L-[methyl- 11C]-methionine ( 11C-MET) predicted glioma prognosis and appeared to outperform 18F-FDG PET and MRI in predicting survival in low-grade gliomas (LGGs), according to a study published online Oct. 10 in The Journal of Nuclear Medicine.
Current strategies for determining tumor grade and prognosis in gliomas are problematic. Invasive histopathologic exams can be prone to sampling error and cannot be repeated frequently. MRI findings are not specific. Although 18F-FDG PET overcomes some limitations of other approaches, its uptake is high in normal brain cortex and low in LGGs.
“ 11C-MET has an advantage for imaging gliomas because, unlike 18F-FDG PET, it has low normal cortical uptake and high uptake in LGGs, enabling tumor visualization and treatment planning,” wrote Tarun Singhal, MD, from Partners Neurology Program at Massachusetts General Hospital in Boston, and colleagues.
Singhal and colleagues conducted a retrospective comparison of the relative grading and prognostic value of 11C-MET PET, 18F-FDG PET and contrast enhancement on MRI in patients with gliomas. The researchers reviewed imaging data for 102 patients with histologically confirmed gliomas who underwent MRI, 11C-MET PET and 18F-FDG PET between 1998 and 2006 at Kettering Medical Center in Dayton, Ohio. All patients underwent the three exams on the same day.
The researchers reported increasing tumor to mean standardized uptake value in contralateral normal cortex (T/N ratio) with increasing tumor grade for PET exams with both tracers. These were higher in high-grade gliomas (HGGs) than in LGGs for both 11C-MET PET and 18F-FDG PET.
“The median survival was 19 +/- 5.4 months for all patients with a mean T/N ratio greater than 1.51 for 11C-MET PET and 26.7 months for those with a mean T/N ratio less than 1.51,” wrote Singhal et al.
On 18F-FDG PET, median survival was 19 +/- 4.19 months in patients with a mean T/N ratio greater than 0.51 and significantly lower than the median survival in those with a T/N ratio less than 0.51.
MRI contrast enhancement did not predict survival, according to the researchers. However, 18F-FDG PET predicted survival in patients with MRI contrast enhancement, and 11C-MET PET predicted survival in patients without contrast enhancement. For patients with MRI contrast enhancement, median survival was 47 months for those with a mean T/N ratio greater than 0.51 on 18F-FDG PET and 17 months for patients with a ratio less than 0.51. For patients without MRI contrast enhancement, median survival was 12 months in patients with a T/N ratio greater than 1.51 on 11C-MET PET, which was significantly lower than that of patients with a T/N ratio less than 1.51.
The prognostic information provided by 11C-MET PET and 18F-FDG PET “is clinically significant,” Singhal and colleagues wrote. The tracers complement each other and deliver differing value in contrast-enhanced and unenhanced lesions, a finding that supports the practice of scanning patients with both tracers on the same day, according to the authors.
Singhal and colleagues called for future studies to incorporate 11C-MET PET and 18F-FDG PET findings in the prognostic schemata for glioma patients. These should include a prospective randomized controlled trial that compares patients with LGGs treated according to 11C-MET PET uptake criteria with a control arm of patients undergoing conventional management.