“We’re pretty sure that if you did it before eight weeks you will be badly confounded by false positives,” he said. “The closer that PET is to the completion of treatment, the more likely that you won’t have a false positive.” In fact, he said, even histology may be misleading if it’s performed as soon as six to eight weeks of finishing therapy.
Explore this issue:May 2012
The longer the wait before PET/CT is done, the more reliable the results will be, but that presents a problem, he said. “If you wait four months… the problem that we’ve all struggled with is delay,” he said. “I mean, we worry if we wait a long time we will lose the opportunity to cure the patient. And wouldn’t it be better to find that persistent disease sooner rather than later?”
Still, he said, he is unconvinced of the benefit of earlier imaging. “[With] these persistent cancers that have not [completely] responded to the chemoradiation, finding them early has not been demonstrated to change outcome.”
The question of how long PET/CT is needed in surveillance is not known, he said. But his group in Pittsburgh does them every three months. When it’s okay to stop doing PET/CT for surveillance is similarly undefined, he said.
“Once again, the data are not out there,” he said. “All of us have to struggle with this as we go along.”Generally, about 18 months is good guideline, he said.
Dr. Johnson said caution is important when a tumor is found that is negative for fluorodeoxyglucose (FDG) avidity after completion of chemoradiation. He used the example of a patient in whom an FDG-negative N2a neck tumor is found after completion of chemoradiation therapy. “It does not need neck dissection,” Dr. Johnson said. “In fact, neck dissection may be harmful to these patients, in terms of development of severe dysphagia in the future…. Obviously, the tumor is dead but it didn’t go away, at least not yet. And it remains a conundrum and a topic of some debate in some centers.”
At Pittsburgh, his team watches such patients and they “almost never” discover that it was the wrong course to have taken, he said.
Robert Maisel, MD, professor of otolaryngology at the University of Minnesota Medical School, where PET/CT has been used for about seven years, said Dr. Johnson’s address hit important points. In his group, he said, patients get a post-radiation PET/CT at 12 weeks to evaluate the results—that’s “the one thing we’re sure of,” he said—and then a second one is done six months later.