Tumor size alone not always best for gauging treatment response

Both PET and CT useful for judging biologic changes

Not only can positron emission tomography (PET) help evaluate treatment for gastrointestinal stromal tumors (GIST) by revealing biologic changes such as how the tumor processes the fuel that makes it grow, but CT can indirectly reveal biologic changes as well by analyzing the tumor’s density, say researchers from The University of Texas M. D. Anderson Cancer Center in Houston.

For the study, researchers evaluated 173 GISTs–tumors that usually begin in the wall of the GI tract–in 36 patients who had undergone a protein enzyme inhibitor treatment, 29 of which underwent both CT and PET. In those 29 patients, the researchers found a high correlation between the tumor responses based on tumor density and changes in internal characteristics of the tumors shown on CT and the changes in glucose metabolism of the tumors shown on PET.

In general practice, CT is used to gauge tumor response to treatment by revealing changes in the size of the tumor, an anatomic change as opposed to a biologic one, said Haesun Choi, MD, lead author of the study. “Since anticancer therapy is focusing more toward targeting a particular molecule or receptors at a cellular level, we should look into evaluating biologic changes, too, rather than just anatomic changes,” she said.

Current guidelines require monitoring only tumor size to judge if a treatment is working, a method which, according to the study, proved to be unreliable. “Understanding this new concept of tumor response will help ensure that patients are always undergoing the right treatment,” said Dr. Choi.