Reply: Comment on: “Tumor Aggressiveness and Patient Outcome in Cancer of the Pancreas Assessed by Dynamic ¹⁸F-FDG PET/CT”

REPLY: We would like to thank Dr. Salavati and his coauthors for the interesting comment on our study (1). As they mentioned, dynamic PET and PET/CT are more time-consuming and at the moment are therefore confined to research projects for scientific purposes. Furthermore, dynamic PET/CT requires dedicated evaluation software. However, the introduction of new-generation PET/CT scanners has reduced the total acquisition time because of, for example, new detector materials such as lutetium oxyorthosilicate, which improves the counting rate performance, as well as 3-dimensional acquisition protocols. Moreover, new-generation PET/CT scanners also allow dynamic (list-mode) multibed acquisitions. In the future, this technologic improvement will allow for dynamic partial-body PET/CT studies without the need for additional bed positions in static mode, with a shorter acquisition than in our study (2). We agree that an additional limitation hampering the use of dynamic protocols in a clinical environment is the lack of operator-friendly and robust evaluation software—an omission that will hopefully be addressed by manufacturers. The existing software for calculation of transport rates is based on a 2-tissue-compartment model for oncologic studies. This software is not robust enough because it is based on an iterative fitting, like the Levenberg–Marquardt algorithm. We presented a solution that is based on the use of an oncologic reference database and a support vector machine algorithm (3). Routine use of dynamic PET/CT requires that the calculated rates be reproducible—a problem that should be solved in the future.

Ludwig Strauss proposed at the end of the 1980s the use of the standardized uptake value (SUV) as a robust value that can easily be calculated for the evaluation of PET data (4). SUVs are widely used and lead to good results, provided that the values are acquired under standardized conditions, such as at a defined time point after tracer injection, with glucose levels within the normal range, and with the same reconstruction algorithms. John W. Keyes, Jr., wrote an interesting paper in The Journal of Nuclear Medicine in 1995 titled “SUV: Standard Uptake or Silly Useless Value?” In this paper he doubted the usefulness of SUV and discussed the limitations of this semiquantitative approach in detail (5). Nineteen years later, everybody uses the SUV or its derivatives (such as SUVmax, SUVlean, or even total lesion glycolysis) as a first quantitative approach. It remains to be seen how silly or useless dynamic multibed PET/CT (including parametric imaging) in oncology will be in the future.

Dynamic imaging allows the registration of tracer kinetics over time instead of at only one time point after the tracer injection as static images do. Pharmacokinetic studies are helpful not only for the evaluation of new tracers but also for the evaluation of small therapeutic effects, such as the use of ¹⁸F-FDG early after the onset of chemotherapy. Furthermore, the use of kinetic parameters may help to differentiate between benign and less aggressive tumors (e.g., lipomas from low-grade liposarcomas) (6). In a recent paper, we demonstrated a correlation between k₁ and angiogenesis-related genes (7). Based on dynamic datasets, parametric imaging can be applied using different algorithms. Parametric images allow the visualization of dedicated parameters of radiopharmaceutical kinetics, such as perfusion, transport, or phosphorylation in the case of ¹⁸F-FDG. Karakatsanis et al. recently presented some aspects of the use of whole-body PET parametric imaging and, for example, Patlak analysis in addition to SUVs for tumor diagnosis and therapy response monitoring (8).

We agree that several approaches available today may be used for the evaluation of oncologic ¹⁸F-FDG imaging, including metabolic tumor volume and total lesion glycolysis. We decided to use an analysis based primarily on the pharmacokinetic data, and this proved to be successful. We hope our colleagues will succeed as well in future using any other approach they may wish to choose.

• © 2014 by the Society of Nuclear Medicine and Molecular Imaging, Inc.