Reply: Repeatability of Tumor SUV Quantification: The Role of Variable Blood SUV

REPLY: My coauthors and I thank Drs. van den Hoff and Hofheinz for their comment. Our paper (1) focused on commonly used parameters for quantifying tumor ¹⁸F-FDG uptake (SUV_peak and SUV_max). However, we fully agree that there are several other potential ways to normalize tumor ¹⁸F-FDG uptake. Normalizing tumor SUV by arterial blood SUV is supported by tracer kinetic analysis as described by Drs. van den Hoff and Hofheinz in their letter and their previous publications. One caveat, however, is that defining a second region of interest to measure the blood activity concentration introduces an additional source of variability. Also, tumor-to-blood ratios will be more dependent than tumor SUVs on the time after injection, because the activity concentration in the blood steadily decreases with time whereas that in the tumor typically increases.

Therefore, it needs to be determined whether the repeatability of tumor-to-blood ratios is better than the repeatability of SUVs. The image data of our trial are stored at the American College of Radiology Imaging Network, and data access can be requested to evaluate the repeatability of other quantitative parameters of tumor glucose metabolism. We encourage Drs. van den Hoff and Hofheinz to apply their interesting approach to our data and compare the repeatability of SUVs and tumor-to-blood ratios.

• © 2015 by the Society of Nuclear Medicine and Molecular Imaging, Inc.