Case Author(s): Lester Johnson, M.D., Ph.D. and Farrokh Dehdashti, M.D. , 06/07/00 . Rating: #D3, #Q4

Diagnosis: Radiation necrosis

Brief history:

38-year-old male with history of astocytoma


Axial FDG-PET images of the brain are shown

View main image(pb) in a separate image viewer

View second image(pb). A coronal FDG-PET image through the temporal lobes is shown

View third image(mr). A sample T2-weighted coronal MRI image through the temporal lobes is shown

View fourth image(mr). A sample T1-weighted gadolinium-enhanced coronal image through the temporal lobes is shown

Full history/Diagnosis is available below

Diagnosis: Radiation necrosis

Full history:

This 39-year-old male has a history of astrocytoma diagnosed 16 months ago, treated with radiation therapy ending 13 months ago. He now presents with new onset seizures and ataxia. His MRI demonstrates enlargement of the right temporal lobe associated with extensive abnormally decreased signal on T1-weighted and increased signal on T2-weighted images and two small areas of abnormal enhancement with gadolinium, all changes which are compatible with recurrent astrocytoma versus radition necrosis. PET imaging is requested to evaluate for evidence of recurrence versus radiation necrosis.


10 mCi F-18 fluorodeoxyglucose (FDG)


There is a large area of decreased metabolic activity involving the entire right temporal lobe, which is enlarged. There are no foci of abnormally increased metabolic activity, including in the two sites of abnormal enhancement on MRI, to suggest recurrent malignancy. These findings are consistent with radiation necrosis.


One of the first recognized valuable clinical uses of PET imaging was to distinguish between recurrent brain malignancy and radiation necrosis. This distinction is often impossible to make by MRI. Abnormal gadolinium enhancement, mass effect, and edema are findings usually associated with tumor before radiation therapy. After radiation therapy, however, these findings are non-specific and can be seen due either to recurrence or radiation necrosis. It should be noted that these findings can also be dramatically delayed after the radiation has been completed, occurring years later, so the timing of the findings does not reliably allow a distinction to be made. FDG-PET imaging uniquely assesses metabolic activity. Radiation fibrosis usually demonstrates metabolic activity which is equal or less than that seen in white matter activity. On the other hand, with tumor recurrence, areas of abnormal gadolinium enhancement on MRI also show high metabolic activity greater than that in the white matter or equal to or greater than the highly metabolic cerebral cortex.

Differential Diagnosis List

Radiation necrosis vs. tumor recurrence.

ACR Codes and Keywords:

References and General Discussion of PET Brain (Nontumor) Imaging Studies (Anatomic field:Skull and Contents, Category:Inflammation,Infection)

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Case number: pb007

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