18F-FDG PET Assessment of Malignant Pleural Mesothelioma: Total Lesion Volume and Total Lesion Glycolysis—The Central Role of Volume

Cancer survival is related to tumor volume. (18)F-FDG PET measurement of tumor volume holds promise but is not yet a clinical tool. Measurements come in 2 forms: the first is total lesion volume (TLV) based on the number of voxels in the tumor, and the second is total lesion glycolysis (TLG), which is the TLV multiplied by the average SUL (i.e., SUV normalized for lean mass) of the tumor (SUL(average)). In this study, we measured tumor volume in patients with malignant pleural mesothelioma (MPM). Methods: A threshold-based program in Interactive Data Language was developed to measure tumor volume in (18)F-FDG PET images. Nineteen patients with MPM were studied before and after 2 cycles (6 wk) of chemoimmunotherapy. Measurements included TLV, TLG, the sum of the SULs in the tumor (SUL(total), a measure of total (18)F-FDG uptake), and SUL(average). Results: Baseline TLV ranged from 11 to 2,610 cm(3). TLG ranged from 32 to 8,552 cm(3) g/mL and correlated strongly with TLV. Although tumor volumes ranged over 3 orders of magnitude, SUL(average) stayed within a narrow range of 2.4–5.3 units. Thus, TLV was the major component of TLG, whereas SUL(average) was a minor component and was essentially constant. Further evaluation of SUL(average) showed that in this cohort its 2 components, SUL(total) and TLV, changed in parallel and were strongly correlated (r = 0.99, P < 0.01). Thus, whether the tumors were large or small, (18)F-FDG uptake as measured by SUL(total) was proportional to the TLV. Conclusion: TLG equals TLV multiplied by SUL(average), essentially TLV multiplied by a constant. Thus TLG, commonly considered a measure of metabolic activity in tumors, is also in this cohort a measure of tumor volume. The constancy of SUL(average) is due to the fact that (18)F-FDG uptake is proportional to tumor volume. Thus, in this study, (18)F-FDG uptake was also a measure of volume.