Differentiation of recurrent astrocytoma from radiation necrosis: a pilot study with ¹³N-NH₃ PET

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.