PET imaging to diagnose brain tumor and monitor recurrence after treatment is an evolving field of research. Investigators at the RSNA meeting presented studies revolving around five tracers, as well as various permutations of imaging combinations such as FDG-PET with MR spectroscopy. While results are promising, challenges remain before any of these research avenues becomes clinically routine.
"We don't have definitive answers as to the best way to diagnose and monitor treatment for brain tumors," Dr. Satoshi Minoshima, vice chair for research in radiology at the University of Washington, Seattle, told Diagnostic Imaging.
In one study, Dr. Yuka Yamamoto, a radiologist at Kagawa Medical University in Japan, and colleagues found that the radiotracer carbon-11 methionine was more sensitive than carbon-11 acetate and FDG in detecting brain tumors. They didn't totally discount C-11 acetate, however, saying it has potential. The researchers observed C-11 methionine uptake in all 14 malignant tumors in the study, C-11 acetate uptake in 12, and FDG uptake in five.
Dr. Ashok Muthukrishnan, an assistant professor of radiology at the University of Pittsburgh, reported that uptake of F-18 fluorothymidine (FLT) was associated with untreated tumor, as well as treated residual/recurrent tumor, despite a very low cell proliferation index. Muthukrishnan reported on the results of a subset of three patients from a larger ongoing study. FLT would bring some clinical advantages to brain tumor imaging, as it is more readily available than carbon-based analogs and has a longer half-life.
The Pittsburgh group also tested the ability of MRS to help detect recurrent brain tumor after radiation and/or chemotherapy. While the technique showed a high number of false positives, Muthukrishnan said that it can be complementary to FDG-PET. Researchers evaluated 14 patients with treated brain tumors. In six high-grade tumors, MRS was positive for four with recurrence, but it also was positive for the two without recurrence. PET had three true positives and two true negatives.
In five of the eight low-grade tumors without recurrence, MRS was positive in four, with only one true negative. PET was true negative for all five, with two true positives.
Early assessment of therapeutic efficacy helps determine whether treatment should be continued. Dr. Michael Paldino, a radiology resident at Duke University, and colleagues found prognostic significance between FDG uptake and contrast-enhanced MRI findings in patients undergoing intracavitary radiation therapy for high-grade gliomas.
From 10 patients, researchers evaluated 37 coregistered PET and MRI studies. Enhancing lesions with a volume of more than 15 cc and a mean activity ratio greater than 1.2 on PET were associated with decreased survival.
"We are able to define specific characteristics of MR and PET imaging associated with a decrease in survival in a therapeutic setting. These results raise the possibility that a quantitative analysis of CE-MRI and FDG-PET may be able to help identify treatment failure in patients with high-grade gliomas," Paldino said.
