Enhanced accuracy in differential diagnosis of radiation necrosis by positron emission tomography-magnetic resonance imaging coregistration: Technical case report

OBJECTIVE AND IMPORTANCE: To demonstrate the usefulness of positron emissio n tomography-magnetic resonance imaging (MRI) coregistration for differenti ation of radiation necrosis and recurrent tumor in stereotactic planning. CLINICAL PRESENTATION: T1-weighted MRI scans of a 43-year-old woman re veal ed a contrast-enhancing lesion 4 years after open removal of a recurrent Fi ght parieto-occipital Grade II oligodendroglioma and subsequent external ra diation therapy. The suspected contrast-enhancing lesion revealed only mode rate tracer uptake (1.3 times the uptake in the contralateral normal cortex ) in a coregistered [C-11]methionine positron emission tomographic scan. Ap proximately 15 mm posterior and mesial to the center of the contrast-enhanc ing lesion, however, an area of higher tracer uptake was found (1.8 times t hat of the contralateral normal cortex), which exhibited only very minor co ntrast enhance ment on MRI. TECHNIQUE: The coregistered images were used for planning stereotactic seri al biopsies, from the contrast-enhancing lesion as well as from the area wi th higher methionine uptake. Histological examination demonstrated that the contrast-enhancing lesion with low methionine uptake was necrotic tissue, and the nonenhancing area with high methionine uptake was recurrent tumor, CONCLUSION: High-resolution positron emission tomography and modern coregis tration techniques allow differentiation of contrast enhancement and methio nine uptake in irradiated brain tissue within small areas. High methionine uptake is typical for recurrent tumor tissue and can be differentiated from minor tracer accumulation resulting from disruption of the blood-brain bar rier or macrophage activity within the necrotic area.