Meningiomas

Meningiomas constitute the most common extra-axial neoplasm of the brain (Box 3-1). This lesion commonly affects middle-aged women. However, because of its incidence, it represents a significant proportion of the extra-axial neoplasms in men and in adults of all age groups. The most common locations for meningioma (in descending order) are the parasagittal dura, convexities, sphenoid wing, cerebellopontine angle cistern, olfactory groove, and planum sphenoidale. Ninety percent occur supratentorially. One percent of meningiomas occur outside the central nervous system (CNS), presumably from embryologic arachnoid rests. The most common sites for these “extradural” meningiomas are the sinonasal cavity, parotid gland, deep tissues, and skin. Because meningiomas arise from arachnoid cap cells, they can occur anywhere that arachnoid exists.

On unenhanced computed tomography (CT) scans, approximately 60% of meningiomas are slightly hyperdense compared with normal brain tissue (Fig. 3-2). You may see calcification within meningiomas in approximately 20% of cases. Rarely, cystic, osteoblastic, chondromatous, or fatty degeneration of meningiomas occurs. The specific histologic subtypes of meningioma—transitional, fibroblastic, and syncytial—cannot be readily distinguished on imaging.

Figure 3-2 Meningioma. Axial unenhanced computed tomography scan shows a hyperdense (arrows) extra-axial mass in the middle cranial fossa.

Magnetic resonance imaging is superior to CT in detecting the full extent of meningiomas, sinus invasion or thrombosis, vascularity, intracranial edema, and intraosseous extension. The typical MR signal intensity characteristics of meningiomas consist of isointensity to slight hypointensity relative to gray matter on the T1-weighted image (T1WI) and isointensity to hyperintensity relative to gray matter on the T2-weighted image (T2WI). The “cleft sign” has been described in MR to identify extra-axial intradural lesions such as meningiomas. The cleft usually contains one or more of the following: (1) cerebrospinal fluid (CSF) between the lesion and the underlying brain parenchyma, (2) hypointense dura (made of fibrous tissue), and (3) marginal blood vessels trapped between the lesion and the brain. One may see vascular flow voids on MR within and around a meningioma. Avid enhancement after contrast is also seen (Fig. 3-3), but occasionally meningiomas may have necrotic centers or calcified portions, which may not enhance. With MR you may be able to identify the dural tail, enhancement of the dura trailing off away from the lesion in crescentic fashion, which is typical of meningiomas and has been exhibited in up to 72% of cases (Fig. 3-4). There has been a debate in the radiologic literature as to whether the dural tail always represents neoplastic infiltration of the meninges by the meningioma or, alternatively, a reactive fibrovascular proliferation of the underlying meninges. In the typical situation where a differential diagnosis of vestibular schwannoma, meningioma, or fifth nerve schwannoma is debated, the dural tail may be a useful sign to suggest meningioma rather than other diagnoses (Table 3-1). The dural tail is not usually seen with schwannomas, although dural metastases may demonstrate a similar finding.

Figure 3-3 Parasagittal meningioma. A, This meningioma is really dark on T2-weighted image but the adjacent bright cerebrospinal fluid (CSF) demonstrates its extra-axial location nicely (CSF cleft sign designated by arrows). B, Add a dural tail (arrows) and strong enhancement, and you should be dictating this case as a meningioma.

Figure 3-4 Another piece of tail in a meningioma. Coronal enhanced T1-weighted image of this tentorial meningioma shows enhancing tails (arrowheads) coursing medially and laterally from the main portion of the tumor. Note that tumor extends to either side of the tentorium.

Table 3-1 Differential Diagnosis of Meningioma versus Schwannoma

Meningiomas may encase and narrow adjacent vessels. This finding helps in the sella region, because pituitary adenomas, the other culprit in this location, virtually never narrow the cavernous carotid artery.

The degree of parenchymal edema is variable in meningiomas. Although it is true that larger meningiomas tend to have a greater degree of parenchymal edema, there are exceptions, in which smaller meningiomas incite a large amount of white matter edema. The degree of edema seems to correlate with location, because meningiomas adjacent to the cerebral cortex tend to incite greater edema than those along the basal cisterns or planum. Edema associated with meningiomas may be caused by compressive ischemia, venous stasis, aggressive growth, or parasitization of pial vessels. Venous sinus occlusion or venous thrombosis can also cause intraparenchymal edema from a meningioma.

Intraosseous meningiomas may appear as expansions of the inner and outer tables of the calvarium or may even extend into the scalp soft tissues (Fig. 3-5). No dural component may be present at all. This type of meningioma strongly resembles a blastic osseous metastasis. Intraventricular meningiomas typically occur around the choroid plexus (80%) in the trigone of the lateral ventricle and have a distinct propensity for the left lateral ventricle (Fig. 3-6). Only 15% of intraventricular meningiomas occur in the third ventricle, and 5% occur in the fourth ventricle. Intraventricular meningiomas calcify in 45% to 68% of cases, and their frequency is higher in children. Multiple meningiomas are associated with neurofibromatosis type 2 (NF-2).

Figure 3-5 Osseous involvement by meningioma. Enhanced T1-weighted image demonstrates evidence of an osseous meningioma expanding the left calvarium outward as well as inward. Arrows depict the dural tail on either side of this lesion. There is a soft-tissue component extending under the galeal layer.

Figure 3-6 Intraventricular meningioma. A, Sagittal T1-weighted image shows a well-defined large mass in a dilated left lateral ventricular trigone. B, The mass, typical of a meningioma, is isointense to gray matter. It is centered on the choroid. C, Classic meningioma!

Bony changes associated with meningiomas may be hyperostotic or osteolytic and occur in 20% to 46% of cases. Hyperostosis is particularly common when the tumor is at the skull base or anterior cranial fossa, and here it may resemble fibrous dysplasia or Paget disease. The presence of bony reaction may be a helpful feature to distinguish meningiomas from other extra-axial masses, particularly schwannomas, which do not elicit a bony reaction. Bony changes, according to the neuropathology literature, may be due to actual tumor infiltration of the marrow space with osteoblastic metaplasia or merely due to the involved dura inciting hypervascularity of the periosteum and subsequent benign osteogenesis. Therefore, if the hyperostosis is along the inner table only, you cannot say whether it is due to neoplastic invasion or reactive changes. If the outer table is transgressed, tumor is most likely.

Meningiomas are one of the few tumors where angiography still may play an important role. Meningiomas diagnostically appear as lesions with an angiographic stain (tumor blush) and have both dural and pial blood supply. The characteristics of the stain are classically compared with an unwanted guest who comes early and stays late. Depending on the location of the tumor, you may have to perform internal carotid, external carotid, and vertebral injections (Table 3-2). In the typical convexity or sphenoid wing meningioma, the middle meningeal artery is enlarged (seen by primitive radiologists before angiography as enlarged meningeal grooves on the lateral skull film). At present preoperative embolization of meningiomas is sometimes performed to decrease the vascularity of the tumor. Make your referring neurosurgeon’s day by decreasing the intraoperative blood loss. Polyvinyl alcohol particles are most commonly used as the embolant (100 to 250 micromillimeters in size), but Gelfoam, cyanoacrylate, and trisacryl gelatin microspheres (Embospheres) are also employed by some.

Table 3-2 Blood Supply to Meningiomas

Skull base meningiomas in particular often require preoperative embolization. The skull base is the one region where meningiomas can become unresectable because of collateral damage to vital structures (e.g., the cranial nerves and carotid artery in the cavernous sinus meningioma, the vertebrobasilar vessels for foramen magnum lesions, the optic nerves at the optic canal).

On magnetic resonance spectroscopy (MRS), meningiomas are characterized by high levels of alanine and absent N-acetyl aspartate (NAA). No glutamine is seen. One can use the presence of taurine or gamma-aminobutyric acid in schwannomas to distinguish meningiomas from schwannomas.

Melanocytic Lesions

Within this category one finds diffuse melanocytosis, melanocytoma, neurocutaneous melanosis, and malignant melanoma. The melanin-containing cells are leftovers from neural crest origin. These diagnoses are difficult to make because metastatic melanoma to the dura does occur and must often be excluded. The melanocytoma is the most common of the lot and is seen in adults, whereas melanocytosis is more of a pediatric disorder (see Fig. 9-53). Although the latter is a diffuse process, the former usually presents as a posterior fossa mass. Hyperintensity on T1WI is the only hope for sealing this diagnosis, but the presence of this finding varies with melanin content. Spread of melanocytosis through the Virchow-Robin spaces is possible.

Malignant melanoma of the meninges may bleed or spread from the dura to the adjacent nerves, brain, or skull. Neurocutaneous melanosis shows melanocytic nevi of the skin (especially about the face), syringomyelia, and CNS lipomas. Prognosis is poor as is that for malignant melanoma of the meninges with distant metastases.

Schwannomas

The three neurogenic tumors (schwannomas, neurofibromas, and neuromas), are similar in appearance. Histologically schwannomas arise from the perineural Schwann cells. These cells may differentiate into fibroblastic or myelin-producing cells. Two types of tissue may be seen with schwannomas: Antoni A and Antoni B tissue. The Antoni A tissue consists of densely packed palisades of fibrous and neural tissue and typically has a darker signal on T2WI because of the compactness of the fibrils. Antoni B tissue is a looser, myxomatous tissue that is typically brighter on T2WI. Note that, depending on the degree of Antoni A and B tissue within schwannomas, the signal intensity of these lesions may directly simulate that of meningiomas. Other terms used for schwannomas include neurilemmomas and neurinomas, but the most accurate term is schwannoma.

Cellular, plexiform, and melanotic varieties of schwannomas have been described. Fifty percent of patients with psammomatous melanotic schwannomas have Carney complex, a syndrome characterized by facial pigmentation, cardiac myxomas, and endocrinologic disorders, including Cushing syndrome, acromegaly, pheochromocytoma, or adrenal hyperplasia (NAME syndrome: nevi, atrial myxoma, mucinosis of skin, and endocrine overactivity). More than 10% of melanotic schwannomas may become malignant. Malignant schwannomas may also be seen in patients with NF-1.

The distinction between meningiomas and schwannomas is a common one that radiologists must make (see Table 3-1). In some instances it is impossible to distinguish between the two. Both meningiomas and schwannomas may track along the course of the nerves; therefore the extent of the tumor is not helpful in distinguishing the two. As described previously, the dural tail is a helpful sign in suggesting a diagnosis of meningioma. In 81% of cases the border of a vestibular schwannoma makes an acute angle with the petrous bone; meningiomas usually make an obtuse angle. A curious finding has been described with vestibular schwannomas; arachnoid cysts coexist in 7% to 10% of cases, usually with the larger tumors (Fig. 3-7). These must be distinguished from cystic degeneration (and preponderance of Antoni B tissue) of the tumor, which is also a frequent phenomenon. Schwannomas may therefore be very bright on a T2WI, unusual for meningiomas. Vestibular schwannomas show microhemorrhages on susceptibility weighted scans in a high proportion—something absent in meningiomas.

Figure 3-7 Vestibular schwannoma with arachnoid cyst. A cyst (C) associated with enhancing acoustic schwannoma (T) is seen on this enhanced T1-weighted image.

One of the features distinguishing vestibular schwannomas from meningiomas is the expansion of the internal auditory canal (IAC) seen in schwannomas. The porus acusticus (the bony opening of the IAC to the cerebellopontine angle cistern) is typically flared and enlarged with vestibular schwannomas, whereas the amount of tissue seen in the IAC with meningiomas is usually small or absent. Vestibular schwannomas account for more than 90% of purely intracanalicular lesions, but only 5% to 17% of them are solely intracanalicular (Fig. 3-8). Approximately 10% to 20% of vestibular schwannomas present only in the cerebellopontine angle cistern without an IAC stem (Box 3-2). Approximately 75% of vestibular schwannomas have a canalicular and cisternal portion (Fig. 3-9).

Figure 3-8 Left intracanalicular vestibular schwannoma. Postgadolinium enhanced T1-weighted coronal scans show an intracanalicular mass (arrows) on the left side. Perform a fat-saturated scan to ensure that this does not represent fat.

Figure 3-9 Cerebellopontine angle and intracanalicular vestibular schwannoma. Note the brightly enhancing mass with both a cisternal (open arrow) and intracanalicular (arrowheads) portion. The ipsilateral prepontine cistern is enlarged.

On MR, schwannomas are usually isointense to slightly hypointense compared with pontine tissue on all pulse sequences. Enhancement is nearly always evident and homogeneous in approximately 70% of cases. Peritumoral edema may be seen in one third of cases, usually in the larger schwannomas. Less common features of schwannomas include calcification, cystic change, and hemorrhage. Subarachnoid hemorrhage is a rare presentation of vestibular schwannomas. Hydrocephalus may occur due to mass effect or obstruction of CSF outflow at the arachnoid villi level from “humours” from the “tumours.”

Schwannomas occur most commonly along cranial nerve VIII. The superior vestibular branch of cranial nerve VIII is the most common origin of the vestibular schwannoma (not “acoustic neuromas”), slightly more common than the inferior vestibular nerve. Nonetheless, patients typically have hearing loss. After lesions of cranial nerve VIII, schwannomas of cranial nerves VII (Fig. 3-10) and V are the most common site of intracranial neurogenic tumors, although ANY cranial nerve may be involved (Fig. 3-11).

Figure 3-10 Facial nerve schwannoma. A, Sagittal T1-weighted image (T1WI) shows a markedly thickened descending portion of the facial nerve (arrows). B, The mass is difficult to appreciate on the coronal T2WI were it not for the well-placed arrows delineating its course. (This is a new feature on the Geimensillip scanner.) C, The course of this enhancing mass suggests a facial nerve schwannoma by virtue of its tympanic segment and descending portion (star).

Figure 3-11 Fourth nerve schwannoma. FLAIR image through the brain stem demonstrates an extra-axial mass along the right side of the upper pons. One can faintly see the normal fourth cranial nerve (open arrows) on the left side coursing around the brain stem. The right-sided lesion (solid arrows) represents a fourth nerve schwannoma.

Postoperatively, it is not unusual to see linear gadolinium enhancement in the IAC after vestibular schwannoma resection as dural reaction. This can be followed expectantly. For those cases with progressive, nodular, or masslike enhancement, more careful follow-up is required to exclude recurrence.

Trigeminal schwannomas may arise anywhere along the pathway from the pons to Meckel’s cave to the cavernous sinus, to and beyond the exit foramina (ovale, rotundum, and superior orbital fissure). Outside the brain, the schwannomas of cranial nerve V most commonly occur along the second division. These tumors present with facial pain that is burning in nature.

Jugular schwannomas more commonly grow intracranially than extracranially and typically smoothly erode the jugular foramen. The border of the bone is sclerotic, as opposed to the paraganglioma, which has a much more irregular and nonsclerotic margin. Schwannomas compress the jugular vein, whereas paragangliomas (glomus jugulare tumors) invade the vein. Growth into the posterior fossa is the rule. Jugular foramen schwannomas most commonly present with hearing loss and vertigo rather than cranial nerve IX symptoms. Whether the schwannoma arises from one cranial nerve or the next, the imaging appearance is similar.

Neurofibromas

Neurofibromas, strictly speaking, refer to the tumors associated with neurofibromatosis. They are classified as WHO grade I, and all ages and sexes are represented. They may occur sporadically as well, though less commonly than schwannomas. The skin and subcutaneous tissues are affected more often than peripheral nerves; spinal nerve neurofibromas are rare, and cranial nerve involvement is uncommon. These lesions contain Schwann cells, perineural cells, and fibroblastic cells and may occur in a plexiform aggressive subtype, which appears as a network of diffusely infiltrating masses. Once again, Antoni A or B tissue may predominate in the lesion and will alter the T2WI characteristics. Do not think about neurofibromas for NF-2, only NF-1. NF-2 is a misnomer. Classically, in “neurofibromatosis” type 2 bilateral vestibular schwannomas are seen. Plexiform neurofibromas in the cranial nerves or peripheral nervous system may occur in NF-1 and are one of the seven criteria for diagnosis of this disorder. Plexiform neurofibromas, because of their extensiveness, can be distinguished from neuromas and schwannomas. They are generally found in the extremities or in the soft tissues of the head and neck. They have a significant rate of malignant dedifferentiation.

Classic descriptions say that neurofibromas can sometimes be distinguished from schwannomas by their eccentricity and more fibrous tissue signal on MR, but in many cases the two look the same.

Neuromas

By strict pathologic definition neuromas refer to a posttraumatic proliferation of nerve cells rather than a true neoplasm. The perineural lining and fibroblastic tissue seen in the other lesions just described are not present. These lesions are less common than schwannomas. They are usually seen in the cervical spine when nerves are avulsed or in an operative bed. Again, one must know where to look for these lesions. Unfortunately, in the common vernacular most people mean “schwannoma” when they say “neuroma” (e.g., “acoustic neuroma,” which is a double misnomer as these are truly “vestibular” lesions). On scanning, these masses look like small schwannomas.

Dural Metastases

Dural metastases usually spread out along the dura as hematogenously disseminated en plaque lesions from extracranial primary tumors (Fig. 3-12). Lung, breast, and prostate cancer, as well as melanoma, are known to produce dural metastases. Some of the dural metastases may arise from spread of adjacent bone metastases. Breast carcinoma is the most common neoplasm to be associated with purely dural metastases. Lymphoma is next most common but is unique in that the dural lymphoma may be the primary focus of the neoplasm (Fig. 3-13). Dural plasmacytomas will look nearly identical to dural lymphoma (Fig. 3-14). In children dural metastases are most commonly associated with adrenal neuroblastomas and leukemia. These tumors are also famous for lodging in the cranial sutures, widening them in an infant.

Figure 3-12 Metastasis with a dural tail. Postgadolinium-enhanced T1-weighted scan in a patient with adenocarcinoma of the lung demonstrates a metastasis peripherally in the right temporal lobe. Although a dural tail (arrows) is suggestive of a meningioma, it is not pathognomonic for it.

Figure 3-13 Dural lymphoma. A, See the low signal dural mass on the T2-weighted image that is eliciting the intraparenchymal edema? No? B, Then you cannot miss it on the enhanced scan. Lymphoma will nearly always enhance (unless the neurosurgeons are reluctantly treating it with massive doses of steroids). Include meningioma, sarcoidosis, plasmacytoma, and other dural metastases in the differential diagnosis.

Figure 3-14 Plasmacytoma of the dura. A, The gadolinium-enhanced scan looks just like one would expect for a meningioma with marked enhancement and a dural-based lesion. The irregular margins (saw-toothed appearance) suggesting pial spread would be funky for a meningioma though. B, Even the T2-weighted image shows low signal that simulates a meningioma or lymphoma or sarcoidosis. The way to score on this shot is to have a history of a plasma cell dyscrasia.

Occasionally, one can identify an adjacent parenchymal metastasis with dural spread (Fig. 3-15); alternatively, an osseous-dural metastasis (breast, prostate primaries) occasionally invades the parenchyma (Fig. 3-16). On MR the T1WI and T2WI characteristics are variable; however, typically the lesion is hypointense on T1WI and hyperintense on T2WI (Fig. 3-17 as an exception to the rule). On CT these lesions are identified as isodense thickening of the meninges. Contrast enhancement is prominent. This is a diagnosis where contrast-enhanced T1WI or fluid-attuenuated inversion recovery (FLAIR) scans can readily demonstrate the abnormality.

Figure 3-15 Leiomyosarcoma with dural-based metastasis. Postgadolinium coronal scan shows a high left frontal intraparenchymal metastasis, which demonstrates dural invasion (arrows) along its superomedial margin. (Ignore the phase-ghosting artifact.)

Figure 3-16 Intraparenchymal growth of renal cell carcinoma metastasis. A, T1-weighted scans show a lesion (L) centered on the calvarium with extracalvarial as well as epidural spread. B, The T2-weighted scan again demonstrates the center of the lesion to be at the bone. C, However, with contrast, at a more inferior location, there is a medial nodule, which extends through the dura into the intraparenchymal compartment (open arrows). Dural enhancement more peripherally is denoted by small black arrows.

Figure 3-17 Mucinous adenocarcinoma metastatic to the dura. A, Coronal unenhanced T1-weighted image (T1WI) shows a dural-based mass with low signal centrally and peripheral high signal intensity (arrowheads). The lesion was calcified on computed tomography. (Obsearvant readers will note the subcutaneous sebaceous cyst in the right upper part of the scalp.) B, On axial T2WI the lesion has low intensity but incites high signal intensity edema.

Inflammatory lesions that may simulate dural metastases include granulomatous infections (mycobacterial, syphilitic, and fungal), Erdheim-Chester disease, sarcoidosis, and Langerhans cell histiocytosis.

Subarachnoid Seeding

Usually one is dealing with tiny nodules of implanted tumor seedings. When the process is diffuse we use the term sugar-coating of the subarachnoid space because the whole pial surface is studded with sugar granules. A combination of sugar-coating and focal nodules may also occur. Subarachnoid seeding may occur with primary CNS tumors or primary tumors of other origins (Table 3-3). Lymphoma and leukemia are the most common tumors to seed the CSF. However, because they only rarely invade the meninges and do not incite reactions in the CNS, lymphomatous clusters are infrequently identified by neuroimaging techniques; the diagnosis is usually made by multiple spinal taps for CSF sampling. CSF seeding is associated with a mean survival of 1 to 2 months without and 6 to 10 months with treatment. The differential diagnosis could include arachnoiditis, Guillain-Barré syndrome, sarcoidosis, infectious granulomatous meningitides, Lyme disease, and cytomegalic inclusion virus (CMV) radiculitis.

Table 3-3 Sources of Subarachnoid Seeding

When a lesion has spread to the subarachnoid space, you may see it only on contrast-enhanced studies, and MR is much more sensitive than CT. However, unenhanced and enhanced FLAIR imaging has been shown to be increasingly effective at identifying subarachnoid disease, including metastatic disease. The malignant cells in the CSF or the associated elevated protein in the CSF will cause the usually low signal of CSF to be bright on a FLAIR scan. Although this may be a difficult diagnosis to make in the basal cisterns where “f” (FLAIR and flow) artifacts abound, the presence of such high signal over the convexities implies subachnoid seeding, subarachnoid hemorrhage, or meningeal inflammation. FLAIR may even be positive in lymphoma and leukemia, where enhanced scans fail most dramatically. FLAIR with contrast enhancement increases the yield even higher!

The typical locations where one identifies subarachnoid seeding are at the basal cisterns, in the interpeduncular cistern, at the cerebellopontine angle cistern, along the course of cranial nerves, and over the convexities (Fig. 3-18). One may see various manifestations of subarachnoid seeding, including the sugar-coated linear appearance to the enhancement of the surface of the brain (especially cerebellum) and spinal cord, or a nodular appearance of tumor deposits on nerve roots, both cranial and spinal. Often one identifies a peripheral intraparenchymal metastasis contiguous with the dural surface of the brain, from which cells are shed into the CSF. With subarachnoid seeding secondary hydrocephalus may be present.

Figure 3-18 Subarachnoid seeding from a brain stem glioma. Postcontrast T1-weighted image shows contrast-enhancing nodules (arrows) in the roof of the lateral ventricle and in the superior vermian cistern. The keen observer will notice the “overly pregnant” belly of the pons.

The other terms you will see for the same entity include meningeal carcinomatosis or carcinomatous meningitis. Clinically the patients present with multiple cranial neuropathies, radiculopathies, or mental status changes secondary to hydrocephalus or meningeal irritation. The cranial neuropathies may be irreversible. Although an initial CSF sample is positive in only 50% to 60% of cases, by performing multiple taps the positive cytology (and headache) rate approaches 95%. Patient survival is usually less than 6 months with this finding except in cases of hematologic malignancies. Breast cancer, lung cancer, and melanoma are the most common non-CNS primaries to seed the CSF.

Chloroma

Granulocytic sarcoma (chloroma) is a tumor of immature granulocytes found in association with myelogenous leukemias. This soft-tissue mass can occur virtually anywhere and may predate the diagnosis of leukemia. In the CNS, the orbit and epidural space are affected most commonly, but dural infiltration or even intra-axial involvement may be seen. The lesion, though eliciting bright vasogenic edema, may be intermediate intensity on T2WI, thought to be due to the high concentration of myeloperoxidase. It enhances. The term chloroma refers to its greenish color akin to chlorophyl. Chloromas portend a blast crisis. They are radiosensitive.

Choroid Plexus Papilloma

Choroid plexus papillomas (WHO grade I) comprise 3% of intracranial tumors in children and 10% to 20% of those presenting in the first year of life. Eighty-six percent of these tumors are seen in patients less than 5 years old. In children 80% occur at the trigone or atria of the lateral ventricles; in adults they are usually seen in the fourth ventricle. Overall, 43% are located in the lateral ventricle, 39% the fourth ventricle, 11% the third ventricle, and 7% in the cerebellopontine angle cistern. Multiple sites are present in 3.7% of cases. If they are seen at the foramen of Luschka, it may be due to extension from the fourth ventricle or the cerebellopontine angle cistern, or from primary involvement in this location from a choroidal tuft. Simian virus 40 (SV40) has been implicated in the evolution of these tumors. This same virus has been implicated in ependymomas.

The tumors present with hydrocephalus and papilledema caused by overproduction of CSF (four to five times normal) or obstructive hydrocephalus caused by tumor, hemorrhage, high-protein CSF, or adhesions obstructing the ventricular outlets. Lately, the obstructionists have gained the majority from the overproductionists as far as the explanation for hydrocephalus. Calcification occurs in 20% to 25% of cases, and hemorrhage in the tumor is seen even more frequently than calcification. Choroid papillomas are typically hyperdense on unenhanced CT, with a mulberry appearance. Tumors are usually of low signal on T1WI and mixed intensity on T2WI unless hemorrhage has occurred. These tumors enhance dramatically (Fig. 3-19). Between the calcification, flow voids, and hemorrhage, the tumor has a heterogeneous appearance, sometimes with a salt-and-pepper appearance from vessel supply.

Figure 3-19 Choroid plexus papilloma. This choroid plexus mass has a lot of peripheral high signal on T2-weighted image (T2WI) (A) and FLAIR (B), signifying edema of the parenchyma. C, Note the hydrocephalus and central necrosis of the tumor, as well as an extraventricular cyst (C) on the enhanced T1WI. D, A mulberry-like shape to the mass, centered on the trigone, is typical of choroid plexus papillomas and carcinomas.

Choroid Plexus Carcinomas

Choroid plexus carcinomas (WHO grade III) are much less common than papillomas, with malignant change occurring in fewer than 10% to 20% of cases. They are usually seen in the lateral ventricles. CSF dissemination is the rule with choroid plexus carcinomas, occurring in more than 60% of cases, but even benign papillomas may seed the CSF. It is difficult to distinguish a benign papilloma from a malignant one. Parenchymal invasion may suggest carcinoma. One can have primary melanomas of the choroid plexus as well.

The 5-year survival for completely resected choroid plexus papillomas is 100%; for carcinomas it is more like 40%.

Choroid Plexus Hemangiomas

Choroid plexus hemangiomas are benign neoplasms of the choroid plexus usually seen in the lateral ventricle. Although the tumor enhances markedly and may calcify, it is usually seen as an incidental finding in an asymptomatic patient. There is an association with Sturge-Weber syndrome. In this syndrome, choroid plexus hemangiomas ipsilateral to the leptomeningeal vascular malformation may be present (see Chapter 9).

Choroid Plexus Xanthogranuloma

Another benign condition of the choroid plexus is the xanthogranuloma. This incidental lesion may have fat density/intensity within it and is also centered on the glomus of the trigone. Curiously, they are frequently bright on DWI scans (Fig. 3-20). They are of no clinical impact, only rarely causing visual disturbance. Box 3-3 lists common choroid plexus neoplasms. For choroid plexus inflammatory processes, consider Box 6-10.

Figure 3-20 Xanthogranulomas of the choroid plexus. Although the FLAIR scan (A) may be relatively unremarkable, the diffusion-weighted image (DWI) (B) shows the bright signal in the xanthogranulomas of the choroid plexus (arrows). The presence of protein, cholesterol, or other compounds has been cited for why these are bright on DWI.

Epidermoids

There is some confusion involved with putting epidermoids and dermoids into a “tumor” chapter because most people think of these lesions as congenital epidermal inclusion cysts and dermal inclusion cysts. They really are not truly neoplastic and merely reflect two entities of ectodermal origin, one with just desquamated skin (epidermoid) and one with skin appendages such as hair follicles and sebaceous cysts (dermoid). Epidermoids and dermoids grow slowly and are histologically benign. Teratomas, usually lumped in the same category, are true neoplasms of multipotential germ cells, however. So despite their variable origins, we have placed these lesions in the neoplasm chapter to be consistent with other authors.

Epidermoids are collections of epithelium with desquamated debris (keratin and cholesterin) resulting from inclusion of ectodermal rests at the time of neural tube closure early in embryonic development. The walls are lined by simple stratified squamous epithelium, and the lesion has a pearly appearance. Men and women are affected equally, with a peak incidence in the 20- to 40-year age range. Epidermoids often occur in the cerebellopontine angle cistern (where they may present with trigeminal neuralgia and facial paralysis), the suprasellar cistern, the prepontine cistern, or the pineal region (Fig. 3-21). Extradural epidermoids are nine times less common than intradural ones and can arise within the diploic space, the petrous bone, and the temporal bone, where they appear as well-defined bony lesions with sclerotic borders (Fig. 3-22).

Figure 3-21 Epidermoid. A, Sagittal T1-weighted (T1WI) scan without contrast demonstrates a low intensity mass (E) similar to cerebrospinal fluid (CSF) anterior to the brain stem. Note the scalloped margin to the lesion at the pons–medulla–cervicomedullary region. B, On the T2-weighted scan, the lesion again has signal intensity similar to CSF. Note Meckel’s cave enlargement (funky arrow)—is it involved or merely dilated? C, No enhancement (no arrow) is seen on the postgadolinium T1WI. D, Aha! The diffusion-weighted scan shows that there is restricted diffusion within the mass and it is dissimilar to CSF based on its very high signal intensity. (Compare with CSF in fourth ventricle.) This is classic for an epidermoid.

Figure 3-22 Extradural epidermoids. Axial computed tomography scan shows a bony lesion scalloping the outer table of the skull (arrows) with a small superficial soft-tissue mass.

Epidermoids are typically of low density on CT. This is to be distinguished from dermoids or teratomas, which may have fat, bone, calcification, or other dermal appendages associated with them. Epidermoids expand to fill the interstices of the CSF space. An epidermoid is a lesion that is quite aggressive in insinuating itself around normal brain structures and often has scalloped borders. CT demonstrates a nonenhancing lobulated lesion. Sometimes epidermoids are hard to distinguish from arachnoid cysts, particularly in the cerebellopontine angle cistern (Table 3-4). Classically, epidermoids do not enhance. A stereotypical appearance on CT is that of displacement of the brain stem posteriorly by what appears to be just a dilated cistern anteriorly (see Fig. 3-21). In fact, this cistern is really a CSF density epidermoid with mass effect.

Table 3-4 Differentiation of Epidermoid and Arachnoid Cyst

Magnetic resonance has been very helpful in distinguishing between epidermoids and arachnoid cysts, a distinction that is sometimes blurred on CT. On MR these lesions are hypointense on T1WI and hyperintense on T2WI, similar to CSF. The advent of FLAIR imaging has made this an easy diagnosis because epidermoids are bright on FLAIR, whereas arachnoid cysts are as dark as CSF. On DWI these lesions are usually very bright and easily distinguishable from arachnoid cysts, which are dark on DWI. Once again, the lesion does not demonstrate enhancement unless it has been previously operated or secondarily infected. Rarely, epidermoids may be bright on T1WI—this usually is due to high protein and viscosity in the lesion.

Dermoid

The high intensity of fat and signal void of calcification on T1WI suggests a diagnosis of a dermoid or teratoma (Fig. 3-23). Dermal appendages, such as hair follicles, sebaceous glands, and sweat glands, are found histologically in dermoid cysts. These lesions more typically occur in the midline as opposed to epidermoids, which are generally off the midline. Male patients are more commonly affected, and patients are younger than those with epidermoids. The presence of fat may be suggested by an MR chemical shift artifact seen as a hyperintense and hypointense rim at the borders of the lesion in the frequency-encoding direction. Fat suppression scans decrease the intensity on T1WI (see Fig. 3-23C). The possibility of a ruptured dermoid should be considered when multiple fat particles are seen scattered on an MR or when lipid is detected in the CSF. (Rule out Pantopaque droplets.) Usually the lesions are very well defined.

Figure 3-23 The many faces of dermoids. A, This fatty mass seen on computed tomography scan could represent a meningioma with fatty degeneration, a teratoma, or a dermoid. It is extra-axial in location and scallops bone (arrowheads). B, T1-weighted scan (T1WI) of a different ruptured dermoid shows high signal intensity along the superior surface of the cerebellum (arrows). C, Fat-suppressed T1WI confirms that lesion is fatty. D, A larger mass is seen in the Meckel’s cave region anteriorly (asterisk). Chemical shift artifact (arrows) is seen along the inferior margin of the fat droplets extending posteriorly. E, This T2-weighted fat-suppressed scan demonstrates the dark signal suppressed fat (star) in the left Meckel’s cave region as well as dark signal intensity large (black arrows) and small (white arrows) fat deposits along the cerebellar folia, representing rupture of this dermoid tumor.

Teratoma

Teratomas are congenital neoplasms seen in neonates containing ectodermal (skin, brain), mesodermal (cartilage, bone, fat, muscle), and endodermal (cysts with aerodigestive mucosa) elements.

The pineal and suprasellar regions are common sites for intracranial teratomas. One will see a lesion of mixed density and intensity. The presence of an enhancing nodule in this neoplasm may help distinguish it from a dermoid. Infected or postoperative dermoids and epidermoids may show some peripheral enhancement, however. Teratomas are also common sacral neonatal masses.

Lipoma

Lipomas also probably should not be placed in a chapter on neoplasms because they are most frequently developmental or congenital abnormalities associated with abnormal development of the meninx primitiva, a derivative of the neural crest. Lipomas are particularly common in association with agenesis of the corpus callosum, and 60% are associated with some type of congenital anomaly of the associated neural elements (see Fig. 9-19). The most common intracranial sites for lipomas are the pericallosal region, the quadrigeminal plate cistern, the suprasellar cistern, and the cerebellopontine angle cistern. Because the tissue is histologically normal but located in an abnormal site, lipomas should best be termed choristomas, not neoplasms. Fat defines the lipoma; look for low density on CT, high intensity on T1WI, and low intensity on conventional T2WI that suppresses even further when fat suppression techniques are applied.

Grade I: Circumscribed Astrocytomas

Juvenile Pilocytic Astrocytoma

Cerebellar juvenile pilocytic astrocytomas (JPAs) are the most common infratentorial neoplasm in the pediatric age group and are classified as a WHO grade I astrocytic tumor (Table 3-5). They are seen in children. They are benign. They have a solid central piece and a separate peripheral portion. In general, pilocytic astrocytomas are well outlined from normal brain, are usually round, and usually are not ominous in appearance (Fig. 3-24). The lesions, when removed completely, are associated with an excellent prognosis (5-year survival rate >90%). Sixty percent of pilocytic astrocytomas occur in the posterior fossa, but they also favor the optic pathways and hypothalamus. Anaplasia is less common when the tumor is cystic than solid; therefore, the prognosis varies according to tumor morphology. The typical cerebellar astrocytoma in the pediatric age group is cystic (60% to 80%), whereas in older patients it is more likely to be solid. A mural nodule may be present with a similar appearance to the hemangioblastomas of adults. Occasionally the astrocytomas in the posterior fossa are solid, without a cystic component, and may simulate other pediatric posterior fossa masses (Table 3-6).

Table 3-5 WHO Classification of Astrocytic Tumors

Tumor	Grade	Peak Age (years)
Pilocytic astrocytoma	I	0–20
Subependymal giant cell astrocytoma	I	10–20
Pleomorphic xanthoastrocytoma	II	10–20
Diffuse astrocytoma	II	30–40
Fibrillary	II	30–40
Protoplasmic	II	30–40
Gemistocytic	II	30–40
Anaplastic astrocytoma	III	35–50
Glioblastoma	IV	50–70
Giant cell glioblastoma	IV	40–50
Gliosarcoma	IV	50–70

Figure 3-24 Cyst of pilocytic astrocytoma. A, The large cystic mass in the posterior fossa that causes cerebellar tonsillar herniation (squiggly arrow) and hydrocephalus in this child with a pilocytic astrocytoma. B,C, T2-weighted image shows the dominant feature of the cyst with a small mural nodule (n), which enhances in C.

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