MR_of_CNS_Tumors_and_Tumor_Mimics.pdf

1A D V A N C E D M R I 2 0 0 2 F R O M H E A D T O T O E

MR of CNS Tumors & Tumor Mimics


Blake A. Johnson

The diagnostic evaluation of brain tumorsand other mass lesions once requiredinvasive surgery or indirect diagnosticmodalities such aspneumoencephalography and angiography.Direct cross-sectional imaging capabilitiesnow allow for superior detection,localization and characterization of CNSlesions. As imaging equipment continues toadvance in sophistication, so too must thecapabilities and knowledge of neuroimagersin the realm of neuroanatomy, pathology,and imaging principles.

Although neuroangiography is no longerused as a primary diagnostic tool forevaluating brain lesions, it does serve anadjunctive role for the evaluation ofneoplasms, especially if embolizationtherapy is utilized prior to surgery.Determining the degree of tumor vascularityand the blood supply is best performed withconventional angiography. However the artof tumor localization, based on theangiographic appearance of vasculardisplacement, is no longer propagated withthe same intensity because of the currentreliance on more accurate and noninvasiveimaging modalities.

Computed tomography (CT) is the initialimaging study performed in many cases ofsuspected intracranial pathology due to itsavailability and utility as a diagnosticscreening tool. Direct imaging is generallyaccomplished in the axial or coronal plane,and the tissue contrast provided is based ontissue density. Brain morphology is welldepicted on CT images, although not aselegantly as with magnetic resonance (MR)imaging. Because CT is more sensitive forthe detection of calcification and bonydestruction, it is often employed as anadjunct to MR for the assessment of skeletalchanges and tumoral calcification.

Magnetic resonance imaging hassupplanted CT as the modality of choice forimaging CNS tumors. The multiplanarcapabilities of MRI provide superiorlocalization of brain lesions. In addition,superior contrast resolution is provided viamultiple sequences which exploit the varioustissue characteristics of normal andpathologic structures. MR is exquisitelysensitive for the detection of both solid andcystic lesions within the brain, and it isexcellent for characterizing secondary effectsof tumors such as edema, necrosis,hemorrhage and infarction. Multiplanarimages provide superior evaluation of mass

effect and associated anatomic distortion.

MR interpretation demands a greaterunderstanding of the imaging principles tooptimize image quality and avoid diagnosticpitfalls inherent to this powerful technique.While CT images provide a single electrondensity-weighted sequence, multiple se-quences must be interpreted whenperforming an MR examination. While thisgreatly expands the capabilities of thetechnique to characterize tissue, it alsodemands a higher level of expertise from theneuroimager. Familiarity with principles suchas relaxivity, susceptibility and flowphenomenon are essential for accurate MRinterpretation. Several technical pitfalls, orartifacts, must be also considered whenreading MR studies. Thus, a workingknowledge of neuroanatomy, pathology andMR imaging principles (including artifacts)constitute the prerequisites for accuratecranial MR interpretation.

Functional imaging techniques are rapidlymoving from a research tools to clinically-proven diagnostic modalities. Positronemission tomography (PET) to assessmetabolism is useful for distinguishinghypermetabolic lesions such as some tumorsfrom entities such as radiation necrosis,which may have similar CT and MR features.Functional MR allows for improved temporalresolution, and is already employed to mapeloquent cortex prior to surgery on adjacentlesions.

Magnetic resonance spectroscopy allows invivo detection of certain inorganic andorganic molecules. Characterization oflesions based on tumor constituents can beeffected by MR spectroscopic mapping.Further refinement of this technology andmore clinical trials will determine theubiquity of MR spectroscopy inneurodiagnosis.

Magnetoencephalography is another recentaddition to the armamentarium ofneuroimagers. Mapping functional braintopography can be utilized to plan thesurgical approach in order to avoideloquent brain such as the motor strip.Simple reliance on morphology in tumorpatients is unreliable, as functional anatomymay be displaced by, or even reorganizeddue to, the offending mass.

In this section, we will review the clinical andimaging features of central nervous systemneoplasms and lesions which mimic tumorson imaging studies. The imaging featuresand keys to differential diagnosis will be

reviewed.

SUPRATENTORIAL NEOPLASMS

Gliomas

Gliomas are the most common primaryintracranial neoplasm, representing 40% to45% of brain tumors. The vast majority ofgliomas are derived from astrocytic celllines. Astrocytomas have been traditionallyclassified by three- or four-tiered systemsbased on the degree of histologicmalignancy. In increasing order of histologicmalignancy, astrocytomas may becategorized as low-grade astrocytoma(LGA), malignant (anaplastic) astrocytoma(AA) and glioblastoma multiforme (GBM).Practical limitations of any grading systemresult from the usual heterogeneity tumorcell populations, sampling errors, and thepossibility of subsequent malignantdegeneration. These limitations are allimportant considerations when evaluatinggliomas.

Low-grade astrocytoma

Clinicopathology

This group comprises 15-20% of gliomas.Despite a less fulminant clinical course andmore organized histology of this tumorrelative to high-grade astrocytomas, a fataloutcome is the rule. Patients may presentwith focal seizures or neurologic deficits.Symptoms often antedate the diagnosis bymore than a year. The five-year survival rateis 20% to 33%. The peak incidence is in thefourth decade for hemispheric tumors,exclusive of cerebellar and brainstemgliomas.

The most common pathologic subtype is thediffuse fibrillary astrocytoma. Microscopi-cally, these tumors consist of cells containingpleomorphic nuclei of variable shape andsize. Astrocytomas tumors lack a well-defined interface with the adjacentparenchyma. Similar tooligodendrogliomas, lobar localizationwithin the brain is proportional to the whitematter volume. Thus, they are mostfrequently located in the frontal lobes,followed by the parietal, temporal andoccipital lobes. Posterior fossa astrocytomasare uncommon in adults, whereas inchildren, brainstem and cerebellarastrocytomas are the most commonintracranial tumor in some age groups.

Imaging

CT images of low-grade astrocytomasreveal a low density lesion. They are mostoften localized to the cerebral hemispheres,usually within the cortex or subcortical whitematter. Perifocal edema is sparse.Enhancement is variable; a majority of low-grade astrocytomas do not enhance. Inapproximately 40% there is faint, patchyenhancement of at least a portion of thelesion. Calcifications are detected in 15% to20% of lesions, and are less common in themore aggressive astrocytomas.Occasionally, the tumors are predominatelycystic, a feature that is typical of childhoodpilocytic astrocytomas.

MR images reveal homogeneouslyincreased signal on long-TR images andintermediate to low signal on the T1-weighted sequences. Local mass effectvaries with lesion size, and perifocal edemais usually minimal. Cyst formation,hemorrhage and necrosis are not commonfeatures of low-grade astrocytomas,although hemorrhage and cystic foci areoccasionally present. Enhancement is mildand irregular when present. The degree ofenhancement demonstrated by a fibrillaryastrocytoma correlates positively withhistologic malignancy. An interval increasein the intensity and amount of enhancementwithin a tumor indicates a more aggressivepopulation of cells, and may reflectprogression to anaplastic astrocytoma orGBM.

The differential diagnosis for low-gradeastrocytomas depends on location andimaging appearance. Other primary brainneoplasms should be considered. Highgrade tumors and metastases are usuallydistinguished by more intense enhancement,a greater tendency to be heterogeneous,and an association with more pronouncedperifocal edema. In addition, magneticsusceptibility effect, best observed withgradient-echo sequences, portends a highertumor grade of glioma. A cortical LGA mayappear similar to an infarct. Cerebritis isoccasionally a diagnostic consideration.

Anaplastic (Malignant)Astrocytoma

Clinicopathology

Anaplastic astrocytomas are intermediate indegree of histologic malignancy betweenlow-grade astrocytomas and glioblastomamultiforme. They represent 30-35% ofgliomas. Presentation is often similar to low-grade astrocytomas, but a shorter interval todiagnosis reflects the more aggressivenature of this lesion. Generally, AAs affecta slightly older population than their lower-

grade counterpart, with a peak incidence inthe fifth decade of life. Prognosis issignificantly worse, with a mean survival ofapproximately two years followingdiagnosis.

Microscopically, anaplastic astrocytomasdemonstrate high cellular density and moremalignant features than LGA, such asgreater nuclear pleomorphism and mitoticfigures. Cells with more abundant cytoplasmare occasionally present. The so calledgemistocytic subtype is diagnosed when asignificant number of these plump cells arepresent. The tumors lack the vascularproliferation and necrosis which characterizethe more malignant glioblastomamultiforme, however. Dedifferentiation ofthese tumors to glioblastoma most likelyoccurs, and the spectrum of malignantfeatures ranges from sparse mitotic figuresto very aggressive lesions, which approachclassification as a GBM.

Imaging

On CT images, anaplastic astrocytomasdemonstrate more heterogeneity than lowgrade astrocytomas due to cyst formationand hemorrhage. Nonetheless, the lesionsusually appear well-defined. Moderate masseffect and edema are characteristic. Thelesions are generally low density relative tobrain parenchyma.

T1-and T2-prolongation characterize thetumors on MR, frequently with signalcharacteristics indicating hemorrhage andfocal cystic degeneration. Calcifications arenot generally seen with anaplasticastrocytomas. It is uncertain whether thoselesions which do contain calcifications arosede novo, or represent a calcified LGA whichdegenerated into an AA. Enhancement isusually focal and irregular, most likelyreflecting the heterogeneity of the tumor cellpopulation. The more malignantcomponents show a higher propensity toenhance.

The differential diagnosis for anaplasticastrocytomas includes other primary brainneoplasms and metastatic tumors,depending largely on patient demographics.Cortical tumors occasionally have a similarappearance to infarcts. Cerebritis or earlyabscess formation may also have a similarappearance.

Glioblastoma multiforme

Clinicopathology

Glioblastoma multiforme (GBM) is the mostmalignant of glial tumors. Unfortunately, it isalso the most common, comprisingapproximately 50% of gliomas and 20% ofall intracranial neoplasms. The tumors often

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present like lower grade astrocytomas, but amore rapid initial appearance of symptomsmay mimic cerebrovascular disease. Thepeak incidence of this lesion is in the sixthdecade. Prognosis is extremely poor, with a6-month median survival a 6% five-yearsurvival rate following diagnosis.

Pathologically, extensive necrosis is theHallmark feature which distinguishes GBMfrom lower-grade astrocytomas. Vascularendothelial proliferation appears in theadjacent brain and even within the meningealstructures. Cells with bizarre nuclei andfrequent mitotic figures invade the adjacentparenchyma, leptomeninges, and dura.These malignant cells may extend along whitematter fiber tracts, including the corpuscallosum. At biopsy, malignant cells may befound beyond the boundaries of imagingabnormalities, centrifugal to both the marginof contrast enhancement and T2-prolongation on MR. Intratumoralhemorrhage is frequent, and hemosiderindeposition may accompany subacute blooddegradation products. Calcifications are nota feature glioblastoma multiforme. Rarely,sarcomatous degeneration of the adjacentvascular elements occurs, resulting in agliosarcoma. This compound tumor isindistinguishable from a GBM based onimaging studies, but should be suspectedwhen rare extracranial metastases occur.

Imaging

Imaging studies demonstrate extensiveedema and significant mass effectassociated with GBM. The heterogeneousappearance reflects the pathologic featuresdescribed above, with regions of necrosis,hemorrhage, and cystic degeneration. Thetumor is predominately low density on CT.Hyperdense areas within the tumor representmore recent hemorrhage, whereas lowattenuation foci indicate cystic change, oldhemorrhage and characteristic necrosis. Theheterogeneous appearance is augmentedfollowing contrast administration. Enhance-ment is irregular, usually in a ring- orgarland-like configuration due to theextensive central necrosis.

On MR images, tumoral necrosis andhemorrhagic residua are well-characterized,and contribute to the characteristicheterogeneous appearance. The extent ofperifocal edema is better delineated thanwith CT, but it is important to bear in mindthat even MR signal abnormalities do notreliably reflect the perimeter of tumor cells.Imaging-pathologic correlation studies haveshown tumor cells well beyond the marginsof signal abnormality on MR. Thisphenomenon may explain some cases ofmulticentric glioblastomas that aredescribed in as many as 5% of cases.

Enhancement is intense and irregular. Athick, irregular peripheral pattern ischaracteristic, often described as garland-like. Other common, albeit lesscharacteristic, enhancement patterns includemore extensive, irregular enhancement andsmooth rim-enhancement.

The differential diagnosis for glioblastomaincludes metastatic disease and, in theappropriate clinical setting, radiationnecrosis. Distinguishing recurrent tumorfrom radiation necrosis can be problematicfrom a diagnostic and therapeuticstandpoint. If the tumor has a simple rim-enhancing appearance, the differentialdiagnosis is more extensive.

Gliomatosis Cerebri

Clinicopathology

Gliomatosis cerebri (diffuse cerebralglioblastosis) demonstrates a variableclinical course but in general has a poorprognosis. Such patients most often presentwith personality changes and mentaldeterioration, but the presentation isvariable and may include seizures, evidenceof increasing intracranial pressure, or focalneurologic signs. Gliomatosis cerebri tendsto be infiltrative rather than destructive innature, thus the patient's neurologic deficitsmay be mild relative to the extent ofparenchymal involvement at the time ofdiagnosis. Peak incidence for this rareneoplasm is in the third and fourth decadesof life, but gliomatosis cerebri is seen in allage groups. Initial response to radiationtherapy is often favorable, but the disease isusually refractory to treatment uponrecurrence of symptoms.

Pathologically, gliomatosis cerebri isessentially a diffuse, confluent glioma. Thereis neoplastic overgrowth of glial elementswhich tend to infiltrate and expand the whitematter over a large portion of the brain.Grossly, there may be diffuse enlargement ofthe cerebrum, cerebellum, white mattertracts and/or the brainstem. Commonly,there is involvement of the midline structuressuch as the corpus callosum, fornices, andanterior optic pathway. A diffuse array ofundifferentiated astrocytes is demonstratedmicroscopically. There is generallypreservation of the neuronal architecture,and demyelination which is proportional tothe extent of involvement. At the periphery ofthe lesion, the tumor cells characteristicallyassume an elongated configuration.

Imaging

Involvement of the midline structures ischaracteristic, and both hemispheres arefrequently involved. The lesion may be subtle

on CT, and there are several reports ofoccult lesions on this modality. It maymanifest as ventricular effacement,decreased attenuation of the involvedparenchyma or regional expansion.Thickening of the hemispheric white matterand commissural tracts, including the corpuscallosum, fornices and anterior commissure,is characteristic. The septum pellucidum mayalso be involved. Enhancement is rarelydemonstrated on CT.

MR of this rare neoplasm demonstrates ill-defined, high signal intensity on long TRimages generally involving a large portion ofthe brain. Involvement is most pronouncedin the paramedian and midline structures.The short-TR images show a correspondingdecrease in signal, or isointensity withadjacent brain. Hemorrhage, calcificationand enhancement are not characteristicfeatures of this lesion, althoughenhancement has been reported in a patchyor leptomeningeal pattern.

Oligodendroglioma

Clinicopathology

Oligodendrogliomas constitute 5% to 7% ofintracranial neoplasms. They primarily affectadults, with a peak incidence in the fourth tofifth decade. Because of the indolent natureof the tumor, there is frequently a long latentperiod between the onset of symptoms anddiagnosis. With an average 50% to 60%five-year survival rate, many patients survivemore than a decade following diagnosis.

Histologically, oligodendrogliomas arehighly cellular tumors, which may result inspurious overgrading with respect tohistologic malignancy. Imaging findings arethus an important adjunct to thepathological assessment. Patternless sheetsof oligodendrocytes with uniformly roundnuclei are often accompanied by a reticularnetwork of small capillary structures. Focalnecrosis and hemorrhage are common. Ofall glial tumors, oligodendrogliomasdemonstrate the greatest propensity tocalcify. The calcifications associated with thetumors represent both vascular calcificationsand calcospherites within the tumor andadjacent parenchyma. Theseunencapsulated lesions generally display asharp interface with adjacent white matterbut are highly infiltrating at the cortical orgray matter interface.

One-third to one-half ofoligodendrogliomas are mixed cell forms,and may harbor astrocytic and/orependymal elements in addition tooligodendroglial cells. A high degree ofpleomorphism and other aggressive featuresportend a worse prognosis. The more



aggressive oligodendrogliomas tend todisplay a higher percentage of astrocyticattributes. Because such high-grade tumorsare often reported as glioblastomas, the trueincidence of malignant oligodendrogliomasis likely under-reported.

Imaging

Oligodendrogliomas occur most commonlyin the frontal lobe, followed by the parietal,temporal and occipital lobes; a distributionwhich is proportional to the relative volumeof white matter. Hemispheric lesions tend tobe superficial, and these long-standingintraaxial tumors not infrequently(approximately 20%) cause calvarialerosion. Less common locations include thedeep hemispheres, within the cerebellumand the spinal cord.

On CT, oligodendrogliomas are oftenheterogeneous, and iso- to hypodense.Calcifications are visualized in up to 90% oftumors, although most series report anincidence of 50 to 70%. Approximately 20%display focal hemorrhage, with a similarpercentage displaying cystic components.There is generally a paucity of perifocaledema. Enhancement is variable, and ispresent in approximately 50% of cases.

MR images demonstrate a heterogeneousappearance due to the variable componentscited above. T1-weighted imagesdemonstrate a mixed hypointense signalpattern. On T2-weighted images, the lesionsare primarily hyperintense. Cystic andhemorrhagic components are wellcharacterized by these two sequences. Thecalvarial changes, when present, are lesswell seen than with CT. The solidcomponents of the tumor may show mild tomoderate enhancement, furtherdistinguishing them from the cysticcomponents. In the pediatric and adolescentpopulation, the tumors demonstrate a lowerfrequency of enhancement, calcificationsand perifocal edema.

Differential diagnosis includes other glialneoplasms: glioblastoma, astrocytoma, andganglioglioma. When the lesion isintraventricular, central neurocytoma may beindistinguishable by light microscopy. In fact,the similar imaging and histologicappearance of these lesions is probablyaccounts for several cases of intraventricularoligodendrogliomas diagnosed prior to thediscovery of immunohistochemical andultrastructural differences between the twotumors. The dysembryoplasticneuroepithelial tumor is a cortical tumorwhich may have a similar histologic andimaging appearance. Finally, a singlemetastatic lesion will occasionally mimicoligodendroglioma, although there is

generally more edema and intenseenhancement with metastatic disease.

Ganglioglioma

Clinicopathology

Ganglioglioma is a slow-growing tumor withan excellent clinical prognosis, although it isworse for deep or midline lesions, despite arelatively benign histology. Gangliogliomasrepresent approximately 1% of intracranialneoplasms and 3% to 7% of pediatric braintumors. This tumor has a peak incidence inthe second decade, and although generallyconsidered a tumor of childhood and inyoung adults, up to 40% are diagnosed inpatients over 30 years of age. Most of thelesions are hemispheric in location,characteristically in the temporal lobe.Gangliogliomas may also be situated at thefloor of the third ventricle (suprasellar),within the cerebellum, in the brain stem andeven within the spinal cord.

Pathologically, gangliogliomas arecomprised of a mixture of glial cells(primarily astrocytic) and differentiatedneural elements. The astrocytic elementsmay be fibrillary or pilocytic. When theneuronal component is predominant, thelesions are appropriately termedganglioneuromas. A majority of lesionscontain cystic components, and occasionallythe lesions are primarily cystic, especially inthe posterior fossa. Approximately one-thirdcontain calcifications. Mineralization mayoccur within the wall of the tumor vessels oras organization granules within the brainparenchyma. Malignant transformation israrely associated with these relatively benignneoplasms, but some contain less matureganglion cells and neuroblasts. Such tumorsare categorized as ganglioneuroblastomas.

Imaging

CT images demonstrate mostgangliogliomas to be predominantly solid.They may be hypo-, iso- or hyperdenserelative to normal brain parenchyma.Approximately 50% of the lesions enhance;with an even greater percentage in someseries. A majority of lesions have a partiallycystic appearance. However, the actualcontents of the cystic foci often prove to besolid at surgery. Calvarial erosion may beseen with peripheral lesions, an uncommonsecondary finding described inapproximately 5% of cases. There isgenerally a paucity of edema, and irregularenhancement is seen in approximately halfof cases. Occasionally, the lesion may beoccult on CT images, and this has beenreported on MR as well.

On MR images gangliogliomas are

heterogeneous and generally display T1 andT2 prolongation. Occasionally, T1shortening results in hyperintensity on theshort TR images. Enhancement is usuallyirregular, and is demonstrated in greaterthan half of cases. There is usually minimalmass effect or perifocal edema. While focalcalcifications are less well seen than with CT,the extent of tumor is better delineated withMR. This is important from a clinicalstandpoint, as total excision of the lesion isthe treatment of choice; such patients canbe spared radiotherapy if complete excisionis accomplished and they remain free ofrecurrence.

Differential diagnosis includes other glialneoplasms such as low grade astrocytomaand oligodendroglioma. If the lesionappears cystic, consider juvenile pilocyticastrocytoma (which rarely calcifies) and othercystic glial neoplasms. A dysembryoplasticneuroepithelial tumor should also beconsidered, especially in a young patient witha history of seizures. Intracranial cysts may beconfused for cystic gangliogliomas on CT,but the two are easily distinguished on MR.

Dysembryoplastic neuroepithelial tumor

Clinicopathology

Dysembryoplastic neuroepithelial tumors(DNET) are a recently described tumor whichprimarily affects young patients. Primarilylocated peripherally within the cerebralhemispheres, they most often present with along history of partial complex seizures.Prognosis is excellent, although large seriesto assess survival statistics are not available.

Pathologically, the diverse cellularcomposition includes neuronal elements,astrocytes and oligodendrocytes within amucinous matrix characterize the DNET.Minimal neuronal atypia may cause these tosuperficially resemble mixedoligodendrogliomas by light microscopy. Amultinodular morphology, often withmicrocystic degeneration, further typifies thelesion. Cellular atypia is sparse, if present.Adjacent cortical dysplasia is characteristic.

Imaging

On CT images a hypodense cortical mass iswell demarcated, and a vast majority arefound within the temporal or frontal lobes.The lesion may approximate CSFattenuation. At surgery, however, the cystic-appearing component is often found to besolid Enhancement is usually absent, butfaint, irregular enhancement is reported.Like oligodendrogliomas, a smallpercentage of cases show adjacent calvarialerosion, reflecting the long-standing nature



of this indolent lesion.

MR images demonstrate a well-definedcortical mass with T1 and T2 prolongation.A psuedocystic appearance, withhyperintensity on T2-weighted images andan isointense appearance on intermediate-weighted scans, may be demonstrated. Truecystic components are occasionally present.Other lesions are hyperintense onintermediate-weighted images. Faintenhancement and calcification are presentin a minority of these tumors.

Other diagnostic considerations includeganglioglioma, oligodendroglioma, mixedglioma and cystic astrocytoma. This lesionshould be considered in patients withimaging features described above, as suchpatients may be spared radiation and otheraggressive therapeutic regimens.

Primary CNS Lymphoma

Clinicopathology

Primary central nervous system lymphomanow accounts for approximately 2% ofintracranial neoplasms. There has been asignificant increase in the number ofdocumented cases in bothimmunocompetent andimmunocompromised patients, with theAIDS population accounting for a significantproportion of this increase.

Clinical presentation is highly variable,depending on location of the lesion anddegree of mass effect. Patients may presentwith manifestations of increased intracranialpressure, seizures, or focal deficits. Peakincidence of primary CNS lymphoma is inthe sixth decade with males outnumberingfemales by at least 2:1 in theimmunocompetent population. Withimmunocompromised patients, the agedistribution is skewed to younger adults, andeven children with AIDS may harbor primaryCNS lymphoma. A rare case of the diseasein an immunocompetent child has beenreported. It is an aggressive neoplasia, witha grim prognosis. Median survival is lessthan 2 years following diagnosis.

Histologically, densely cellular aggregates oflymphoid cells are characteristically seen ina concentric perivascular pattern. Non-Hodgkins lymphomas seen within the brainare histologically equivalent to the systemiclymphomas and may be classified accordingto the Working Formulation for the Classi-fication of Lymphomas. Most areintermediate to high-grade B-celllymphomas; T-cell lymphomas are rare butare being recognized with increasingfrequency. Extensive central necrosis ischaracteristic of CNS lymphomas in AIDSpatients, but is uncommon in the

immunocompetent patient. Calcificationand hemorrhage prior to therapy are rare.

Criteria for the diagnosis of primary CNSlymphoma includes no evidence of diseaseoutside the brain, spinal cord (intradural),leptomeninges or intraocular structures.Fewer than 10% of CNS lymphomas aresecondary (metastatic).

Imaging

Brain lesions are multiple in approximatelyhalf of cases. There is a higher incidence ofmultiplicity in AIDS patients. Classically, CNSlymphoma has been described as a large,sharply demarcated tumor which is iso- tohyperdense relative to brain parenchyma.The lesions are situated in the deep orcentral structures, including the basalganglia, corpus callosum andperiventricular white matter. It should benoted, however, that up to half of the lesionsare located within the cerebral hemispheres.The degree of mass effect and perifocaledema is variable. Other patterns includediffuse subependymal spread, infiltratingwhite matter lesions or a gyral distributionwithin the cerebral cortex. Enhancement isusually homogeneous and intense followingiodinated contrast administration. In AIDSpatients, there is a higher incidence ofcentral necrosis with concomitant irregularor ring-like enhancement pattern is morecharacteristically seen.

On MR images, the lesions are iso- tohypointense relative to brain parenchyma onT1-weighted images. Long TR images showvariable signal intensity, ranging from hypo-to hyperintense to gray matter. Because mostCNS tumors demonstrate T2 prolongation,the iso- to hypointense appearance on thelong TR sequences should promptconsideration of CNS lymphoma. Despitethis, tissue is essential for the definitivediagnosis because treatment varies greatlyfor the lesions which may have a similarimaging appearance.

Differential diagnosis depends on thelocation and morphology of the tumor. If thelesion is peripheral, the hyperdenseappearance on CT may mimic ameningioma. Hemispheric and deep lesionsmay mimic metastases, primary glialneoplasms, and infarction, depending onthe MR and CT characteristics. For example,metastatic small cell and germ cell tumorsmay also be iso- to hypointense on long-TRimages. If the lesion has central necrosis anda rim-like enhancement pattern, abscessmust also be considered. A frequentdilemma in AIDS patients with anintracranial mass lesion is distinguishingCNS lymphoma from toxoplasmosis, theother common enhancing brain lesion in

such patients. Imaging features which favorlymphoma include subependymal spread,hyper-attenuation on CT and iso- tohypointensity on long TR images. Suchfindings may warrant a needle biopsy ratherthan a trial of empirical toxoplasmosistherapy in these patients.

Choroid plexus papilloma

Clinicopathology

Choroid plexus papilloma it typically en-countered in children, where it is generallysupratentorial in location. In adults,posterior fossa locations are most common,typically within the fourth ventricle orcerebellopontine angle cistern. This raretumor accounts for approximately 0.5% ofintracranial neoplasms overall, and 3% to6% of pediatric brain tumors. A majority ofthe lesions present within the first decade,less commonly in the second and thirddecades. Choroid plexus papillomaspresenting after the third decade are veryrare. In children, the most common locationis within the lateral ventricles. A left-sidedpredominance has been described; howeverlimited reports describe a right-sidedpredominance and statistical variance mayaccount for the laterality cited in thesereports. In the first decade, the lateralventricle is most common; after the firstdecade, the fourth ventricle is the mostcommon site for choroid plexus papillomas.Less commonly, they occur in the thirdventricle. In children, two-thirds to 80% areseen in lateral ventricle, 16% to 20% in thefourth ventricle, and fewer than 5% in thethird ventricle. Patients usually present withmanifestations of hydrocephalus. In thepediatric population, this may includeincreasing head size, lethargy, anddepressed mental status and seizures.Prognosis for the postoperative patient, ifcomplete resection is effected, is excellent. Ifsubarachnoid seeding or brain parenchymainvolvement occurs, prognosis is negativelyimpacted.

Histologically, choroid plexus papillomasreplicate normal choroid plexus with afrond-like pattern of growth in a singleepithelial layer supported by an underlyingvascular stroma. Despite the ultrastructuralsimilarity, a tendency toward columnarepithelial cells and a more compact cellarmatrix provide clues for a diagnosis ofchoroid plexus papilloma. The hallmark ofthis lesion is hydrocephalus, which is usuallysevere and may persist after shunting. Themechanism is generally accepted to beexcessive CSF production: up to five timesgreater than normal. The result is prominentventricles and extraaxial CSF spaces. Theinvolved ventricle may be asymmetrically



enlarged relative to the remaining ventricles.Other mechanisms that have been cited asan underlying etiology for hydrocephalusinclude an elevated protein content due tothe tumor, depressed CSF absorption by thearachnoid granulation secondary torecurrent hemorrhage, ventricularobstruction, diffuse meningeal spread of thetumor, and adhesions obstructing the outletforamina of the fourth ventricle. Even smallchoroid plexus papillomas, too small toproduce obstruction of the ventricle, mayresult in significant hydrocephalus. Thediagnosis should thus be suspected whensuch an appearance is encountered. Afibrovascular pedicle attaches the tumor tothe parent choroid plexus. This allows formotility of the tumor within the ventricles.The pliable lesion may even extend throughthe outlet foramina of the involvedventricles, a characteristic feature when it isencountered.

Imaging

CT images reveal a lobulated iso- tohyperdense mass within the ventricle. Inchildren, the lesions show homogeneousattenuation as a rule. In adults,heterogeneous appearance due to centralcystic degeneration is not uncommon. Suchregions show low attenuation relative to theadjacent remainder of the iso- tohyperdense mass. Invasion of the adjacentbrain parenchyma is uncommon but mayoccur as a late manifestation if resection isnot performed. Following contrastadministration, there is intense enhancementof the vascular lesions. Calcifications withinthe lesion range from finely stippled tocoarse in nature. Calcifications are morecommon in adult tumors, but have beenreported in up to 80% of pediatric series.Overall, calcifications occurred inapproximately 24%. Hemorrhage is lesscommonly identified.

With MR imaging, choroid plexuspapillomas are seen as well marginated,lobular lesions within the ventricle. Thechoroid plexus is engulfed by the tumorrather than displaced. T1-weighted imagestypically show an isointense lesion, whereason long TR images tumors range from iso-to hyperintense. Curvilinear signal voidsresulting from tumor vascularity,calcifications and/or hemorrhagic residuamay also be noted. As with CT, intenseenhancement is demonstrated following theadministration of intravenous contrast agent.MRI is very sensitive for the demonstration ofperiventricular interstitial edema whichfrequently accompanies the pronouncedhydrocephalus associated with these tumors.

Cross-sectional imaging modalities allow foran early diagnosis in the appropriate clinical

setting. Diagnosis prior to brain invasion orcompromise of the brain parenchyma byhydrocephalus or hemorrhage allows forfavorable outcome. The lesions may recur ifnot entirely resected, and there is also thepossibility of subarachnoid seeding prior totumor resection. Thus, there should be a lowthreshold for obtaining postoperativeenhanced imaging studies in such patients.

Choroid plexus carcinoma

Clinicopathology

Malignant degeneration of choroid plexustumors may occur. Approximately 10% to20% of choroid plexus tumors overall aremalignant, are associated with a muchworse prognosis. Carcinomatousdegeneration is most common in the lateralventricle and the prognosis is much worsewhen malignant features are present.Clinically, the hydrocephalus tends to be lesssevere than with choroid plexus papillomas.Histologically, an infiltrative growth patternwith a dense cellular matrix and an atypical(pseudostratified columnar) epithelial matrixis demonstrated. Aggressive features notseen in papillomas, such as mitoses,necrosis, and poor margination of thelesion, are demonstrated. Irregular growthof tumor neovascularity is also identifiedmicroscopically.

Imaging

CT and MR images demonstrate aheterogeneous lesion which invades theadjacent brain parenchyma. In children, theheterogeneity is often indicative ofmalignant degeneration; however in adultslack of homogeneity is frequently seen inbenign choroid plexus papillomas. Morepronounced cerebral edema and masseffect are demonstrated than are seen withchoroid plexus papillomas.

Differential diagnosis for choroid plexustumors includes papillary ependymoma,which is most commonly in the fourthventricle in children. Medulloblastomas tendto be centered within the vermis in children,with a greater tendency to involve thecerebellar hemispheres in adults. Rarely,degenerative changes in choroid plexus canmimic tumor; however hydrocephalus is nota feature when this is encountered. A rareintraventricular astrocytoma or ameningioma can also occur. The latter isgenerally seen in young to middle-agedadults, however. Intraventricular hematomaand colloid cysts should be distinguished bytheir imaging characteristics and location,respectively. Rarely, intraventricularmetastases are reported, however clinicalhistory facilitates the diagnosis.

Metastatic tumors

Clinicopathology

Secondary neoplasms involving the brainare relatively common, accounting for atleast 35% of intracranial neoplasms. Thereported incidence is increasing, likely dueto improved survival of cancer patients aswell as increased detection rates whichaccompany advancements in diagnosticimaging. Despite such improvementshowever, 10% to 15% of brain metastasesdevelop from unknown primary tumors.Patients may be asymptomatic at the time ofdiagnosis, or may present with seizures,increased intracranial pressure, focalneurologic deficits, mental status changes orobtundation. Peak incidence of brainmetastases parallels the peak incidence ofthe extra CNS tumors, which are mostcommonly encountered in the fifth throughthe seventh decades. Survival rates areunfortunately low and vary with the primarytumor site and degree of extra CNSpathology.

Microscopically, metastatic tumors oftenprovide clues regarding the source of theneoplasm, because the metastasis generallyrecapitulates the histology of the primarytumor. It is not uncommon, however, for thelesion to mimic a primary glial neoplasm orto be otherwise indeterminate with respect tosite of origin. Secondary tumors generallydemonstrate sharper demarcation from theadjacent brain parenchyma than that seenwith primary brain neoplasms.

Pulmonary neoplasms are the most commontumor to metastasize to the brain, followedby breast carcinoma and melanoma.Combined, these tumors constitute 70% to80% of brain metastases. Eighty to ninetypercent of such metastases aresupratentorial. Approximately 10% ofintracranial metastases result fromgastrointestinal and genitourinary tractneoplasms. Almost half of these metastasesoccur in the posterior fossa, making theoverall ratio of supra- to infratentorialmetastases approximately 4:1.

Imaging

The imaging appearance of metastaticlesions is highly variable, depending upontumor type, the presence of hemorrhage,cystic change and necrosis. Edema is usuallyextensive due to lack of a blood brainbarrier (BBB). Hemorrhage is seen inapproximately 20% of metastatic lesions,most commonly in malignant melanoma,lung, breast and renal cell carcinoma.Although multiplicity is the Hallmark ofmetastatic disease, almost half of patientshave a solitary metastases at the time of



diagnosis in some series. Most report a 60-70% incidence of multiplicity, however.Lesions occur most commonly in the middlecerebral artery distribution, concentrated inthe watershed zones and at the gray/whitematter junction. Although a vast majority oflesions occur in the cerebrum, posteriorfossa lesions are seen in 15% to 20%, withrare occurrences in the brain stem, pinealregion, and parasellar regions.

On CT images, metastatic lesions may behypo-, iso- or hyperdense. Moderate tosevere perifocal edema is present. Calci-fications are rare prior to therapy. Followingcontrast administration, intenseenhancement is noted, which may be solidor rim-like in distribution.

MR images better characterize the variabletumor constituents. Hemorrhage and otherparamagnetic substances, necrosis, andother pathologic features are welldelineated. Gadolinium-enhanced, T1-weighted images are the most sensitivesequence for the detection of metastases,and may be positive even prior to theappearance of abnormalities on long TRimages if sufficient perifocal edema or masseffect have not yet developed. This isespecially true if the lesions aresubarachnoid, rather than parenchymal.Double- and triple-dose enhanced imaging(exceeding the standard 0.1 mmole/kg)shows even higher sensitivity for thedetection of subtle metastatic foci.Magnetization transfer contrast imagingenhances the target-to-background ratiowithout the addition of additional contrastmedia.

PEDIATRIC INFRA-TENTORIAL NEOPLASMSBetween the second year of life and themiddle of the second decade, a majority ofpediatric brain tumors are located within theposterior fossa (65-75%). The most commonlesions are the cerebellar astrocytoma andmedulloblastoma (PNET), followed byependymoma and brainstem glioma. Duringthe first year or two of life, and afteradolescence, supratentorial lesions are mostcommon. In the pediatric population as awhole, intracranial tumors are equallydistributed between supratentorial andposterior fossa tumors.

Cerebellar Astrocytoma

Clinicopathology

Cerebellar astrocytomas present mostcommonly during the first decade of life.Prognosis is excellent, with a greater than90% 20-year survival rate. There is no

apparent gender predilection. Althoughthese tumors are relatively indolent, laterecurrence is not uncommon, sometimesseveral years after apparent eradication ofthe tumor. Pathologically, this well-circumscribed mass is usually partially cystic.Numerous microcysts, imparting a gelatin-like consistency, may also be encountered.The vast majority of pediatric cerebellargliomas are juvenile pilocytic astrocytomas(JPA). A minority are infiltrating fibrillaryastrocytomas, which may be low grade oranaplastic. Fibrillary astrocytomas have aless favorable prognosis and tend to presentin the second decade of life. The histologicfeatures of pilocytic and fibrillaryastrocytomas readily distinguish them fromthe other common pediatric tumors foundwithin the posterior fossa.

Imaging

Approximately half of cerebellarastrocytomas display the characteristicimaging appearance of a cystic lesion. Thesolid mural nodule shows intenseenhancement, delineating it from theproteinacious fluid within the cyst. The solidcomponent is not as hypointense as the fluidon T1-weighted images, nor as hyperintenseon long TR sequences. Some of the tumorsare predominately solid, with centralcavitation. Enhancement in these lesions isvariable, as the solid tumor componentsmay enhance. The areas of necrosis areappreciated as nonenhancing areas of T1-and T2-prolongation. A minority ofcerebellar astrocytomas are completely solidin appearance. Most lesions are located inthe midline, but up to 30% are located atleast partially within the hemispheres. Thejuxtaventricular lesions efface the 4thventricle, producing obstructivehydrocephalus. Calcification is seen in up to20% of lesions, while intratumoralhemorrhage is rare.

Medulloblastoma

Clinicopathology

Medulloblastoma is the second mostcommon posterior fossa tumor in children,representing 15% to 20% of intracranialneoplasms in the pediatric population. Insome series, it is the most common pediatricinfratentorial tumor. Peak incidence is in thefirst decade; however there is a seconddemographic peak early in the third decade.In fact, a full 25% to 30% of these so-calledpediatric neoplasms are found in adults.Male victims outnumber females. Medullo-blastomas are seen with increased frequencyin patients with Turcots syndrome, ataxiatelangiectasia and basal cell nevussyndrome. This aggressive posterior fossa

neoplasm has a propensity for CNS

dissemination via the subarachnoid space.

Pathologically, medulloblastomas are

generally considered to arise from the

pleuripotential cells which originate in the

germinative zone of the posterior medullary

velum. In normal cerebellar development,

they migrate superolaterally to form the

external granular layer of the cerebellar

hemispheres. Although medulloblastomas

are midline tumors in the majority of cases,

they may also occur lateral to the vermis

within the cerebellar hemispheres. An

increasing propensity to do so in older

patients may relate to the temporal aspects

of the tumor's pathogenesis, given the

developmental sequence described above.

The midline or vermian tumors may also

extend secondarily into the cerebellar

hemispheres. Histologically, the densely

cellular tumor contains a vascular stroma.



Imaging

On imaging studies, medulloblastomas aregenerally well-defined, homogeneous,midline lesions. They are hyperdense on CTand show variable enhancement followingcontrast administration. In children, cysticfoci and necrosis are uncommon.Hemorrhage and calcification are seen in aminority of cases. Tumors generallydemonstrate T1 prolongation and are iso- tohypointense on T1-weighted images. Onlong TR images, the signal characteristicsare variable. There is generally less T2prolongation than is demonstrated by otherbrain tumors. Thus, the lesion may be iso-,hyper- or hypointense relative to brainparenchyma.

In young adults, the imaging features aremore variable. Differences in the pathologicfeatures such as a higher prevalence ofnecrosis, cystic degeneration, anddesmoplastic variants provide anexplanation for the variability. Lesions inadults tend to be more heterogeneous inappearance, with cystic foci accompanyingthe solid component of the tumor.Enhancement is less intense and tends to bemore heterogeneous than thosedemonstrated in their pediatric counterparts.In addition, approximately half ofmedulloblastomas in adulthood are laterallysituated within the cerebellar hemispheres. Abroad-based interface with the tentorial ordural surface may even mimic an extraaxialneoplasm.

Ependymoma

Clinicopathology

Ependymomas represent approximately 5%of intracranial neoplasms overall, and 10%of pediatric intracranial tumors. The peakincidence of ependymomas occurs in thefirst decade, with an adult population peakin the 4th decade. Certain series of posteriorfossa ependymomas in children show apeak age at 5 years of age. Two-thirds ofependymomas arise in the posterior fossa,primarily within the fourth ventricle. Thesecond most common location is within thelateral ventricles, with a slight left-sidedpredominance. Adult tumors tend to besupratentorial and may be extraventricular,while the posterior fossa ependymomasoccur most commonly in children. Thereported prognosis for these lesions varieswidely. Five-year survival rates range from14% to 70%. An older age at diagnosis andtotal resection have both been statisticallycorrelated with improved prognosis.

Histologically, approximately three-fourths ofependymomas are low-grade malignancies.Perivascular pseudorosettes are

characteristically seen. Ependymomas maybe mixed tumors, and not uncommonlycontain other glial elements such asastrocytic and oligodendrocytic cell lines.Various astrocytic and oligodendrocytic celllines may be present. This histologicmlange may initially impede attempts ataccurate pathologic diagnosis. Grossly, theintraventricular tumors tend to extendthrough into the outlet foramina, explainingtheir occasional appearance within thecerebellopontine angles and foramenmagnum.

Imaging

CT images demonstrate a midline tumor,most commonly within an expanded fourthventricle. Hydrocephalus is virtually alwayspresent at the time of diagnosis. The lesionstend to be solid and are often hypo- toisodense on short TR images, andhyperintense on long TR images. The tumorsmay be heterogeneous, which helpsdistinguish them from medulloblastomas.Occasionally, a hyperdense appearance willmimic a medulloblastoma. Approximatelyhalf of the lesions demonstratecalcifications, which increases the specificityof the CT appearance. Enhancement isusually intense and may be either uniform orheterogeneous in nature. Hemorrhage is acommon feature of intracranialependymomas, unlike the other posteriorfossa tumors described.

MR images demonstrate an iso- tohypointense lesion with respect to whitematter on T1-weighted images. On long TRimages, T2 prolongation manifests as highsignal intensity relative to brain parenchyma.Cystic changes, more common in thesupratentorial lesions, are well depicted onMR images. Focal calcifications are not aswell identified as on CT. Hemorrhage isdemonstrated within ependymomas in up to13% of lesions. The various stages ofhemorrhage in evolution are well depictedon MR. The multiplanar capabilities of MRare also well suited for demonstrating thepropensity of ependymomas to spread intothe outlet foramina and aqueduct. Intenseenhancement is seen in over 90% of lesions.

Brainstem glioma

Clinicopathology

Astrocytomas arising within the brainstemusually present during the first decade of life.The pons is the most common location,followed in frequency by the midbrain andmedulla. Cranial neuropathy and long tractsigns are characteristic clinical features.Although up to 60% have and exophyticcomponent, these tumors are not resectable.

Usually responsive to radiation therapyinitially, these malignancies tend to recurwithin a 2 year interval. Thus, prognosis ispoor, with a 20% 5 year survival rate.Gliomas in the brainstem are generallyfibrillary astrocytomas, with a significantminority showing histologic high gradefeatures. Up to 20% of lesions are juvenilepilocytic astrocytomas. While the low gradefibrillary lesions demonstrate enlargement ofthe brainstem without a focal mass in mostcases, high grade lesions show characteristicnecrosis and hemorrhage.

Imaging

Brainstem gliomas display T1- and T2-prolongation, characteristic of most brainneoplasms. Enlargement of the brainstemwith effacement of the adjacent cisterns and4th ventricle are common. Ventral expansionaround the basilar artery is a typical featureof this lesion. Cystic changes andhemorrhage are well depicted on MR.Enhancement is variable, and is seen inabout 50% of cases. The enhancementpattern is often sparse, and may be absent inlow grade fibrillary astrocytomas. Aprominent exophytic component, withexpansion of adjacent cisterns, may impartan extraaxial appearance to these lesions.Careful attention to signal abnormalitieswithin the brainstem, the effect on the 4thventricle and the MR characteristics on pre-and post-contrast images will facilitateaccurate assessment of such lesions.

ADULT INFRATENTORIALNEOPLASMS The most common posterior fossaneoplasms in adults are secondary(metastatic) tumors, which are described inthe previous section. The followingdiscussion includes the most commonprimary infratentorial tumor in adults, andsome uncommon lesions found in thiscompartment. Occasionally, the pediatricposterior fossa tumors described abovepresent in young adults, thus these lesionsmust be considered in the appropriateclinical setting as well.

Hemangioblastoma

Clinicopathology

Hemangioblastoma is the most commonprimary posterior fossa neoplasm in theadult. Only metastases are more commonoverall in the adult population. This tumor isan important component of the autosomaldominant phakomatosis, von Hippel-Lindaudisease. Thus, when a hemangioblastoma isencountered, other manifestations of thisneuroepithelial disorder should be excluded.



Hemangioblastomas comprise 1% of intra-cranial neoplasms, and 7% of posteriorfossa neoplasms in the adult population.Forty percent of patients withhemangioblastomas have retinalhemangioblastoma or other manifestationsof von Hippel-Lindaus disease.Approximately 15% of von Hippel-Lindaupatients have hemangioblastomas. Peakincidence of this lesion is in the fifth to sixthdecade, but it presents earlier in patientswith von Hippel-Lindaus disease. There is amale predominance.

Pathologically, this highly vascular tumor iswell demarcated but lacks a true tumorcapsule. The lesion is most commonly foundin the cerebellar hemispheres. Lesions inother locations such as the vermis,brainstem and cervical cord are rare, andare usually associated with von Hippel-Lindau disease. Approximately two-thirds ofthe lesions are cystic. The cystic tumors tendto be larger than solid lesions and usuallycontain a well vascularized peripheral tumornodule. This vascular nodule abuts the pialsurface and is comprised of a fine meshworkof blood channels and capillaries with anintervening stroma. The gelatinous cystcontents may contain hemorrhage.Associated polycythemia is occasionallypresent and is thought to be the resulttumoral erythropoietin production. One-third of the lesions are solid. The solidtumors have a higher frequency in the brainstem and supratentorial compartments.

Imaging

Angiographically, the mural nodule or solidtumor reveals its vascular nature with intensecontrast accumulation via the prominent,serpiginous vessels which supply it. The cystis hypodense relative to brain on CT images,whereas the solid components are isodense.The enhanced study displays the feedingvessels in a majority of cases with intenseenhancement of solid tumors and muralnodules within cystic lesions.

On MR imaging, T1 and T2 prolongationresults in a hyperintense appearance onlong TR images and hypointensity on T1-weighted images. The margins and extent oftumor are better depicted than with CT.Lesion detection, especially with the solidtumors, is also more sensitive utilizing thismodality. Differential diagnosis based onimaging appearance includes pilocyticastrocytoma, especially if the lesion ispartially cystic. In adults, solid lesions mustbe distinguished from metastases. Peripheralsupratentorial lesions may have a similarappearance to angioblastic meningioma.

Subependymomas

Clinicopathology

Subependymomas are often considered asubcategory of ependymomas. The lesionswere described in 1945 by Scheinker andare often incidental lesions which arediscovered during imaging studies or atautopsy, with peak incidence in the sixthdecade. These indolent tumors affect anolder population than ependymomas andare usually found along the ventricularsurface, most commonly adjacent to theseptum pellucidum or floor of the fourthventricle.

Histologically, subependymomas resemblenormal periventricular glial tissue. They arecomprised primarily of astrocytes andependymal tubules. Three-fourths of thelesions occur within the fourth ventricularmargins.

Imaging

Imaging studies show such lesions to beindistinguishable from ependymomas. Thelesions tend to be homogeneous; thus grosscystic changes favor a diagnosis ofependymoma, as does a younger age atpresentation. Calcifications are common.The lesions tend to be isodense to theadjacent brain parenchyma on CT images.Isointensity on T1-weighted MR images isalso the rule. T2 prolongation and/orcalcification make the lesions easy torecognize. Differential considerationsinclude ependymoma, subependymalhamartoma, giant-cell astrocytoma, and incertain cases, heterotopic gray matter.

Dysplastic cerebellargangliocytoma

Clinicopathology

This rare entity was described in 1920 byLhermitte and Duclos and may represent adysplasia rather than a true neoplasm.Differentiation and migration anomalies ofthe granule cell precursors underlie thisinteresting disease. There is a femalepreponderance with peak incidence in thethird and fourth decades if a familialtendency has been implicated.

Pathologically, there is a thickenedmolecular layer within the cerebellum withhypermyelination and an abnormal granularlayer. Absence of Purkinje cells completesthe histologic constellation. The grossspecimen demonstrates flat, broad folia withhypertrophy of the involved cerebellarhemisphere. Surgically, the margins of theabnormality may be difficult to distinguishfrom normal adjacent cerebellum.

Imaging

CT images demonstrate low-density masslesions without enhancement. Cysticchanges, hemorrhage and calcifications arenot features of the cerebellargangliocytoma. Erosion of the adjacentcalvarium may be present. Obstructivehydrocephalus may result from fourthventricular compromise.

MR images demonstrate an inhomogeneousmass with a disorganized, serpiginouspattern of low signal structures having theappearance of folia. On long TR images,the tumor has well defined margins anddisplays relatively homogeneoushyperintensity. Mass effect is welldemonstrated on multiplanar MR images.

Although low-grade astrocytic neoplasmsand other posterior fossa tumors such aslateral medulloblastoma may be consideredin the differential diagnosis, the MR featuresincluding lack of enhancement generallyallow for a specific diagnosis to besuggested when this rare tumor isencountered.

DIFFERENTIAL DIAGNOSISThe differential diagnosis for intracranialneoplasms includes cerebral abscess, whichmay solitary or multiple. Radiation injurymay also produce mass lesion(s) which aredifficult to distinguish from primary andsecondary tumors within the brain.Occasionally, demyelinating disease mayproduce rim-enhancing lesions. Theimaging appearance of these conditions arediscussed in detail in another section.Vascular lesions and hematomas can alsocause intracranial masses. Cavernousmalformations can be recognized on thebasis of blood degradation products withinand bordering the lesion. Hematomaformation may follow trauma or bespontaneous in such conditions as amyloidangiopathy. The task of the imager is toexclude an underlying lesion in addition tothe heme products. A bland or hemorrhagicinfarction may also be mass-like inappearance. The location of the lesion, itsconfiguration, and enhancement patternmay provide morphologic clues to itsidentity. Advanced techniques such asdiffusion imaging or spectroscopy providethe means to assess the physiological andbiochemical properties of various forms ofpathology, and will be used with increasingfrequency as they mature.

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