How to Compare Sequential Exams with Very Different Background Activities

Further complicating the comparison of SUV’s on subsequent exams is the fact that the background metabolic activity of the two exams may be significantly different.

For example, if the baseline background activity on a follow up exam is much higher than on the prior exam, we should expect all structures (including malignant lesions) on the second exam to have higher SUV values than the structures on the first exam.

In such cases, readers must be exceedingly cautious in the manner in which SUV’s are reported.

Example:

If only the SUV’s of the lung mass are analyzed, the lesion appears unchanged, with a stable SUV of 2.4

If, however, background metabolic activity is also analyzed, we see that the second exam’s background activity is 50% higher than on the first exam – and that the lesion’s metabolic activity is no longer above this background uptake, as it was on the first exam.

Consequently, this mass actually demonstrates marked improvement, despite no change in its calculated SUV. [Fig.1]  [Fig.2]

HOW THEN DO WE COMPARE SUCH CASES?

In such cases, we recommend a qualitative comparison of the lesion by visual assessment (after manually equalizing liver intensity on both exams on one’s reading monitors). The beginning of the report should clearly explain why comparison will be made in this manner:

Due to significant differences in background metabolic activity between today’s study and the prior exam, comparison of metabolic activity is best made qualitatively rather than quantitatively.”

For the example above, we would report that “the lung mass has markedly decreased in intensity, no longer demonstrating uptake above background metabolic activity within the liver.”