Radiography
Findings
Plain-film radiography does not play a role in the diagnosis of astrocytoma
Computed Tomography
Grade II astrocytoma in a 27-year-old woman. Nonenhanced CT scan shows a heterogeneous, ill-defined, hypoattenuating area in the right temporal lobe.
Grade IV astrocytoma in a 73-year-old man. Top row (left to right), Nonenhanced CT images and fluid-attenuated inversion recovery (FLAIR) MRI. Bottom row, Axial nonenhanced and enhanced and coronal enhanced T1-weighted MRIs. CT demonstrates an inhomogeneous area of abnormal attenuation in the right temporal lobe that extends to the parietal region, with surrounding edema and mass effect. Enhanced MRI demonstrates heterogeneous enhancement, extensive vasogenic edema, and mass effect. Note the ependymal and subependymal enhancement involving the adjacent lateral ventricle and enhancement of the adjacent dura; this finding is consistent with spread.
Recurrent grade IV astrocytoma in the region of the right caudate and putamen in a 76-year-old man. Top row (left to right), Nonenhanced CT scan, nonenhanced T1-weighted MRI, T2-weighted MRI, and fluid-attenuated inversion recovery (FLAIR) MRI. Second row from top (left to right): Diffusion-weighted MRI, apparent diffusion coefficient (ADC) map, and axial and coronal contrast-enhanced T1-weighted MRIs. Third row from top: Perfusion-weighted MRIs show increased flow in the caudate and putamen but not in the other areas of abnormal signal intensity. Bottom row: Axial spectroscopic image shows the 2 regions of interest in the right caudate corresponding to the multivoxel spectra. Note the large, infiltrating mass centered in the right basal ganglia and extending to the right frontal lobe, temporal lobe, and insula. Image shows thick peripheral enhancement and central necrosis. Multivoxel spectroscopy demonstrates decreased N-acetylaspartate (NAA), elevated choline, and elevated lactate values in thecaudate.
A, Image in a patient after resection of a left frontoparietal, high-grade astrocytoma. Positron emission tomography (PET) demonstrates increased activity in this region, consistent with recurrence. B, Image in another patient being evaluated for recurrence of a high-grade astrocytoma. Image shows no abnormally increased activity to suggest recurrence.
Findings
The appearance of astrocytoma on CT partly depends on its grade. Low-grade astrocytomas typically appear as homogeneous areas of decreased attenuation. They are relatively well circumscribed, and 20% have associated calcification. Although low-grade tumors usually do not enhance, rare tumors demonstrate minimal enhancement.
Specific low-grade tumors have imaging characteristics that can increase the specificity of CT. Pilocystic astrocytomas often appear as a cystic lesion with an eccentric mural nodule that strongly enhances after the administration of contrast agent. Subependymal giant-cell astrocytomas are typically near the foramen of Monro and usually occur in patients with tuberous sclerosis. Pleomorphic xanthoastrocytomas are typically supratentorial, cortically based masses with strong heterogeneous enhancement. The adjacent dura and meninges often enhances, creating the dural-tail appearance.
Grade III astrocytomas appear more heterogeneous. Edema is often appreciated, calcification is rare, and the enhancement pattern is usually more pronounced.
Grade IV astrocytomas are even more heterogeneous than tumors of other grades on CT, and they almost always enhance strongly. Calcification is uncommon. Hemorrhage and necrosis is common. Extensive edema and mass effect are usually appreciated. This grade often involves both hemispheres by spreading by means of the corpus callosum or commissures.
Magnetic Resonance Imaging
Pilocytic astrocytoma in a 20-year-old man. Top row (left to right), Sagittal, coronal, and axial contrast-enhanced T1-weighted MRIs. Bottom row: Axial fluid-attenuated inversion recovery (FLAIR), diffusion, and apparent diffusion coefficient (ADC) images. Note the cystic mass with an intensely enhancing mural nodule in the inferior cerebellar vermis, as well as the mass effect on the brainstem, upper cervical cord, cerebellum, and fourth ventricle.
Grade II astrocytoma in a 30-year-old man. Nonenhanced T2-weighted MRI shows a well-circumscribed area of increased signal intensity in the left temporal lobe.
Grade II astrocytoma. Left, Fluid-attenuated inversion recovery (FLAIR) image demonstrates an area of increased signal intensity in the parietooccipital region. Right, Perfusion MRI demonstrates decreased relative cerebral blood volume (rCBV), consistent with a low-grade neoplasm. The final pathologic diagnosis was a grade II astrocytoma.
Grade III astrocytoma in a 71-year-old man. Top row (left to right), Axial nonenhanced and contrast-enhanced T1-weighted, proton density–weighted, and fluid-attenuated inversion recovery (FLAIR) MRIs. Bottom row (left to right), Sagittal nonenhanced and contrast-enhanced T1-weighted MRIs, axial diffusion-weighted images, and axial apparent diffusion coefficient (ADC) map. Images show a cystic, well-defined lesion in the left parietal region with surrounding vasogenic edema and a thick rim enhancement on enhanced images. Diffusion and ADC images shows no evidence of acute restriction.
Findings
MRI has increased the sensitivity and specificity in imaging astrocytomas. With the advent of the new techniques (MRS, perfusion-weighted imaging, and diffusion-tensor imaging), specificity has further improved.
MRS allows cerebral metabolites to be assessed by suppressing the signal of water and by interrogating for entities, including NAA, Cho, creatine (Cr), lactate, and lipids. The 2 main MRS techniques are single-voxel spectroscopy and chemical-shift imaging. Single-voxel spectroscopy is used to detect the signal from a single region during 1 measurement. Chemical-shift imaging uses additional phase-encoding pulses to obtain signals.
With cerebral gliomas, MRS is used to assess the spectral pattern, metabolite intensities, and ratios to help grade the tumor and/or predict treatment response. MRS can also help in evaluating for tumoral recurrence and treatment response. The intensity of NAA is correlated with neuronal density and viability. Cho is involved in the turnover of cell membranes and neurotransmitters. Cr serves as a reserve for high-energy phosphates in the cytosol of neurons. Cerebral lactate is always abnormal and indicates ineffective cellular oxidative metabolism. Free lipids are present in areas of necrosis. Compared with normal tissues, cerebral gliomas consistently show lowered NAA intensity, elevated Cho (indicating increased membrane metabolism), and a stable or reduced Cr concentration. An Ins peak is described in certain low-grade tumors.
Perfusion-weighted imaging involves several image acquisitions during the first pass of a bolus of contrast agent. This method allows the imager to determine the relative cerebral blood volume (rCBV). In general, the greater the rCBV, the higher the grade of tumor. Lack of notable flow indicates a nonneoplastic etiology with abnormal signal intensity, such as demyelination. Of note, mixed oligodendrogliomas can have low rCBV. Besides the prognostic information it provides, perfusion-weighted imaging can increase the yield of brain biopsy and help in differentiating recurrent neoplasm from radiation necrosis.
Diffusion-tensor imaging is an experimental sequence that allows the imager to evaluate the structure and orientation of the white matter tracts. This sequence takes advantage of the fact that myelin restricts diffusion of water molecules in directions perpendicular to the fiber orientation. This sequence can help in determining whether neoplasm involves white matter pathways, improving the precision of surgical planning and the placement of radiation ports.
Although not used in the diagnosis of astrocytomas, functional MRI (fMRI) deserves mention because it can be an important part of presurgical planning. A blood oxygen–dependent sequence is applied as the patient performs various tasks involving motor, sensory, visual, auditory, and language functions. Increased blood flow to a part of the brain is correlated with increased metabolic activity. The results are used determine whether tumor involves vital structures (eloquent areas), a finding that may possibly affect surgical decisions.
Low-grade astrocytomas are typically hyperintense on T2-weighted images. On T1-weighted images, most low-grade astrocytomas are hypointense relative to white matter. Contrast enhancement may be absent or, at best, mild. Exceptions include the mural nodule of pilocytic astrocytoma and the strong heterogeneous enhancement of pleomorphic xanthoastrocytomas. Astrocytomas are often associated with enhancement of the adjacent dura and meninges, giving the dural-tail appearance. MRS may show an elevated Cho peak and decreased NAA peak. An elevated Cho-Cr ratio or a depressed NAA-Cr ratio suggests tumor. This holds true for all high-grade tumors and many, but not all, low-grade tumors. (Some low-grade tumors may not have an elevated Cho peak.) Perfusion MR studies fail to demonstrate increased rCBV.
Grade III astrocytomas often invade structures without destroying them, causing their ill-defined borders. The mass is inhomogeneous and bright on T2-weighted images. Surrounding edema and/or tumor infiltration is usually appreciated. Enhancement is usually seen. MR perfusion demonstrates increased relative cerebral flow volume.
Grade IV astrocytomas (GBM) are usually discovered as bulky disease, and necrosis is a hallmark of this grade. These lesions usually enhance peripherally, in a nodular and irregular manner, and they cause a large amount of mass effect and edema. These tumors often cross the corpus callosum, giving them a typical butterfly shape. Areas of hemorrhage and necrosis are common, and spectroscopy demonstrates high Cho, high lactate, high lipid, and low NAA values. Short–echo time (TE) studies demonstrate an absent or low myo-inositol peak. Perfusion studies demonstrate elevated rCBV.
Ultrasonography
Findings
Ultrasonography plays a role in the intraoperative assessment of nonsuperficial masses. Using ultrasonography, the surgeon can plan the best surgical approach before excision or biopsy.
Nuclear Imaging
Findings
Primary brain tumors generally have increased glucose metabolism on [18F]fluorodeoxyglucose (FDG) PET studies. The degree of metabolic activity is correlated with both the grade of the tumor and the patient's prognosis. Low-grade tumors may demonstrate little to no increased uptake, whereas grade IV lesion often have uptake that overshadows that of the gray matter.
The heterogeneous nature of grade III or IV lesions is a specific feature for which PET helpful. Obtaining samples from active areas is of vital importance for accurate grading. PET can be used to guide stereotactic biopsy to obtain a representative sample.
PET has also been used to assess the response to therapy, and it can be used to detect transformation of a low-grade lesion to a high-grade one.9
Angiography
Findings
Angiography is being used in several ongoing trials to assess the intratumoral treatment of grade III and IV astrocytoma.
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References
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Further Reading
Clinical guidelines
Radiotherapy for newly diagnosed malignant glioma in adults: a clinical practice guideline.
Program in Evidence-based Care - State/Local Government Agency [Non-U.S.]. 2000 Sep 19 (revised 2005 Nov 2). 32 pages. NGC:004954
Gliadel wafers in the treatment of malignant glioma: a clinical practice guideline.
Program in Evidence-based Care - State/Local Government Agency [Non-U.S.]. 2006 Aug 15. 19 pages. NGC:005649
Adjuvant systemic chemotherapy, following surgery and external beam radiotherapy, for adults with newly diagnosed malignant glioma: a clinical practice guideline.
Program in Evidence-based Care - State/Local Government Agency [Non-U.S.]. 2004 Mar 10 (revised 2006 May 8). 23 pages. NGC:005092
Clinical trials
Intracerebral Clysis in Treating Patients With Recurrent Primary Brain Tumors
Temozolomide and Radiation Therapy in Treating Patients With Newly Diagnosed Glioblastoma Multiforme or Anaplastic Astrocytoma
Antineoplaston Therapy in Treating Children With Low-Grade Astrocytoma
Related eMedicine topics
Astrocytoma (Pediatrics)
Glioblastoma Multiforme (Neurology)
Glioblastoma Multiforme (Radiology)
Glioblastoma Multiforme (Oncology)
Low-Grade Astrocytoma
Keywords
brain astrocytoma, primary intra-axial brain tumors, primary intraaxial brain tumors, gliomas, supratentorial tumors, grade I astrocytomas, pilocytic astrocytoma, subependymal giant cell astrocytoma, giant-cell astrocytoma, pleomorphic xanthoastrocytoma, grade II astrocytomas, protoplasmic astrocytomas, gemistocytic astrocytomas, fibrillary astrocytomas, grade III astrocytomas, anaplastic astrocytomas, grade IV astrocytomas, glioblastoma multiforme, GBM, giant cell glioblastomas, giant-cell glioblastomas, gliosarcoma variants, low-grade astrocytomas, PTEN/MMAC1, DMBT1, EGFR, p53, TP53, p16, PDGFR, retinoblastoma cell-cycle regulatory genes
