Low-Grade Astrocytoma Workup
- Author: George I Jallo, MD; Chief Editor: Tarakad S Ramachandran, MBBS, MBA, MPH, FAAN, FACP, FAHA, FRCP, FRCPC, FRS, LRCP, MRCP, MRCS more...
No specific laboratory test is available for the diagnosis or follow-up of low-grade gliomas. There are promising studies which aim to detect circulating tumor DNA in human malignancies. Although this technology hasn't been applied to low-grade gliomas it could potentially be implemented in the future as a screening, diagnostic or follow-up tool.
Both CT scan and MRI can aid in the diagnosis of low-grade glioma. Generally, MRI is considered the study of choice. However, in an emergency setting a noncontrast CT scan may be ordered first.
Patients with new-onset headache, seizure, weakness, or numbness frequently undergo a CT scan first. Typical CT findings of a low-grade glioma show lower attenuation than the surrounding brain (see image below). A mild mass effect may be noted. Obstructive hydrocephalus can be confirmed. Low-grade gliomas also may show evidence of calcification (more common in oligodendroglial tumors). Low-grade astrocytomas are usually non-enhancing lesions, although the presence of contrast enhancement doesn´t preclude their diagnosis.
PET and SPECT
Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging sometimes are used to try to differentiate low-grade gliomas from either high-grade tumors or other types of pathology. Typically, low-grade gliomas show hypometabolism via PET or SPECT while high-grade gliomas are hypermetabolic. This information may be useful in guiding further therapy.
Magnetic resonance imaging
On MRI, low-grade astrocytomas show decreased signal relative to surrounding brain on T1 sequences (see following images).
On T2 sequences, higher signal reflects both the tumor and surrounding edema (see following images). Pilocytic astrocytomas often are associated with a cyst, which may be particularly prominent on T2-weighted sequences.
MRI of the spinal cord is also the study of choice if an intramedullary low-grade astrocytoma is suspected. On MRI, widening of the spinal cord and frequently an associated cyst are noted. The tumor may show a variable degree of enhancement.
Functional magnetic resonance imaging (fMRI) can provide information about the relationship and localization of a low-grade glioma and eloquent structures such as speech centers and motor pathways. This may help in planning the operation. Recently, the use of digital tractography has allowed to pre-operatively assess the relationship of tumors to important white matter tracts (e.g. descending corticospinal tract) prior to resection.
Electroencephalography (EEG) may be performed on a patient with new-onset seizures; however, no EEG findings are specific to low-grade glioma. However, generalized, diffuse slowing and/or epileptogenic spikes can be seen over the area of the tumor.
Lumbar puncture is generally contraindicated in patients with elevated intracranial pressure, which may occur in the setting of a brain tumor. Cerebrospinal fluid (CSF) studies do not aid in the diagnosis of low-grade astrocytomas.
The histologic findings in low-grade astrocytomas vary according to the specific tumor type. As reviewed before, the absence of high-grade lesion characteristics like necrosis, microvascular proliferation and high mitotic indices are common to the group as a whole.
Pilocytic astrocytomas show the presence of bipolar piloid cells with long hair-like processes and Rosenthal fibers. Pilomyxoid astrocytomas are dominated by the presence of a mucoid matrix, monomorphous bipolar cells and an angiocentric cell arrangement. Pleomorphic xanthoastrocytomas have a variable histological appearance, hence the name. The term xanthoastrocytoma is derived from the presence of xanthomatous cells which show intracellular accumulation of lipids. Diffuse astrocytomas are composed of well differentiated fibrillary or gemistocytic neoplastic astrocytes on the background of a loosely structured microcystic tumour matrix.
There are currently no valid staging systems in clinical use.
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