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Neuroradiology Case of the Week

Case 194

Samuel Madoff, MD and Per-Lennart Westesson, MD, PhD, DDS

Clinical Presentation: Patient is a 69-year-old female status post-radiation and chemotherapy for multi-focal glioblastoma multiforme (GBM).

Radiological Findings: On the initial CT (Figs. 1 & 2), heterogeneous lesions of the right parietal and left frontal lobes demonstrate areas of low density, probably due to necrosis. Extensive regions of low attenuation surround these lesions, consistent with vasogenic edema.
     Post-treatment FLAIR images demonstrate persistent lesions with marked surrounding edema (Figs. 3 & 4). A post-contrast T1 sequence reveals a thick, irregular rim of enhancement (Figs. 5 & 6). On perfusion imaging (CBV), the lesion of the corpus callosum splenium shows increased perfusion (Fig. 7). Contrastingly, the left frontal lesion exhibits decreased perfusion (Fig. 8).

Diagnosis: Residual tumor (right-sided lesion) and radiation necrosis (left-sided lesion)

Discussion: The differential diagnosis of radiation necrosis includes recurrent tumor (usually GBM), metastatic disease, abscess, multiple sclerosis, vascular dementia and progressive multifocal leukoencephalopathy. This brief discussion will be confined to the imaging distinctions between delayed (non-acute) radiation necrosis and recurrent tumor. Differentiating these entities is notoriously difficult based solely on lesion morphology. On T1 and T2 sequences, radiation necrosis may appear hypointense and hyperintense respectively. Involvement of the surrounding white matter is often due to edema and/or demylination. With contrast, T1 imaging demonstrates variable enhancement patterns that include nodular, linear, curvilinear, “soap bubble,” and “swiss cheese” appearances. All of these findings may be associated with recurrent tumor as well.
     Perfusion imaging is a valuable aid in distinguishing radiation necrosis from recurrent tumor. Perfusion processing generates three image sets: cerebral blood flow (CBF), cerebral blood volume (CBV) and mean transit time (MTT). CBF is a measure of the concentration of a tracer (representing blood) passing through a volume of tissue per unit time. CBV quantifies the amount of blood in a given volume of tissue. MTT estimates the rate at which blood passes through an area (volume is not part of this algorithm). The relationship of the three is CBF = CBV / MTT. In regards to interpretation, decreased CBV points to radiation necrosis, while increased CBV indicates tumor.
     MR spectroscopy (MRS) and PET-FDG are two other useful tools. As for MRS, radiation necrosis demonstrates reduced metabolites (i.e. NAA, Cho, Cr), whereas recurrent tumor may show increased Cho. Lactate and lipid are not reliable differentiating factors. PET-FDG reveals areas of radiation necrosis to be hypometabolic, whereas recurrent tumor is hypermetabolic.
     If imaging remains inconclusive, a biopsy may be pursued for definitive diagnosis.

References:

1. Osborn A. Diagnostic Imaging: Brain. 1st ed. Philadelphia: W.B. Saunders, 2004: I-10:46-49.
2. Sorensen AG, Reimer P. Cerebral MR Perfusion Imaging: Principles and Current Applications. Stuttgart: Thieme, 2000:108-117.