln⁡(t)} at 3 different time intervals. ROC analysis of the S parameters was performed to evaluate the optimal cut-off value and their accuracy in classifying the SPN. Results. The SUV in malignant SPN was higher than the corresponding value in benign lesions at all 3 hourly time points (P < 0.003). The S parameters using 3 different time intervals all significantly separated the two groups (P < 0.0005) with an optimal cut-off point near the theoretical value of zero with a high sensitivity of 100% and specificity of 86%. Conclusion. The S can be calculated for SPNs using multiple time-point FDG PET, providing a tumor characteristic metabolic parameter with high discrimination power using a simple positive value representing malignancy.