Individual Tumors

Acoustic Schwannoma

Acoustic schwannomas account for 8 to 10 percent of all intracranial tumors and 70 to 80 percent of CPA tumors. They most frequently arise from the vestibular portion of the eighth cranial nerve. These tumors occur most commonly between the ages of 30 and 70 years, with a peak incidence between ages 40 and 60. Patients with type 2 neurofibromatosis often develop bilateral acoustic schwannomas, which is sufficient to make the diagnosis of the disease. Approximately 5 to 20 percent of patients with solitary intracranial schwannomas have type 2 neurofibromatosis. In the majority of patients, acoustic schwannomas will develop as a sporadic neoplasm. Vestibular schwannomas are thought to arise from the transition point between myelin production from oligodendrocytes (central myelin) and that from schwann cells (peripheral myelin). This point of transition is usually thought to occur at the opening (porus acusticus) of the lAC. For this reason most acoustic schwannomas are thought to arise within the lAC. In one review  (17 percent) acoustic schwannomas were entirely intracanalicular, (10 percent) were purely extracanalicular and (74 percent) occupied both the CPA cistern and the lAC. There are two primary explanations for the purely extra­canalicular lesion, the transition point occurs in the CPA cistern and with growth the distal vestibular nerve is avulsed.

On unenhanced CT, schwannomas are generally isodense to hypodense relative to the pons and adjacent brain and generally protrude into the CPA cistern. This is associated with widening of the ipsilateral CPA cistern and in this location is a diagnostic sign of an extra-axial neoplasm. A mass effect on the cerebellum and fourth ventricle is variable and depends on the size of the tumor. Calcification is exceedingly rare and if demonstrated, should raise the suspicion of a meningioma. A CT bone algorithm with 1.5-mm-thick sections will reveal the characteristic remodeling of the porus acusticus and widening of the lAC. Occasionally, large tumors will be entirely extracanalicular and there will be no bony erosion or lAC widening. In these instances the diagnosis of an acoustic schwannoma is less certain.

Following the intravenous infusion of an iodinated contrast agent most acoustic schwannomas (two-thirds) enhance homogeneously, especially small neoplasms. Larger neoplasms are more likely to enhance around the periphery, while the central portion will be relatively nonenhancing. Areas of inhomogeneous enhancement correspond to necrosis, cystic degeneration or hemorrhage.

High-resolution, thin-section CT is limited in its ability to detect small lesions confined to the internal auditory canal, thus, contrast-enhanced MRI has become the most sensitive technique in the evaluation of acoustic schwannomas, replacing CT and gas CT cisternography for assessment of small intracanalicular lesions.

Thin-section (3-mm) MRI evaluation of the internal auditory structures is the modality of choice. The use of a contrast agent is paramount because prior to the use of gadolinium agents, many of the smaller lesions were missed. On unenhanced T1 -weighted studies most acoustic schwannomas are isointense or hypointense to adjacent brain tissue (pons). Occasionally, the tumor is brighter than the adjacent brain on T1-weighted images: this may occur because of proteinaceous fluid or recent intratumoral haemorrhage.

The appearance on T2-weighted images is more variable, with larger tumors tending to be more heterogeneous in morphology and signal intensity. Acoustic schwannomas are composed of various concentrations of Antoni A and Antoni B type histologic patterns. Antoni type A tissue has a compact texture composed of interwoven bundles of bipolar spindle cells: this usually results in areas of homogeneous signal on T2-weighted images. Antoni type B tissue consists of a loose reticulated myxoid matrix within which degenerative changes, including cyst formation and fatty degeneration are common, this usually results in heterogeneous hyperintensity on T2-weighted images. Tumor vascularity produces well-defined curvilinear areas of signal void. These vessels have the potential for thrombosis and haemorrhage, contributing to mixed attenuation patterns on CT and a mixed signal pattern on MRI.

Oedema of the adjacent brain is best visualized on T2-weighted images and although not marked it can be seen to a mild to moderate degree. Virtually all acoustic schwannomas enhance intensely after gadolinium contrast administration. In one large series, the enhancement pattern was homogeneous in 67 percent, slightly inhomogeneous in 10 percent and heterogeneous in 22 percent.

Of the tumors with a component in the CPA cistern, most are centered at the level of the meatus of the lAC. The angle formed between the tumor border and the petrous bone is generally acute. The fourth ventricle is compressed and displaced contralaterally by large tumors.

Tumor shape is mostly round, oval or poly lobular, irregular margins may be present. Assessment of tumor margin and extent of tumor is improved significantly by the administration of a contrast agent. Peritumoral CSF cysts have been noted to be associated with acoustic schwannomas greater than 25 mm in diameter.

Meningioma

Meningiomas constitute approximately 12.5 percent of all intracranial neoplasms. The CPA is the eighth most common site of involvement, with approximately 5 percent of all meningiomas occurring in this location. After acoustic schwannomas, meningiomas are the second most common CPA tumor, constituting approximately 10 percent of CPA lesions. Meningiomas involving the CPA may originate from the arachnoid granulations associated with the venous sinuses or those that exist in relation to cranial nerves and their foramina. Meningiomas can be divided into five histologic subtypes: fibroblastic, transitional, syncytial, angioblastic and mixed. The fibroblastic subtype may be histologically difficult to distinguish from acoustic schwannomas.

CT of meningiomas in the CPA usually reveals a well-circumscribed spherical or ovoid mass of higher attenuation than brain on a noncontrasted CT scan. Occasionally. calcification is demonstrated within the tumor mass. Generally, the epicentre of the mass is eccentric to the meatus of the lAC. Meningiomas are usually larger than acoustic schwannomas at presentation: hearing loss or vestibular symptoms are not necessarily a major clinical component. These tumors are broad based against the dural surface of the posterior medial petrous bone and may "wrap" around the petrous apex. Most meningiomas show an obtuse angle at the interface between the tumor and the posterior surface of the petrous bone. A few meningiomas have a plaque-like configuration, and a small percentage are rounded and thus mimic acoustic schwannomas. Hyperostosis of the temporal bone is more difficult to detect because of its inherent marked density. Thus, hyperostosis may not be an obvious distinguishing feature as with meningiomas in other locations. Rarely, purely intracanalicular meningiomas have been reported.

On MRI, meningiomas are typically sharply circumscribed masses that are characteristically hypointense to isointense with T1-weighted pulse sequences and isointense to hyperintense with T2-weighted pulse sequences. Following the intravenous injection of a contrast agent, the mass enhances homogeneously. The meningeal sign (the enhancement of a tail of dura adjacent to the main bulk of the tumor following the administration of gadolinium) is seen frequently with meningiomas. This feature is not specific and has also been associated with other neoplasms, including schwannomas and metastases. The enhancement of the dura is caused by tumor infiltration and/or reactive changes.

A wide spectrum of signal intensities may be encountered in meningiomas, reflecting the diversity of the histopathologic features. In one study, meningiomas that were markedly hypointense to cortex on T2-weighted images were composed predominantly of fibroblastic and transitional elements, whereas markedly hyperintense meningiomas demonstrated predominance of syncytial or angioblastic elements. At the tumor margin, blood vessels manifesting as surface flow voids may be seen in meningiomas. Calcification may be present and appears as areas of low signal on T2-weighted images. Oedema in the adjacent brain parenchyma may be seen as increased signal on T2-weighted images. The criteria for malignancy of a meningioma include: (a) significant peritumoral oedema, (b) absence of calcium deposition, (c) nonhomogeneous contrast enhancement, (d) cysts within the lesion and (e) poorly defined, irregular borders.

Angiography typically shows one or more large meningeal feeding vessels supplying the lesion and a prominent and persistent tumor blush. Although not necessary for diagnosis, angiography may occasionally be selected for preoperative tumor embolization to minimize surgical blood loss.

Epidermoid Tumor

Epidermoids are the most common intracranial embryonal lesion and third most common CPA tumor, constituting 0.2 to 1.8 percent of all intracranial tumors and 5 percent of masses in the CPA. Epidermoids of the CPA are thought to arise from either the proliferation of multi potential embryonic cell rests or the transplantation of epithelial cell rests by the laterally migrating otic capsule or developing neural vasculature. They are characterized by a clinically protracted course with slow growth of tumor and progression of symptoms. The epidermoids spread along normal cleavage planes, extend into and occupy more than one intracranial compartment and envelop vital neurovascular structures. This makes complete resection difficult, at times impossible, without causing significant neurological deficits. Malignant degeneration of an epidermoid into carcinoma is rare.

In the CPA cistern, epidermoids most commonly present with a long history of hearing loss and tinnitus, with vestibular symptoms being less common. Rarely, they may present with symptoms of headache, hemifacial spasm or trigeminal neuralgia. Aseptic meningitis may occur secondary to rupture of the epidermoid lining with spillage of keratin debris into the subarachnoid space. This may occur spontaneously or as a result of surgical intervention.

Computed tomography of an epidermoid demonstrates a well­demarcated lesion with irregular or scalloped margins and homogeneous low density. This density is often similar to that of CSF, sometimes making identification of the exact extent of tumor difficult. The relative hypodensity is thought to be due to the high cholesterol and keratin content of the desquamated debris. Rare hyperdense epidermoids have been reported. Suggested causes for the hyperdense appearance include high protein content, prior haemorrhage, abundance of leukocytes, saponification of debris to calcium soaps and ferrocalcium or iron-containing pigments. Epidermoid tumors usually show no contrast enhancement because of their avascular nature, but enhancement at their margins can sometimes be seen. The introduction of a water-soluble contrast agent into the subarachnoid space has been used to outline the extent of the tumor and to identify its surface interstices, revealing its characteristic lobulated, frond-like appearance. These surface features help to distinguish a hypodense epidermoid from a hypodense arachnoid cyst.

On MRI, epidermoid tumors have a similar intensity to that of CSF, both on T1- and T2-weighted studies. For this reason small lesions can be overlooked. Careful windowing will often show internal heterogeneity and a difference in the intensity between the tumor and the CSF. On T1-weighted and proton-density images, the lesion tends to be of slightly greater intensity than CSF. Proton­density images usually provide the most specific information, the tumor may have a mixed signal, both hypointense and isointense or even hyperintense, showing the multilobular morphology of the lesion. Also, some lesions may be surrounded by a hyperintense rim that is thought to represent CSF trapped between the lesion and the brain. The use of multiple pulse sequences and multiple planes make MRI the procedure of choice for the evaluation of epidermoid cysts. It provides the best delineation of the lesion and its effect on adjacent neurovascular structures. Only in rare circumstances, when the lesion is small, there is no expansion of the subarachnoid space and MRI is inconclusive, it may still be necessary to perform CT after the intrathecal instillation of a contrast agent.

Distinguishing an epidermoid tumor from an arachnoid cyst can be difficult because the signal intensity of an epidermoid parallels that of CSF (low T1 signal, high T2 signal). Heterogeneity of the signal within the lesion is more frequent with epidermoids (i.e., arachnoid cysts are more homogeneous) and these features may be useful in differentiating between the two lesions. Diffusion imaging and steady-state free-precession sequences can also help distinguish an epidermoid tumor from an arachnoid cyst. Occasionally, an atypical epidermoid that is bright on T1-weighted images and hypointense on T2-weighted images is seen and thus may be confused with a fatty tumor.

Arachnoid Cyst

Arachnoid cysts are benign malformations of the arachnoid that are characterized histologically by duplication of the arachnoid. They comprise approximately 1 percent of all intracranial lesions; however, their true incidence is uncertain. The middle cranial fossa is the most common site of arachnoid cyst formation, followed in frequency by the posterior fossa. The etiology remains largely unknown but was previously attributed to trauma, mastoiditis, central nervous system infection such as meningitis and subarachnoid haemorrhage. The natural history of arachnoid cysts is unclear. Although most remain unchanged with time, there is a subgroup of cysts that may expand.

Arachnoid cysts can be diagnosed easily on CT or MRI. On CT scans, the lesions appear as cystic, extra-axial mass lesions of the same density as CSF. The lesions do not enhance with contrast administration and can be confused with epidermoid tumors. On MRI, arachnoid cysts characteristically parallel CSF in signal intensity on all pulse sequences. Occasionally, arachnoid cysts may have a higher signal intensity than that of CSF if they have been complicated by internal haemorrhage or if they have a higher protein content. Distinction from an epidermoid can generally be made because epidermoids have a slightly higher signal intensity than CSF, lobulated borders and heterogeneous internal architecture (especially on proton-density-weighted MRI) rather than the homogeneous internal appearance as seen in arachnoid cysts. Diffusion or steady-state free-precession MR images can be helpful in distinguishing the two entities. On diffusion imaging, arachnoid cysts are lower in signal because of the presence of a large number of mobile protons, whereas epidermoids are higher in signal intensity. On steady-state free-precession MRI, arachnoid cysts remain homogeneous, whereas epidermoid tumor signal heterogeneity is accentuated.

Neurinomas of Other Cranial Nerves

Nonacoustic schwannomas are differentiated from acoustic schwannomas by their location. The most important in this group is the trigeminal schwannoma, which arises from the intradural portion of the nerve root in the CPA or from the gasserian ganglion in Meckel's cave. A trigeminal schwannoma may straddle the middle and posterior cranial fossae via the incisura, resulting in an "hourglass" configuration. As the tumor grows along the second or third trigeminal division, there may be concomitant enlargement of the foramen rotundum or the foramen ovale, respectively. Trigeminal schwannomas, especially when large, can have a heterogeneous appearance because of their tendency to become cystic. On unenhanced CT, they may be isodense or slightly hypodense. On contrast-enhanced CT, the enhancement pattern is generally inhomogeneous because of the presence of cysts and necrosis. On MRI, they tend to be hypointense or isointense on T1-weighted images and hyperintense on T2-weighted images. Enhancement is marked after the administration of a contrast agent.

Other nonacoustic schwannomas include facial nerve schwannomas and jugular fossa schwannomas arising from the glossopharyngeal, vagus, spinal accessory, or hypoglossal nerves. They cause specific cranial nerve symptoms and signs that are helpful in arriving at the correct diagnosis.

Metastatic Tumors

Metastatic tumors presenting as CPA lesions are rare. Reported types of metastatic tumors to the CPA include small cell anaplastic carcinoma of the lung, adenocarcinoma of the breast, squamous cell carcinoma of the oropharynx, adenocarcinoma of unknown primary, malignant lymphoma, malignant fibrous mesothelioma, malignant fibrous xanthoma, renal cell carcinoma, and carcinoma of the prostate. Metastatic melanoma presenting as a CPA lesion has also been reported and is noted to have a special predilection for haematogenous seeding of the lAC. In contrast to AS and meningiomas, metastatic tumors of the CPA usually have an acute onset of audiovestibular symptoms associated with other cranial neuropathies, as well as headache and postauricular or mastoid pain. Ataxia can be seen with cerebellar invasion. Facial palsy is common and may be diagnosed erroneously as Bell's palsy. The rapid onset of symptoms and signs involving the neurovascular structures traversing the lAC, jugular foramen, or hypoglossal canal as well as a previous history of malignancy should suggest the diagnosis of a metastatic lesion to the CPA.

The CT appearance of metastatic lesions of the CPA is variable. The tumors usually enhance with the intravenous administration of a contrast agent because of their hypervascularity, but they may be heterogeneous in appearance if there are associated cysts or haemorrhage. On MRI, their appearance is variable, although they may appear remarkably similar to acoustic schwannoma. There may be enhancement of the lesion with gadolinium although usually to a lesser degree than with acoustic schwannoma. Oedema of the adjacent brain parenchyma, seen as increased intensity on T2-weighted images, may be noted. Frequently, the brain-tumor interface appears indistinct because of tumor invasion. If metastasis is suspected preoperatively, a thorough metastatic evaluation is indicated with special attention to the lungs, kidneys, breasts, liver, and prostate. Carcinomatous meningitis should be suspected in any patient with unexplained progressive cranial nerve palsies.

Lipoma

Intracranial lipomas are rare lesions that constitute 0.1 to 0.5 percent of primary brain tumors. They are found in 0.06 to 0.30 percent of all CT examinations. They are congenital malformations that result from abnormal persistence and maldifferentiation of meninx primitiva during the development of the subarachnoid cisterns. They are not neoplasms or hamartomas. Histologically, mature adipose cells are seen, accompanied by a varying degree of fibrovascular tissue. Most (80 to 95 percent) intracranial lipomas occur at or near the midline, Supratentorial lipomas frequently have associated anomalies, such as dysgenesis of the corpus callosum, encephaloceles and frontal lobe anomalies. Most lipomas are asymptomatic and are discovered on an imaging study or at autopsy as an incidental finding. Intracranial nerves and vessels typically course through the tumor rather than over its surface, as in most other benign lesions.

Lipomas constitute less than 0.14 percent of all CPA tumors, with only about 30 reported cases in the literature. Of note, CPA lipomas have not been associated with other brain malformations. In contrast to other lipomas, lipomas of the CPA frequently produce neurological symptoms by intimately involving the vestibulocochlear nerve. Lipomas with neurological symptoms can be divided into two categories: those limited to the lAC and those involving both the lAC and the CPA. Of 30 reported cases, nine involved the lAC only and 21 involved the lAC and CPA.

On CT. lipomas within the CPA cisterns are easily overlooked because they are homogeneous, hypodense and nonenhancing. Furthermore. beam-harding artifact frequently limits lesion conspicuity. Radiologic evidence of lAC erosion has been described in five of nine patients with lipomas limited to the lAC.  On MRI, lipomas follow the signal intensity of other fatty tissues: they are bright on T1-weighted images before contrast and isointense or hypointense on T2-weighted images. Fat-suppression techniques can be used to confirm the diagnosis.

Vascular Lesions

Vascular lesions such as vertebrobasilar dolichoectasia, vascular loops, and aneurysms are rare and account for only 2 to 5 percent of CPA masses. They may mimic neoplasms in the posterior fossa and CPA cistern. Vertebrobasilar dolichoectasia (elongation and dilation of the vertebral and basilar arteries) may cause compressive symptoms of the posterior fossa cranial nerves. Elongation of the basilar artery may be considered to be present if any portion of it extends lateral to the margin of the clivus or the dorsum sellae, or if the artery bifurcates above the plane of the suprasellar cistern. Ectasia is diagnosed if the diameter of the basilar artery is greater than 4.5 mm on CT. The vertebrobasilar system is superbly demonstrated on MRI by its signal void. The most common symptoms reported are hemifacial spasm and/or facial paresis, and trigeminal neuralgia.

The vascular loop represents a second vascular lesion. A loop caused by a tortuous anterior inferior cerebellar artery may be located in the CPA cistern at the porus acousticus and in the medial intracanalicular portion of the temporal bone, and may cause acoustic nerve symptoms. including vertigo.

Aneurysms of the anterior inferior cerebellar artery are rare, representing less than 1 percent of all intracranial aneurysms. Other aneurysms may originate from the vertebral and basilar arteries and extend into the CPA as a mass lesion. They may be non thrombosed or completely thrombosed, but are most commonly partially thrombosed. On CT, they present with an enhancing rim, an isodense nonenhancing mural thrombus of various size, and an enhancing lumen. A calcified rim may be present. Non­thrombosed aneurysms enhance homogeneously. MRI demonstrates a flow void in the patent lumen. The laminated thrombus reveals various signal intensities and may show a low T1 and T2 rim secondary to blood degradation products.

Jugular Fossa Lesions

The most common tumor of the jugular fossa is the glomus jugulare tumor (paraganglioma). The tumor is three times more common in women and usually presents in the fourth and fifth decades. The clinical presentation is primarily determined by the pattern of growth. More commonly, lateral growth into the middle ear cavity results in conductive hearing loss, pulsatile tinnitus and a reddish, retrotympanic mass. Medial growth results in specific deficits of the glossopharyngeal, vagus and spinal accessory nerves. When the tumor is large, atrophy of the ipsilateral tongue may result from hypoglossal nerve involvement. Radiologic evaluation can be performed with CT, MRI, and angiography. High­resolution CT with bone detail is superior to MRI in detecting bone destruction and frequently superior for surgical planning.

MRI is helpful for a number of reasons: its multiplanar capability allows for accurate localization and assessment of the posterior fossa and intracranial extent and it allows for detection of a characteristic "salt-and-pepper" pattern of hypointensity and hyperintensity. The areas of hypointensity usually represent flow voids from multiple small and medium vessels traversing the tumor. MRA may be used to determine extent of jugular vein thrombosis. The intravenous administration of a contrast agent results in moderate to intense enhancement.

Angiography is frequently utilized for lesion characterization and for presurgical embolization. Paragangliomas are characteristically hypervascular, with enlarged arterial feeders and early draining veins. The tumor blush is coarse, being intermediate between the blush of meningiomas and arteriovenous malformations. Paragangliomas are most commonly fed by branches of the ascending pharyngeal artery.

Tumors of the jugular foramen other than paragangliomas are unusual. The most common of these would include schwannomas of the caudal cranial nerves, meningioma, metastatic carcinoma and chondrosarcoma. These lesions generally cause expansion of the jugular foramen or destruction of the skull base.

Intra-axial Tumors with Extension to the CPA Cistern

Intra-axial posterior fossa tumors may arise from the brain stem or cerebellum and secondarily extend into the CPA cistern. Tumors that may do this are astrocytomas or other gliomas, generally occurring in children or young adults, or metastases. On CT, they are isodense or hypodense, and show moderate or no enhancement, depending on cell type. The brain stem is usually enlarged and the fourth ventricle is displaced posteriorly. There may be exophytic growth extending into the CPA cistern. Tumors of the cerebellum may arise from the vermis or the hemisphere. Vermian tumors (mainly medulloblastoma in childhood) and hemispheric tumors (astrocytomas, hemangioblastomas. metastases) rarely extend into the CPA cistern.

Extra-axial Tumors with Extension to the CPA Cistern

Choroid plexus papillomas and ependymomas arise from the fourth ventricle and may grow through the foramen of Luschka into the CPA. Childhood papillomas arise most commonly in the lateral ventricle, followed in frequency by the fourth ventricle. In adults, these tumors arise most commonly within the fourth ventricle, from which they can extend into the CPA through the foramen of Luschka. Primary CPA choroid plexus papillomas are rare, but have been reported, they arise from a choroid tuft outside of the foramen of Luschka.

CT demonstrates an enhancing mass that may have a cystic component and calcification. On MRI, T1-weighted images reveal a lobulated tumor that is isointense to gray matter. On T2-weighted images the tumor is usually heterogeneously hyperintense. Linear and ovoid areas of marked hypointensity may result from tumor vascularity or calcification.

Conclusions

1.Most CPA tumors are best evaluated with MRI because of the lack of artifact from bone and the ability to obtain coronal images directly. MRI also helps in characterizing the lesion and is superior for detecting small intracanalicular lesions.

2. CT is better for identifying calcium and the extent and type of bone destruction and alteration.

3. MRA can assist in evaluating the degree of tumor vascularity and in differentiating vascular lesions from neoplasms. MRA can also assess the extent of venous compromise from CPA mass lesions.

4. Angiography is performed when a vascular lesion cannot be evaluated fully by MRA and for preoperative embolization in the case of a very vascular neoplasm.