The dose uptake ratio as an index of glucose metabolism: useful parameter or oversimplification?

The dose uptake ratio (DUR) has been used as a quantitative index of glucose metabolism for tumor classification and monitoring response to treatment. In order to provide consistent results, DUR measurements should be made when the concentration of tracer has reached a plateau. The time of this plateau cannot be identified from a single static acquisition.

In this study, we investigated the changes in DUR as a function of time in eight patients with stage III lung cancer. All patients underwent a quantitative dynamic 18F-FDG PET study before and after treatment and the data were analyzed with a three-compartment model. Using the fitted model parameters, the DUR was predicted at the plateau and intermediate times.

Tumor concentrations of 18F-FDG did not reach a plateau within the 90 min of imaging in any of the pre-treatment studies and only in one case post-treatment. The average time to reach 95% of the plateau value pre-treatment was 298 +/- 42 min (range: 130-500 min); in post-treatment, it was 154 +/- 31 min (range: 65-240 min). The difference between the plateau DUR and the 60-min value was 46% +/- 6% pre-treatment and 17% +/- 5% post-treatment.

These data indicate that DUR can vary widely with the time of measurement and that DUR should be interpreted with caution in any individual patient.