Oncology—Brain
Primary brain tumors are classified according to the cell of origin (e.g., neu- roepithelial, meningeal, etc.). Neuroepithelial tumors in adults compose more than 90% of primary brain tumors. These tumors are collectively called gliomas and include astrocytoma, oligodendriogliomas, and ependymomas. An increas- ing grade of tumor indicates an increasing degree of malignancy.
CT and MRI are commonly performed imaging procedures to delineate the location, size, and vascularity of the tumor. Gliomas are often heteroge- neous with intermixed areas of low- and high-grade neoplastic cells, hemor- rhage, and necrosis. Primary treatment usually involves surgical resection followed by observation, adjuvant external beam radiotherapy, radiosurgery, brachytherapy, or chemotherapy. Anatomic imaging is typically employed for follow-up and assessment of therapy response. However, there are serious limitations of anatomic imaging in differentiating residual or recurrent tumor from post-therapy anatomic alterations. Functional imaging such as PET improves on this distinction noninvasively and leads to cost-effective impact
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Figure 1: A 24-year-old male with brainstem glioma with subsequent radiation therapy.
Intense hypermetabolism in the brainstem, right cerebral peduncle is consistent with recurrent high grade tumor.
on patient management and clinical outcome (Figs. 1–3). Many radiotracers have been employed with PET in the imaging evaluation of brain tumors.
However, the bulk of the literature, similar to that for other tumors, has been with FDG.
The patient preparation for a FDG PET scan is similar regardless of the tumor type. Patients fast for at least four to six hours and should be well- hydrated. Hyperglycemia results in decreased cerebral FDG uptake and poor- quality images. For this reason, in patients with diabetes mellitus, the blood glucose level should be managed so that optimally the fasting blood glucose level is in the normal range. If the level is more than 200 mg/dl, the PET images may be degraded. Corticosteroids, which may be employed in patients with brain tumors for symptomatic relief of cerebral edema, can result in diminished brain glucose utilization independent of concurrent anticonvulsant medica- tion, blood glucose level, cerebral atrophy, and any therapy. Sedatives may also reduce cerebral glucose metabolism.
FDG is administered intravenously while the patient rests in a quiet room with dim lights and minimal environmental stimulation during the uptake phase of 30–40 min before imaging acquisition. Cerebral cortical and subcorti- cal gray matter and the cerebellar cortex demonstrate physiologic high glucose metabolic activity. For image interpretation, the metabolic activity of a lesion is compared relative to the physiologic low activity of the white matter with the
Figure 2: A 53-year-old female with glioblastoma multiforme treated with resection followed by cyber knife, gamma knife, and chemotherapy. PET-CT shows rim of moderately intense hypermetabolism surrounding an area of hypometabolism in the right posterior parietal region consistent with recurrent/residual tumor in the surgical bed.
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centrum semiovale of the contralateral cerebral hemisphere used as reference.
High grade tumors (e.g., anaplastic astrocytoma, glioblastoma multiforme) demonstrate hypermetabolism while low-grade tumors show less than, equal to, or slightly higher than white matter metabolic activity.
Cerebellar diaschisis may also be seen with asymmetric cerebellar hemi- spheric hypometabolism contralateral to the remote primary supratentorial cerebral tumor. The mechanism of this observation is an interruption of the corticopontocerebellar pathway from the cerebral hemispheres through the ipsilateral pons to the opposite cerebellar cortex. There is no clinical correlate to the cerebellar diaschisis and it may be reversible in patients with stroke but often remains persistent in patients with brain tumors.
FDG PET has been shown to be useful in grading tumors, directing biopsy site and resection, assessing for malignant transformation of low-grade tumors, evaluating treatment response, and assessing for cerebral metastases in patients with other malignancies such as melanoma. FDG PET may also be useful in localizing hypermetabolic epileptogenic zones adjacent to the site of original tumor which are unrelated to recurrent tumor but responsible for focal or glob- al neurologic impairment and nonconvulsive status epilepticus in patients with brain tumors.
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Figure 3: A 31-year-old male with a history of grade II left frontoparietal lobe oligodendrioglioma that was treated with radiation therapy and gamma knife surgery.
The patient now presents with aphasia and cognitive impairment. PET-CT shows an area of hypermetabolism in the left paramedian parietal lobe at the location of dense calcification seen on CT consistent with recurrent oligodendroglioma. There is decreased radiotracer activity in the left parietal cortex secondary to radiation therapy.
Assessment for treatment response and detection of residual or recurrent tumor is the most common current utility of FDG PET in patients with brain tumor. Although it is not entirely clear when after therapy to perform a FDG PET scan to assess for response, reduction in lesion glucose metabolism and favorable response may be seen in as early as one week after chemotherapy. The detection of residual or recurrent tumor after high doses of local radiation with a gamma knife may occasionally be difficult due to the intense inflammatory response, which is induced as a result of the pathologic injury of radiation necrosis. However, even the histopathologic assessment may also be difficult due to heterogeneous distribution of distorted potentially viable tumor cells and necrotic cells. Further therapeutic decisions are best made in the individual clinical context.
Other PET radiotracers have also been investigated in evaluating brain tumors but none are currently used routinely. These tracers include but are not limited to [C-11]-methionine and [F-18]-fluorotyrosine (amino-acid trans- port), [F-18]-fluoromisonidazole (tumor hypoxia), [C-11]-thymidine (DNA incorporation), and [C-11]-choline (lipid metabolism). As PET imaging tech- nology and PET radiochemistry evolve, these and other radiotracers may become more important in the diagnostic and prognostic imaging assessment of patients with brain tumors.
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