Dr. Richard Hoffman showed slides of PET findings previously missed on CT at a Wednesday session. When one PET image illustrated a rather large focal lesion in the shoulder, audience members gasped when they realized it had been missed on CT.Hoffman then showed the CT scan and explained how radiologists could miss this lesion in the periphery when their eyes were focused more centrally. The lesion, though very evident from the FDG uptake, was less conspicuous on the CT scan.
He followed with a slide of the infamous "Where's Waldo" poster with the heading "CT." The next slide had Waldo circled in red, with the heading "PET."
"With PET, we can find Waldo -- with CT we can't," Hoffman said.
Hoffman and Dr. Mitchell Parver of Torrance Memorial Medical Center in Torrance, CA, compared PET scans and reports from high-quality postcontrast CT scans on 215 patients. There were 21 discordant findings.
Diminished conspicuity of lesions in areas of complex anatomy or in peripheral areas of the CT scan accounted for variance in some readings. In other cases, a discrepancy was proved to be due to noncontemporaneous PET and CT scanning in the face of aggressive disease.
When asked, Hoffman said that a hybrid PET/CT scanner would be useful in such cases.
In another study, Drs. Mohei Abouzied and Hani Nabi from the State University of New York at Buffalo found that FDG-PET initiated a management change in 40% of patients with known or suspected colorectal cancer recurrence:
* In 49 patients scanned for metastatic workup, PET depicted new metastatic sites not previously suspected in 24.
* In 43 patients scanned to determine the extent of disease recurrence, PET findings were indicative of malignancy in 18, depicting new sites in 13.
* In 41 patients with equivocal findings, FDG-PET identified the source of carcinoembryonic antigen elevation in 29.
Based on the PET findings, surgery to resect isolated recurrences was initiated in 31 out of 131 patients and precluded in 22.
Although his institution gets paid only for one scan, Dr. Lamk Lamki of the University of Texas at Houston believes dual-phase FDG-PET imaging improves specificity and sensitivity for oncology patients.
In his first study, Lamki and colleagues examined how FDG could determine blood flow in patients taking antiangiogenic drugs. They found that in the initial two minutes following injection, FDG acts as a free tracer, then metabolic action takes over.
If the second scan, taken at 30 minutes postinjection, is equivocal, radiologists can refer to the first scan. In some cases, the early-phase image will reveal vascular action that may help fine-tune interpretation and eliminate false positives and negatives, Lamki said.
In his second study, phases were one hour and three hours postinjection. Patient management was changed in 31 of 47 patients after the first phase. More important, treatment was changed in 25 of 44 patients after the second-phase image was consulted, Lamki said.
The second image is not a total-body scan, is done only in select cases, and can be targeted to the area of interest, he said.
