Differentiation of recurrent astrocytoma from radiation necrosis: a pilot study with 13N-NH3 PET
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Differentiation of posttherapy radiation necrosis from recurrent brain tumor remains a challenging diagnostic problem. The combination of the imaging modalities on the basis of different physiologic mechanisms could improve diagnostic accuracy. The present study assessed the role of 13N-NH3 PET in differentiating recurrent cerebral astrocytoma from radiation necrosis.
Seven patients, who were previously treated with conventional external-beam radiation therapy after surgical resection for cerebral astrocytomas, and showed the enhancing brain lesions on T1-weighted gadiolinium-enhanced MR studies performed in 6 months or above after the radiotherapies, were examined prospectively with 13N-NH3 and FDG PET. Five lesions with tumor recurrence and two with radiation necrosis were histologically verified by either surgical resection or stereotactic biopsy. One lesion of radiation necrosis was confirmed clinicoradiologically.
In all eight lesions the 13N-NH3 PET scans were concordant with the final diagnosis (100%, 8/8). The lesions with recurrent tumor showed moderately to markedly increased 13N-NH3 uptake (grade = 4–5). The lesions with radiation necrosis showed absent or less 13N-NH3 uptake than surrounding area (grade = 1–2). The FDG PET scans were concordant with the final diagnosis in six of eight lesions (75%, 6/8), and there were one false-negative result and one false-positive result. The diagnostic result of 13N-NH3 PET was discordant with FDG PET in two lesions. One lesion with gliosis and radiation necrosis showed slightly increased FDG uptake (grade = 4), but less 13N-NH3 uptake (grade = 2). The other lesion with anaplastic astrocytoma showed moderately increased 13N-NH3 uptake (grade = 4), but slightly less FDG uptake than surrounding area (grade = 2).
The recurrent astrocytomas showed increased 13N-NH3 uptake, and the radiation necrosis showed absent or less 13N-NH3 uptake, and 13N-NH3 seem superior to 18F-FDG for this purpose, suggesting that 13N-NH3 is a promising tracer for separating radiation necrosis from astrocytoma recurrence. However, the patient population in this study was small. Thus, the further studies are needed to settle this issue.
KeywordsPET 13N-NH3 Astrocytoma Tumor recurrence Radiation necrosis
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