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Posterior Fossa Masses:

 Differential Diagnosis

 and

 Radiologic-Pathologic Correlation

James G. Smirniotopoulos, M.D.

Uniformed Services University of the Health Sciences

4301 Jones Bridge Road Bethesda, MD 20814

and,

The Armed Forces Institute of Pathology Washington, DC

The opinions expressed herein are those of the author, and are not to be construed as representative of the University or the Department of Defense.

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Introduction

The differential diagnosis of intracranial lesions begins with an accurate assessment of the lesion location. In fact, perhaps the most important diagnostic feature of an intracranial mass is its location. Intracranial masses are very commonly divided into intraaxial and extraaxial locations. However, there is also great value in localizing masses as being supratentorial instead of being infratentorial. The diagnostic subset of infratentorial masses is quite distinct from the supratentorial location. This will be a short review of the differential diagnosis of infratentorial masses.

Infratentorial 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.
 
 

EXTRAAXIAL MASSES

Let's begin with the discussion of extraaxial masses. Extraaxial masses are external to or outside of the pial membrane that invests the brain, spinal cord, and the proximal portions of the nerve root. Extraaxial masses of posterior fossa typically present in adult patients. The most common location for a posterior fossa extraaxial mass is in the cerebellopontine angle cistern. (Figure 1) This cistern represents an enlargement of the subarachnoid space created by the normal indentation in the brain contour at the angle between the pons and the cerebellum. The most common cerebellopontine angle (CPA) mass, by far, is the vestibular Schwannoma,accounting for roughly 7/9 CPA tumors. REF 1 These masses have been previously described as "acoustic neuromas", however, this is a double misnomer since the lesions are histologically composed of Schwann cells (not neurons) and typically arise from the vestibular (not the cochlear) portion of the eighth cranial nerve. Vestibular Schwannomas are sporadic in more than 95% of cases. However, they may be associated with neurofibromatosis Type II (Wishart Neurofibromatosis). REF 2 In patients who have this genetic disorder( NF II), more than 75% of affected individuals will have bilateral vestibular Schwannomas. Conversely, the presence of bilateral vestibular masses is used in the NIH criteria, as part of the definition of the syndrome of NF II. NF II is caused by a chromosomal deletion on the long arm of chromosome 22. It is inherited as an autosomal dominant disorder without racial or sexual predilection. REF 2 It is quite different in most manifestations from the more common von Recklinghausen's neurofibromatosis (NF I).

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)

 A 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. REF 7
 
 

INTRAAXIAL AND INTRAVENTRICULAR MASSES

The overwhelming preponderance of posterior fossa masses in childhood are glial and will present as either intraaxial or intraventricular lesions. (Table I) The nonglial tumors (Schwannoma, meningioma, and epidermoid inclusion cysts) do not typically present in children. Approximately 1/4 to 1/3 of posterior fossa masses will be PNET (primitive neuroectoderm tumors). In this posterior fossa location, this tumor was formerly known as medulloblastoma. Medulloblastoma (PNET) accounts for approximately 1/4 to 1/3 of posterior fossa masses in childhood. The most common site of origin appears to be either the ventricular roof or the cerebellar vermis immediately behind the ventricular roof. (Figure 4) Although the mythical "medulloblast" has never been identified, the currently proposed cell of origin is the external granular cell. The external granular cell is a normal embryologic precursor cell that invaginates into the developing cerebellar folia to form the mature internal granular cell layer. Medulloblastoma can cause CSF dissemination, and possible subarachnoid enhancement should be carefully evaluated. Roughly "tied" for first place in the hit parade for posterior fossa masses is the astrocytoma, and the most common type here is the juvenile pilocytic astrocytoma (JPA). REF 8 This is a very special type of astrocytoma that has a circumscribed (rather than infiltrating) pattern of growth. The pilocytic astrocytoma also accounts for 1/4 to 1/3 of pediatric posterior fossa masses. Pilocytic astrocytomas are often times readily identified because of their presentation as a partially cystic mass with the neoplastic burden limited to a nodule in the wall of the cell (a "mural nodule"). (Figure 5) REF 8

 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.

 Most 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.
 
 

Adult Parenchymal Masses

Intraaxial and Intraventricular Masses of the Adult Posterior Fossa. We must at the outset distinguish between neoplastic and non-neoplastic masses. The acute presentation of a cerebrovascular accident in the posterior fossa (either hemorrhage or infarction) can be associated with acute mass effect, ventricular compression, and the classic clinical signs of a tumor (nausea and vomiting). (Figure 8) Concentrating on the neoplastic masses of the adult posterior fossa, we have to consider the possibility of metastatic disease, primary benign tumors such as hemangioblastoma, and the glial tumors. Glial tumors are distinctly uncommon in the adult posterior fossa, as compared to childhood. Adult patients usually do not develop the benign pilocytic type of astrocytoma. Instead, an adult posterior fossa astrocytoma, whether it is cerebellar or in the brainstem, is typically a high-grade astrocytoma that is diffusely infiltrating and largely unresectable. Adult patients may also present with ependymomas in the fourth ventricle.

 In 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". REF 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. REF 10 REF 12In this genetic condition, there are multiple cerebellar tumors (hemangioblastoma) as well as multiple lesions affecting the kidneys and other abdominal viscera. REF 12 Most importantly, the kidneys may contain simple cysts, but may also harbor multiple renal cell carcinomas.
 
 

In Summary:

The differential fossa masses can be built upon a matrix of two large age categories (adults and children) and two location categories (intraaxial versus extraaxial). Whenever a posterior fossa mass is encountered, the localization generates a "list of suspects" and then each lesion is compared against the suspect "profiles". If there is a match, we have a likely diagnosis; if not, we must provide a broader differential diagnosis.
 
 

FIGURE LEGENDS


Figure 1. Vestibular Schwannoma.

Axial 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.
 
 
 
 
 
 
 
 
 
 
 
 




Figure 2. Meningioma.

Contrast 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.
 
 
 
 
 
 
 
 
 
 
 
 




Figure 3. Epidermoid Inclusion Cyst.

Axial 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.
 
 
 
 
 
 
 
 
 
 
 
 




Figure 4. Medulloblastoma (PNET).

a) 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.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 




Figure 5. Pilocytic astrocytoma.

a) 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.
 
 
 
 
 
 
 
 
 
 
 
 
 




Figure 6. Pontine astrocytoma.

Axial 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.
 
 
 
 
 
 
 
 
 
 
 
 
 




Figure 7. Ependymoma.

Sagittal 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.
 
 
 
 
 
 
 
 
 
 
 




Figure 8. Cerebellar infarction.

Axial 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.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 




Figure 9. Hemangioblastoma.

Axial 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 hemangioblastomas.
 
 
 
 
 
 
 
 
 
 
 
 




 
 









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