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Findings: CT scan of the head demonstrates bilateral frontal lobe vasogenic edema. The study was followed by a MRI with Gadolinium which demonstrated increased signal on the T2 weighted images diffusely in the frontal lobes. Post Gadolinium T1 images in the axial and coronal planes demonstrate two lesions in the frontal lobes bilaterally, which demonstrate rim enhancement. There is decreased signal seen surrounding the area of enhancement consistent with edema.
Diagnosis: Radiation necrosis.
Discussion: The differential diagnosis in this case include unusual bilateral frontal lobe infarcts with surrounding edema, bilateral frontal lobe tumors, or bilateral frontal lobe abscesses from the frontal sinuses and nonhemorrhagic contusion, among others. Further investigation of the patient's past medical history revealed the patient had a previous squamous cell carcinoma of the maxillary sinus for which he received surgical and radiation treatment. A biopsy showed coagulation necrosis of brain parenchyma and vascular changes is consistent with radiation effect. These above findings are consistent with radiation necrosis.
Radiation necrosis may occur several weeks to years following irradiation. It is usually found in or near the radiated tumor bed, however, can appear in remote areas from the tumor bed. The process can be progressive and fatal. The underlying etiology of radiation necrosis is believed to be damaged to the cerebral vasculature. Vascular alterations include heterogeneous endothelial hyperplasia, fibrinoid necrosis of the penetrating arterials, vascular occlusions, and morphologic changes as seen in large vessel atherosclerosis. There is disruption of the blood-brain barrier, which causes the vasogenic edema as seen on the CT scan as well as the increased signal seen on the T2 weighted images.
There appears to be an increased likelihood of radiation necrosis when the patients receive a standard regimens versus the hyperfractionated doses, the patients with the standard treatment also develop radiation necrosis sooner than patients receiving hyperfractionated doses. There is also a correlation with patients receiving methotrexate therapy in combination with CNS radiation.
While these findings in the clinical history suggest radiation necrosis as the most likely diagnosis, one cannot say that this is not metastatic disease in the frontal lobes, however, bilateral frontal lobe neoplasms would be less likely. Serial MRIs of the brain may show an absence of progression, which would strongly suggest radiation necrosis, however, in such cases where the radiation necrosis is progressive, a tumor cannot be excluded. There is evidence that PET scanning would help in differentiating radiation necrosis from metastatic disease or recurrence of the tumor.
References:
Brandt WE, Helms CA. Fundamentals of Diagnostic Radiology.
Williams & Wilkins, Baltimore; 1994:187-188.
Woodruff WW. Fundamentals of Neuroimaging. W.B. Saunders,
Philadelphia; 1993:243-245.
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