Temporal profile of fluorodeoxyglucose uptake in malignant lesions and normal organs over extended time periods in patients with lung carcinoma: implications for its utilization in assessing malignant lesions

Aim. In this study, the fluorodeoxyglucose (FDG) uptake was prospectively investigated in tumors as well as the normal organs over 8 h in patients with non small cell lung carcinoma (NSCLC). The intent of this study was to collect positron emission tomography (PET) data with regard to the time course of FDG uptake in the primary and metastatic sites and the normal tissues over extended time periods (up to 8 h) after intravenous FDG injection in patients. Methods. Three patients (2 males, 1 female; mean age: 64 years; age range: 57-76 years) with the diagnosis of NSCLC underwent a series of whole body FDG-PET at several time points, beginning at 5 min and extending up to 8 h after the intravenous administration of FDG. We calculated the standardized uptake values (SU Vmax) in the malignant lesions and all organs. SU Vmax was calculated over contiguous slices and the highest value was considered for the analysis. Similar locations were used for the placement of regions of interest in subsequent images. Time activity curves (TACs) were generated utilizing these SU V values for each of these sites. The ratios of the SU Vmax of the malignant lesions to those of normal organs (viz. lung and liver) at specific time points were also calculated and the TACs for these ratios were generated. The blood and plasma decay curves of 18 F activity over time were generated based on the counts obtained from blood sample analysis. The ratios of 18 F activity of blood to plasma were also calculated and the TACs of this ratio were generated. Results. The observed mean SU Vmax at different time points (5 min, 1 h, 2 h, 4 h, 6 h and 8 h) in the organs and the lesions were as follows: 1) heart: 2.9, 2, 1.9, 1.6, 1.3, 1.5; 2) kidney: 3.3, 3.5, 2.6, 2.1, 2, 2; 3) liver: 3.9, 2.2, 1.9, 1.6, 1.5, 1.8; 4) lung: 0.6, 0.5, 0.4, 0.4, 0.4, 0.4, 0.4; 5) large bowel: 2.1, 1.4, 1.8, 1.4, 2, 2.2; 6) small bowel: 2.6, 1.6, 1.4, 1.2, 1.3, 1.5; 7) lung neoplasm: 3.7, 5.1, 6.1, 6.8, 6.9, 6.8; 8) mediastinal lesion 1: 6.8, 8.8, 13, 12.7, 13.8, 12.5; 9) mediastinal lesion 2: 5.5, 8.6, 10.7, 13.2, 11.7, 12.1; 10) adrenal metastasis (starting at 1 h): 3.3, 3.7, 4.7, 4.7, 4.7; 11) right iliac metastasis: 2.6, 2.6, 3.1, 3.5, 3.4. For the right iliac metastasis, we had SU Vmax up to 6 h. The SU Vmax ratios of malignant lesions to those of normal lung and liver and their TACs demonstrate initial rise followed by a delayed plateau. Increasing 18 F count ratios of blood to plasma with time was observed in 2 patients where these data were available. Conclusion. The results from this preliminary study indicate that while the tumor sites show increased uptake of FDG during the course of 8 h, surrounding normal tissues demonstrate declining or stable values with time. This would indicate increasing contrast between the lesion and the background and, therefore, possibly improved sensitivity of the test. While the high SU V at 5 min can be explained by the blood pool activity in the organs and the malignant lesions, the SU Vmax values at the later times decreases or remains the same in normal organs. The observation on the different slopes of the curves among the various malignant lesions can be partly explained by the well known seed and soil theory in cancer biology. The finding of continued increases in the blood to plasma count ratios of 18 F activity is also noteworthy and most likely reflects Glut-1 mediated glucose transport into red blood cells.