The biggest trap in follow-up scans and how to avoid it.

When reporting follow-up studies the usual practice is to compare to the most recent previous study. Although generally this is a safe thing to do, with increased frequency of follow up exams in many disciplines, one runs the risk of not detecting subtle growth. If this occurs repeatedly then a tumor that may have significantly increased over time will have a series of reports all of which state “no change”.

As an example (Fig 1) look at 4 studies in a patient with a partially excised low grade glioma. Each study (A to D) is approximately 6 months apart. Note how hard it is to discern a change between adjacent pairs.

We simply cannot reliably detect subtle change. How much is below our change threshold will depend on many factors, such as the size and shape of the lesion, scan parameters and partial volume effect, slice position and patient position etc… Regardless, there is an amount below which we simply won’t be convinced that any actual change has taken place. This is obvious if you consider what would happen if you were to scan a patient every day. Even the fasted growing mass would look unaltered on sequential scans.

So what is the solution?

Look at older scans and consider what you expect the biological behavior of the process you are looking at to be; the more indolent the process the longer interval you need between scan pairs to detect change.

In the same patient as before, see how easy it is to see that there has been change when comparing scans 2 years apart (Fig 2).

My practice when assessing gliomas for example, is to look at the most recent scan, and then at the oldest valid comparison; one which does not have intervening surgery, and is not in the  immediate postoperative period.

You are then left with three possible outcomes from such a comparison:

1. change is obvious even when just compared to the most recent scan
2. no change when compared to the recent scan but some change when compared to the oldest scan
3. no change when compared to both the recent and oldest scan

In the setting of obvious change, there is no problem, and in fact there is no need to look at older scans.

If there is, however, no change compared to the recent study but change is evident when compared to the older scan, I usually pull up a few of the intervening scans, to try and assess whether growth is gradual, or something has changed in the behavior of this tumor, suggesting dedifferentiation into a higher grade. After all a tumor that had been stable for years but suddenly starts to grow needs, at the very least, closer follow-up and probably also needs to be considered for a change in management. My conclusion then reflects this distinction; e.g. “Although there is little discernable change when compared to the most recent study, when compared to multiple previous studies dating back 4 years, slow steady growth is evident.”

Only if there is no change when compared to the most recent scan, and no change when compared to the oldest scan does my conclusion read “Stable”, and then I append the time period over which I am claiming no change to have occurred; e.g. “Stable, with no appreciable growth over the past 24 months”.

This approach is valid to all comparison studies, regardless of system or underlying pathology. I hope this approach is useful to you, and will stop you from merely concluding with “stable”.