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Glioblastoma multiforme (GBM) is the most common primary brain
tumor in adult patients accounting for approximately 15-20% of all
primary brain tumors. This tumor belongs to the family of glial
cell neoplasms (glioma), which account for 40-50% of all primary
brain tumors. The glioma tumor family is further divided into astrocytomas,
oligodendrogliomas, and ependymomas based on the glial cell type
of origin. Astrocytomas account for 70% of all gliomas and GBM represents
50% of all astrocytomas. The World Health Organization (WHO) classifies
GBM as a grade 4 astrocytoma. Most GBM tumors occur supratentorially
with the frontal lobes being the most common site of involvement,
followed by the temporal lobe, and then the parietal lobe. In children,
the most common sites are brainstem and cerebellum. The tumor tends
to spread by following the white matter tracts, and up to 50-75%
cross the midline through the corpus callosum (i.e. butterfly glioma).
Central nervous system lymphoma is the only other tumor that crosses
the corpus callosum. GBM also has a tendency to break through the
cortex and to disseminate via the subarachnoid space. It is rare
for these lesions to metastasize outside the CNS.
GBM has a slight male predominance (male to female ratio of 3:2)
with peak incidence in the fifth decade of life (45-55 years of
age). Glioblastomas are rare in patients less than 30 years of age.
The two most common presenting symptoms are headache and seizure.
Hemiparesis is the most common neurologic deficit at presentation
occurring in up to 83% of patients. Clinical signs and symptoms
of elevated intracranial pressure progress rapidly over a period
often as short as one month from the development of the tumor. On
gross pathology, GBM is a large, irregular but seemingly well-circumscribed
mass lesion which typically demonstrates central necrosis, hemorrhages
of varying ages, and hypervascularity, often with regions of thrombosed
vessels. These tumors demonstrate extensive mass effect and surrounding
vasogenic edema.
Radiologic imaging is an integral component for the diagnosis of
GBM, treatment planning, and following response to therapy. MR imaging
is an excellent modality for detecting intracerebral gliomas and
demonstrating their extent and spread. In general on MR imaging,
GBM demonstrates marked tumoral heterogeneity, reflecting sites
of hemorrhage, necrosis, and varying degrees of cellularity. On
T1-weighted images these tumors appear as inhomogeneous, hypointense
masses, with even less intense areas of central necrosis. These
masses can have a thick irregular wall, prominent flow voids from
the high degree of vascularization and hemorrhages, and debris-fluid
levels. Intratumoral hemorrhages can have a variety of appearances
depending on the age of the hemorrhage, i.e. with intracellular
and extracellular methemoglobin representing recent hemorrhage and
hemosiderin representing an older hemorrhage. Intracellular and
extracellular methemoglobin appears as areas of high signal intensity
on T1-weighted images. Vasogenic edema that surrounds the more solid
portion of the tumor demonstrates low signal intensity. Linear or
serpentine regions of signal void within the tumor mass on spin-echo
MRI indicate the often prominent angiogenesis that characterizes
GBM. On T2-weighted images, GBM and the vasogenic surrounding edema
demonstrates increased, heterogeneous signal intensity. Intratumoral
hemorrhages of varying ages again demonstrate varying signal intensities
on T2-weighted images. Extracellular methemoblobin appears hyperintense,
where as intracellular hemoglobin or hemosiderin appear hypointense.
On gadolinium enhanced MR images, the pattern of enhancement of
GMB lesions depends on the relative proportions of viable tumor
and tumor necrosis. Generally the enhancement pattern takes the
form of a thick, irregular, and/or nodular rim of enhancement surrounding
a central non-enhancing region. The central non-enhancing region
most often contains necrotic tumor, but cystic areas and areas of
hemorrhage can also be present. Tumor dissemination through the
pial surfaces of the cortex or through the ependymal lining of the
ventricles is best seen on gadolinium enhanced T1-weighted images
where tumor dissemination is demonstrated by enhancement of the
pial surface of the brain and of the ependymal lining of the ventricles.
The differential diagnosis for the imaging appearance of glioblastoma,
especially when only some of the “classic” MRI features
( i.e. intratumoral neovascularity, hemorrhage, necrosis) are identified
include metastasis, anaplastic oligodendroglioma, lymphoma, hemangioblastoma,
radiation necrosis with the appropriate history, abcess/cerebritis
with enhancement, cavernous angiomas with recent hemorrhage, and
reactive gliosis.
Treatment for GBM involves multiple approaches including surgery,
radiation therapy, and chemotherapy. Surgery is used to debulk the
tumor and to provide tissue for diagnosis. However, complete resection
of GBM is rare due to the infiltrative nature of the tumor and the
possibility of damaging adjacent normal brain tissue. If surgery
is not an option, sterotactic biopsy is used to obtain tissue and
a diagnosis before treating with radiation therapy and chemotherapy.
Radiation therapy is traditionally performed using external beam
radiation with a focal irradiation field to treat residual disease
post surgery. Some institutions use interstitial radiation therapy
(brachytherapy) as an alternative. For chemotherapy treatment, Carmustine
(BCNU) is one of the most effective chemotherapeutic agents to date.
GBM has a particularly poor prognosis despite all forms of therapy,
with the average survival after diagnosis of approximately eight
to twelve months.
References:
- Atlas, Scott W. Magnetic Resonance Imaging of the Brain
and Spine, 3rd Ed. Philadelphia: Lippincott Williams &
Wilkins, c2002. pp. 348-352.
- Behin A, Hoang-Xuan K, et al. Primary Brain Tumours in
Adults. Lancet. 2003; 361: 323-31.
- Sator K. MR Imaging of the Brain: Tumors. European
Radiology. 1999; 9: 1047-1054.
- Nelson SJ and S. Cha. Imaging Glioblastoma Multiforme.
Cancer Journal. 2003; 9(2): 134-145.
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