masses should be subdivided into a matrix of two large categories by location
and two large categories by age (Table
I). The vast majority of posterior fossa masses in adult patients are
in the extraaxial location. Conversely, in childhood, the majority of posterior
fossa masses are intraaxial or intraventricular. In addition, even if a
mass does present at the wrong age for its location or in the wrong location
for the patient's age, the differential diagnosis may also change.
The second most common CPA mass is the meningioma, representing approximately 1/9 tumors. (Figure 2) Meningiomas are benign tumors derived from arachnoid cap cells. REF 3 Meningiomas typically grow with a broad base of dural attachment, and are usually attached to the dura of the tentorium or to the dura overlying the petrous bone. They are usually not, however, related to any of the cranial nerves. Meningiomas are usually hyperattenuating compared to brain on plain CT, but may be similar in signal intensity on multiple MR pulse sequences. REF 4 The "dural tail" (curvilinear enhancement at the lesion margin) is often associated with meningiomas, but is not specific. REF 5,REF 6
The third most common cerebellopontine angle mass is the epidermoid inclusion cyst. (Figure 3) About 1/18 CPA massse is an epidemoid, and these inclusion cysts are developmental abnormalities that, unlike many other "cystic" intracranial lesions, are true cysts (i.e., they are lined by an epithelium). The lining of an epidermoid inclusion cyst is formed only by squamous epithelium. The lesion enlarges very slowly over years and decades by the desquamation from the simple thin lining. The contents of the epidermoid inclusion cyst consist of cellular debris including proteinaceous material (keratin) and occasionally some lipid material (cholesterin) from cell membrane breakdown. The epidermoid inclusion cyst is thought to arise from a failure of complete separation of the surface ectoderm from the underlying neuroectoderm, as the neural tube closes. Because of the presence of cholesterol crystals within epidermoid inclusions, they are sometimes nicknamed "cholesteatomas" or "congenital cholesteatomas". However, they should be distinguished from acquired inclusion cysts that arise from retraction pockets of the pars flaccida of the tympanic membrane: those are typically found in the middle ear cavity and are associated with episodes of middle ear infection.
Three important features help the process of the differential diagnosis of cerebellopontine angle lesions: lesions morphology, pattern of enhancement; and, site of origin. REF 7 The most common masses, the Schwannoma and the meningioma, are solid neoplastic lesions that almost invariably show contrast enhancement on either MR with gadolinium or CT with iodine. (Figure 1,Figure 2) The distinguishing absence of contrast enhancement is a feature of the epidermoid inclusion cyst, or the less common arachnoid cyst. (Figure 3) Occasionally an epidermoid inclusion cyst may show a faint or thin rim of enhancement but never shows solid enhancement. Small Schwannomas almost invariably show homogeneous enhancement. Large Schwannomas undergo a pathologic change of cystic degeneration, and therefore may show a heterogeneous pattern of enhancement. Meningiomas, on the other hand, almost invariably show contrast enhancement that is usually homogeneous regardless of lesion size.
The second distinguishing feature for cerebellopontine angle masses is the site of origin and relationship of the lesion to the adjacent structures. Since the cell of origin for the Schwannoma is the Schwann cell, Schwannomas will invariably be related to cranial nerves. Again the most common cranial nerve affected will be the vestibular portion of the eighth cranial nerve. Schwann cells produce peripheral myelination; whereas oligodendrocytes produce central myelination. The transition zone for the myelination for the fibers of the vestibulo-coclear nerve is at the mouth or orifice of the internal auditory canal (IAC). (Figure 1) Since the Schwann cells are only found inside the IAC, vestibular Schwannomas almost invariably begin as an intracanalicular mass. However, the growth vector for these intracanalicular lesions is to escape the IAC and growth as a roughly spherical mass in the fluid-filled cistern of the CPA. (Figure 1)
distinguishing factor for meningiomas, is the fact that they may not be
related to a cranial nerve and that they tend to form an oblique angle
as they grow with a broad surface of attachment to the adjacent dura. (Figure
2) Therefore, these lesions tend to be
ovoid or hemispheric masses, rather than spheres, like the Schwannoma.
Children may also present with "brainstem gliomas". These are more properly referred to as astrocytomas since most of them are astrocytic, and, they typically arise within the pons rather than the medulla or the mesencephalon. Pontine astrocytomas, unlike the pilocytic astrocytoma, are typically diffusely infiltrating masses. They usually present with relatively minor symptoms since the infiltration is typically not accompanied by destruction of the structure of brainstem; and, cranial nerve findings, may be relatively minimal or absent. (Figure 6) Pontine astrocytomas may either be low-grade tumors or high-grade tumors, and the presence of contrast enhancement may suggest a higher grade lesion. (Figure 6c) Most pontine astrocytomas are ventrally exophytic. However, dorsally exophytic brainstem gliomas are more likely to be the pilocytic type of astrocytomas.
intracranial ependymomas present in childhood (about 70%), and most are
in the posterior fossa (about 70%). (Figure
7) These are benign glial tumors that
arise from the ependymal lining within the fourth ventricle. In contrast
to the other midline tumor (the medulloblastoma) the ependymoma typically
arises from the ventral portion of the fourth ventricle (the ventricular
floor). REF 3 REF 9Therefore, a
more ventral or anterior location may suggest an ependymoma, whereas a
more posterior location or a relationship to the ventricular roof, may
favor the diagnosis of medulloblastoma. Another important feature of the
ependymoma is that it tends to assume the shape of the fourth ventricle,
forming a cast of the dilated ventricular lumen. Medulloblastoma is often
a more spherical mass. The ependymoma may also send extensions or tongues
of tissue out along the lateral foramina of Lushka or down to and through
the midline foramen of Magendie. Ependymomas tend to be more heterogeneous
than medulloblastomas in the posterior fossa, often with several small
cysts, and chunk-like calcification. Like medulloblastoma, ependymoma can
spread through CSF, therefore enhanced scanning and spinal evaluation for
drop mets are usually indicated.
the adult posterior fossa, although metastatic disease is relatively common,
we must also consider the possibility of a primary hemangioblastoma. This
is a benign low-grade, circumscribed lesion, whose cell or tissue of origin
is still uncertain. REF 3 Hemangioblastomas are usually
solitary lesions, but in about 5-20% of patients, may be multiple in von
Hippel-Lindau disease. The presence of multiple cerebellar lesions may
suggest a diagnosis of metastatic disease. Therefore it is important that
we discuss the features that are suggestive of hemangioblastoma. The hemangioblastoma,
like the pilocytic astrocytoma, may be a partially cystic mass. However,
the range of morphology is much greater in the hemangioblastoma with approximately
1/3 of lesions being completely solid and about 1/3 of the lesions having
a complex morphology that cannot be simply described as a "cyst with nodule".
10 The hemangioblastoma is an extremely vascular lesion that may have
evidence of remote hemorrhage and serpentine flow voids, suggesting neovascularity,
can be identified on the imaging studies.
REF 11 (Figure
9) Multiple hemangioblastomas are almost
invariably part of the inherited syndrome of von Hippel-Lindau disease.
10 REF 12In this genetic condition, there are
multiple cerebellar tumors (hemangioblastoma) as well as multiple lesions
affecting the kidneys and other abdominal viscera.
12 Most importantly, the kidneys may contain simple cysts, but may
also harbor multiple renal cell carcinomas.
T1WI MR after gadolinum contrast infusion. There is enlargement of the
internal auditory canal (IAC). There is a funnel-shaped component of the
mass partially embedded within the IAC, however, most of the lesion is
a spherical mass in the cerebellopontine angle cistern. The mass is homogeneous
and shows uniform enhancement. Notice how the lesion has enlarged the lateral
pontine cisternal space on the same side.
enhanced axial CT. There is an oval-hemispheric (sector-shaped) mass attached
to the right leaflet of the tentorium. The lesion has homogeneous enhancement.
Axial T1WI MR after contrast infusion. There is a low-signal intensity
mass in the right cerebellopontine angle cistern. The mass does not enhance.
Notice that the lesion has an irregular and "undulating" contour medially,
with a sharp indentation.
T1WI MR after gadolinium (a) shows a non-enhancing CPA mass with an undulating
medial border. The signal intensity is only slightly greater than CSF.
The axial T2WI image (b) shows the mass as basically isointense to CSF.
Axial proton-density weighted MR, without contrast. There is a rounded,
homogeneous, midline lesion. The center of the mass straddles the expected
location of the fourth ventricle. b) Axial CT, without contrast. There
is a central, rounded, homogeneous mass with high attenuation.
Sagittal T1WI MR after gadolinium enhancement. This is the "classic" appearance
of a juvenile pilocytic astrocytoma(JPA): a "cystic" mass with an enhancing
"mural nodule" in the characteristic cerebellar location. b) Axial T1WI
MR of a JPA. Note that the wall or "lining" of the cyst does not enhance,
except in the lump of tissue that is the mural nodule. c) Sagittal T1WI
MR after gadolinium. This is also a JPA. However, this mass has a very
complex morphology that cannot be described as a "cyst with mural nodule".
Pilocytic astrocytomas do not always demonstrate the "classic" pattern.
CT (a) and enhanced T1WI MR (b) show an expanded pons with abnormal attenuation
and signal intensity. The mass has extended exophytically in the ventral
direction, to partially surround the basilar artery. However, the mass
does not enhance. At this time, the histology (from a biopsy) and the imaging
were both consistent with a low grade diffuse fibrillary astrocytoma. Several
months later (c) the mass has progressed in size, and has developed diffuse
contrast enhancement. At this time a second biopsy showed a tissue diagnosis
of glioblastoma multiforme (GBM). It is common for a diffuse astrocytoma
to undergo progressive transformation into higher grades of neoplasia.
spin-density MR shows a mass forming a cast of the lumen of the fourth
ventricle. The mass has extended out of the ventricle and dorsally to reach
the cisterna magna.
T2WI MR shows remarkably bright signal intensity to most of the left cerebellar
hemisphere. Although the signal is abnormally high, the folia architecture
of the cerebellum has been preserved. Signal abnormality with minimal mass
effect and minimal architectural distortion are characteristics of infarction.
T1WI MR (a) there is a heterogeneous solid intraaxial mass with multiple
focal areas of hyperintensity (blood products - methemoglobin) and multiple
curvilinear hypointensities (flow voids). Only about one-third of hemangioblastomas
will have the "classic" cyst-with-nodule morphology. The remaining two-thirds
may be solid or complex masses. The angiogram (b) shows a hypervascular
nodule, with persistence of contrast into the venous phase (slow or delayed
transit time). Hypervascularity with slow transit is a characteristic of
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