PET could become go-to modality for gauging response to therapy

June 1, 2005

It's always exciting to report advancements in imaging that have tremendous potential to improve the quality of patient care. Such advancements seem to be imminent as researchers explore the possibility of using PET imaging to assess early response to cancer therapy.

It's always exciting to report advancements in imaging that have tremendous potential to improve the quality of patient care. Such advancements seem to be imminent as researchers explore the possibility of using PET imaging to assess early response to cancer therapy.

As documented by Senior Editor James Brice in our cover story ("PET identifies early response to cancer therapy," page 42), PET could become the go-to modality for assessing how well strategies such as chemotherapy are succeeding in killing tumors. In some cases, PET imaging has managed to trim weeks off the time needed to determine whether a cancer therapy is actually working.

Until now, the best guesses at whether therapy was on track were based on anatomic observation, usually with CT. If the tumor was shrinking, we assumed it was responding to therapy. The problem is that it sometimes takes months to document tumor shrinkage. Also, new drugs are killing tumors without always shrinking them. In the meantime, patients often remain on expensive and sometimes uncomfortable therapeutic regimens. And if it turns out in the end that the therapy is not working, it may be too late to try something else.

PET has the potential to change all that. A functional modality, it reveals the tumor's metabolic activity. FDG, the old PET standby, is being put to this use, but new PET probes are in development that may prove superior in specific applications. Another refinement is the use of the standard uptake value (SUV) measurement to better quantify the tumor's metabolic activity.

In the best case, this will give the oncologist and the patient an early appraisal of how a therapy is working as well as how best to manage the therapy, including what type, how much, how often, and when it has been successful and can be ended. These are important considerations at a time when therapeutic options are increasing.

All these potential benefits, however, need to be tempered with some cold realism, the biggest element of which is that oncologists need to be absolutely convinced that their therapeutic decisions are based on the best available evidence. That will take a lot of statistically valid, evidence-based research that is now just in preliminary stages.

Several other factors will play a role in shaping PET's impact on early assessment of therapy.

PET tends to be more expensive than CT, the predominant modality for evaluating changes in tumor size. Imaging costs are receiving increasing attention, and modality-to-modality comparisons will become more important. If a particular PET scan can suggest which therapy is best and save thousands of dollars in treatment costs, it's a clear winner-but not if CT or MR can do the same thing at less cost.

New cancer therapies are appearing every day. It wasn't that long ago that oncologists would try one round of therapy, and it would work or not. Now second-, third-, and even fourth-line therapies are being proposed. Some of them are very expensive, with costs running into the tens of thousands of dollars. The ability to assess early response to these therapies can be a cost issue as well as a treatment issue.

How PET will fare within this mix remains to be determined, but it seems to have established a solid foundation on which to build. We've known for some time that PET is a great modality for diagnosing and staging cancer. Now we see that PET can be just as valuable in quantifying the processes that underlie tumor growth and is perhaps the most direct measure to do so.