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 (13)N-NH(3) PET in differentiating recurrent cerebral astrocytoma from radiation necrosis.
Methods: 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 (13)N-NH(3) 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.
Results: In all eight lesions the (13)N-NH(3) PET scans were concordant with the final diagnosis (100%, 8/8). The lesions with recurrent tumor showed moderately to markedly increased (13)N-NH(3) uptake (grade = 4-5). The lesions with radiation necrosis showed absent or less (13)N-NH(3) 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 (13)N-NH(3) 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 (13)N-NH(3) uptake (grade = 2). The other lesion with anaplastic astrocytoma showed moderately increased (13)N-NH(3) uptake (grade = 4), but slightly less FDG uptake than surrounding area (grade = 2).
Conclusions: The recurrent astrocytomas showed increased (13)N-NH(3) uptake, and the radiation necrosis showed absent or less (13)N-NH(3) uptake, and (13)N-NH(3) seem superior to (18)F-FDG for this purpose, suggesting that (13)N-NH(3) 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.