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tect a chordoma with CT, MR has been proven to be considerably superior, in particular the T2-weighted sequence, in the delineation of the tumor extent, and is therefore the most pertinent study for treatment planning purposes. On MR imaging, chordomas show a low to intermediate T1 and high T2 signal intensity with small focal areas of decreased T2 attenuation caused by tumoral calcifications or small foci of intratumoral hemorrhage (Fig. 13.19c,e). Moderate to marked heterogeneous contrast enhancement is typically seen (Fig. 13.19c,d). Total surgical resection is the optimal treatment but is usually not feasible in the skull base region. Therefore, post-surgical radiation therapy is usually utilized to improve patient’s prognosis, which is primarily affected by the amount of tumor tissue left behind within the surgical bed (Erdem et al. 2003). Overall, the reported survival rate is poor with a 5-year survival rate of approximately 50% (Forsyth et al. 1993).

13.5.7

Mimics of Malignant Central Skull Base Lesions

Cephalocele is a rare entity in which brain tissue and or meninges protrude through a bony defect from intracranial to extracranial. In the central skull base

region, intra-sphenoidal and trans-sphenoidal cephaloceles have been encountered (Jabre et al. 2000). Trans-sphenoidal cephaloceles are usually congenital in nature. They are typically midline in position and therefore may present on imaging as a midline bony gap associated with a cystic nasopharyngeal mass. This type of cephalocele is usually complex in nature containing optic nerves, hypothalamus or other adjacent midline structures. These patients might present with pituitary gland dysfunction due to protrusion of the pituitary gland into the meningoencephalocele or more commonly because of associated maldevelopment of the sella and pituitary gland. The intra-sphe- noidal cephaloceles are typically lateral in location and most commonly post-traumatic in nature. These usually contain a variable amount of temporal lobe tissue and may present with seizures or rhinorrhea, bearing increased risk of meningitis. Therefore, they are often treated with surgery. On CT, a bony gap in the sphenoid body and partial or complete opacification of one or both sphenoid sinuses is seen (Bolger and Reger 2003). On CT, they can be mistaken for mucoceles or aggressive skull base masses when associated with mass effect within the sphenoid sinus. On MR the diagnosis is easier as the extracranial brain tissue and/or meninges are directly demonstrated, in particular on T2-weighted images (Fig. 13.20). On

CT, however, the location of the bony gap, the pattern of bone remodeling rather than destruction and potentially associated mild bowing of the involved bone towards the sphenoid sinus provide important diagnostic clues that warrant further evaluation with MRI, or CT with intrathecal contrast injection.

13.5.8

Malignant Posterior Skull Base Lesions

Chondrosarcomas are rare malignant cartilaginous tumors of variable aggressiveness (Evans et al. 1977). Their aggressiveness depends upon the tumor subtype and grade. The dedifferentiated and mesenchymal variants are more aggressive than the classic variant. The classic variant is the most common and represents a low grade chondrosarcoma. This type of chondrosarcoma shows the least aggressive growth pattern, minimal risk of metastatic disease and is often difficult to distinguish from chordomas histologically and clinically (Bourgouin et al. 1992). As with chordomas, headaches and diplopia related to cranial nerve VI palsy are the most common symptoms at presentation. However, chondrosarcomas usually occur in younger patients with a peak incidence in the 2nd and 3rd decade, and even more importantly they are off midline (petroclival) in location (Hassounah et al. 1985). Their paramedian location also predisposes these patients for lower cranial nerve deficits,

in particular hearing loss. CT images typically demonstrate a destructive mass centered in the petrous apex with ring and arcs in it reflecting the chondroid nature of this tumor (Fig. 13.21). Variable amount of soft tissue component is seen, typically isodense to the adjacent brain parenchyma before, and showing some but variable enhancement following intravenous contrast administration. Chondrosarcomas have low to intermediate signal intensity on T1, and high signal intensity on T2-weighted sequences, with a variable enhancement pattern (Fig. 13.22). As with chordomas, total surgical resection is the most desirable treatment in combination with radiation therapy if positive resection margins are present. Despite the histological and clinical similarities, the classic type of chondrosarcoma has been reported to have a better prognosis than chordomas with a 5-year survival rate of approximately 90%. This, however, is not true for the other chondrosarcoma variants (Forsyth et al. 1993; Evans et al. 1977).

Endolymphatic sac tumors are also very rare. They may occur sporadically or in association with von Hippel-Lindau disease (Luff et al. 2002). The patients typically present with hearing loss, tinnitus, vertigo and or facial nerve dysfunction (Choyke et al. 1995). Bilateral involvement has been reported in von Hippel-Lindau disease patients. The majority of these tumors are low grade adenocarcinomas or papillary cystadenomas that are indistinguishable from each other based on imaging (Mukherji and

Castillo 1996). On CT, local destruction is seen in the retrolabyrinthine petrous bone, at the aperture of the vestibular aqueduct, with characteristic intratumoral bone spicules that might be associated with an enlarged vestibular aqueduct. The T1-weighted images demonstrate characteristic hyperintense foci within the tumor substance with extensive enhancement following intravenous contrast administration. Endovascular embolization prior to surgical resection may be required.

Paragangliomas are rare tumors of neural crest origin (Rao et al. 1999). In the posterior skull base, they are most commonly seen in the jugular fossa and are called glomus jugulare tumors. They arise from the

paraganglion cells of the Jacobson or Arnold nerve which are branches of the cranial nerve IX and X, respectively (Rao et al. 1999). The majority of the paragangliomas are benign in nature.Up to 13% of glomus jugulare tumors have been reported to be malignant. Both types will be discussed in this section together as they both demonstrate a locally aggressive growth pattern making them indistinguishable from each other (Figs. 13.23 and 13.24). Demographic data are not helpful either as both forms most commonly occur in middle aged females that typically present with tinnitus (Brewis et al. 2000). It has been suggested that the incidence of pain is higher and the occurrence of hearing loss lower with the malignant

variants. The main differentiating feature, however, is the existence of metastatic lesions in the malignant form (Fig. 13.24b). Metastatic disease most commonly spreads to the bone, lung and cervical lymph

nodes and might present even years after the initial diagnosis and treatment (Fig. 13.24b). While nuclear medicine examination with Indium 111 labeled somatostatin analogues is an excellent way to evaluate

for metastatic disease, CT and or MR are primarily utilized to evaluate the primary tumor within the jugular fossa. On CT, a glomus jugulare tumor causes destruction of the walls of the jugular foramen and adjacent petrous bone giving the so-called mouth eaten appearance (Figs. 13.23 and 13.24). On MR, a hypervascular mass with flow voids and extensive enhancement is seen in the jugular fossa region. On both studies, the tendency of the glomus jugulare to grow into the middle ear cavity and posterior fossa are seen, although the intracranial involvement is better demonstrated with MR (Figs. 13.23 and 13.24). In approximately 10% of patients, multi-focal and/or bilateral involvement is seen. The benign forms of glomus jugulare can be treated with surgical resection following intraarterial embolization, radiation therapy or both as the tumor is known to be radiosensitive and total surgical resection is often difficult. Since radiation therapy is known to leave a residual mass of variable activity behind, treatment of the malignant forms with radiation therapy alone is currently not recommended but rather surgical resection with post-operative radiation therapy. Utilization of Iodine 131 labeled MIBG for treatment of the metastatic lesions has been reported with mixed preliminary results (Bomanji et al. 2001).

Malignant schwannomas of the cranial nerve VII through XII demonstrate the same rarity, behavior

and imaging findings as the malignant schwannomas of the cranial nerve III through IV (see Sect. 13.5.5 for details).

13.5.9

Mimics of Malignant Posterior Skull Base Lesions

Congenital cholesteatoma is a rare disease arising from aberrant ectoderm that is trapped during embryogenesis within the temporal bone (Robert et al. 1995). Histologically, the lesion is lined with stratified squamous epithelium and filled with debris. It typically presents in children and young adults. It can occur anywhere within the temporal bone; when located in the petrous apex it might simulate a malignant lesion of the skull base. Petrous apex congenital cholesteatomas present with facial nerve palsy, sensorineural hearing loss and/or vertigo. On CT, a hypodense, non-enhancing, expansile mass is seen with variable degrees of bony destruction. Congenital cholesteatoma is usually low to intermediate in signal intensity on T1and high signal intensity on T2weighted images. Presence of restricted diffusion on DWI has also been reported which is not surprising as cholesteatomas histologically resemble epidermoid tumors (Aikele et al. 2003). Surgical excision is the treatment of choice and recurrent disease is common

with subtotal resection. In such patients, DWI might be a helpful tool for evaluation of residual or recurrent disease if the lesion is at least 5 mm in size.

Cholesterol granuloma represents an accumulation of granulation tissue intermixed with blood products and cholesterol crystals within the middle ear or petrous apex (Farrior et al. 1981). Patients with cholesterol granuloma classically have a long standing

history of chronic otitis media. Lesions in the petrous apex are often quiet large at the time of diagnosis and may therefore present with hearing loss, tinnitus and/or cranial neuropathies (Goldofsky et al. 1991). On CT, cholesterol granulomas present as expansile, well-defined masses (Fig. 13.25a) (Greenberg et al. 1988). When large, the site of origin might be difficult to determine making distinction from other types of

petrous apex lesions, in particular petrous apex aneurysm, difficult. As cholesterol granulomas are often large at the time of initial scanning, CT images often show bony gaps that may be mistaken for bone destruction or erosions suggesting an aggressive or malignant lesion. On MRI, cholesterol granulomas are classically of increased signal intensity on T1and T2-weighted images, with no or minimal peripheral enhancement following contrast administration (Fig. 13.25b). Cholesterol cysts are essentially identical to cholesterol granulomas but occur in the absence of chronic middle ear disease. Mucoceles of the petrous apex share the same imaging characteristics as cholesterol granulomas on CT, but usually demonstrate decreased signal intensity on T1-weighted images. Mucoceles of the petrous apex are very rare and are felt to be related to post-inflammatory obstruction. All of these lesions are usually treated with surgical drainage.

Internal carotid artery aneurysms can occur within the cavernous sinuses, causing remodeling of the adjacent sphenoid bone, or within the petrous bone. The cavernous sinus aneurysms are more common and usually do not demonstrate a diagnostic dilemma. Petrous segment internal carotid aneurysms, in contrast, are rare. They can be congenital, post-traumatic or post-infectious in etiology (Liu et al. 2004). Aneurysms in this location are usually asymptomatic and are incidentally found on CT or MR studies performed for other indications. In symptomatic patients, symptoms are primarily non-specific in nature (headaches) or related to local mass effect (cranial neuropathies, Horner’s syndrome or tinnitus). Embolic events have not been associated with petrous segment aneurysm even with larger mural thrombus formation (Liu et al. 2004). On CT, they usually present as an expansile mass with marked enhancement following intravenous contrast administration. The degree and homogeneity of enhancement depends upon the amount of mural thrombus and the size of patent lumen (Fig. 13.26). If no intravenous contrast is given, they may mimic a cholesterol granuloma, cholesterol cyst or a petrous apex mucocele. Close search for the internal carotid canal within the petrous bone and evaluation of the vector of the associated bone remodeling usually results in the correct diagnosis. Contrast enhanced CT and/or CT angiography might then be performed to confirm the diagnosis and to classify the type of the aneurysm (Fig. 13.26). On MR, petrous apex aneurysms usually present with mixed signal intensity and enhancement pattern, related to the turbulent flow within the aneurysmal segment (Fig. 13.26). Even MR angiogra-

phy is usually difficult to interpret as the turbulent flow causes signal drop out due to reentry phenomenon or thrombus formation that underestimates the size or makes the aneurysm difficult to appreciate at all. This constellation of imaging findings may result in misdiagnosis of the aneurysm as a non-vascular benign and or malignant skull base lesion. In the instance of present flow voids within the aneurysm, the aneurysm might be completely missed as the difference in signal intensity to adjacent petrous apex, in particular when the petrous apex is aerated, might be very small and therefore not appreciable to the radiologist’s eye. The bottom line is that internal carotid artery aneurysms in this location might pose a diagnostic dilemma and the radiologist should consider it every time a petrous apex lesion is observed on CT or MRI to prevent a potential biopsy with usually fatal outcome. The treatment of petrous segment aneurysm remains challenging, in particular when the patient has no or minimal symptoms. Currently, a “wait and see” approach is applied to this latter patient group as these type of aneurysms do not suffer life-threatening bleeding episodes. Endovascular intervention (balloon or coil occlusion of the internal carotid artery and/or stent placement) or surgical trapping with revascularization bypass is currently reserved for symptomatic patients (Liu et al. 2004).

Petrous apicitis is usually caused by otitis media that extended anteromedially into the aerated petrous apex cells without or with progression to osteomyelitis in the early and delayed stages, respectively. Usually, the patients present with fever reflecting the acute nature of the infection in combination with some or all of the symptoms of the Gradenigo’s triad (see Sect. 13.3) (Gradenigo 1904). In the early stages, obliteration of the petrous apex air cells with fluid in association with otomastoid disease is seen. In the later stages, additional bony destruction with trabecular breakdown and cortical erosions is present, reflecting the degree of underlying osteomyelitis (Fig. 13.27a). All these changes are readily visible on CT. Petrous apicitis is prone to intracranial complications such as epidural or brain abscess formation, subdural empyema, meningitis and sinus thrombosis or thrombophlebitis (Fig. 13.27b). These possible complications are usually better delineated with MR imaging. Petrous apicitis, in particular when associated with osteomyelitis or intracranial complications, represents a surgical emergency requiring debridement of the petrous apex and drainage of possible abscesses or empyemas. However, recently, successful conservative treatment of uncomplicated petrous apicitis has been reported (Burston et al. 2005).

13.6

Imaging Protocol

In view of the broad variety of lesions that can involve the skull base, it is not surprising that there is no single imaging protocol that fits all these entities. The imaging protocol will primarily depend upon availability, presenting symptoms, patient’s history, and extent of tumor. Overall, CT is usually the study of first choice as it is widely available and easily accessible. Based on symptoms, MR should be the initial diagnostic study for patients with cranial neuropathies, while CT is primarily reserved as a screening tool for more nonspecific symptoms such as headaches and dizziness. Presence of risk factors for a certain disease process (e.g. chronic otomastoiditis or neurofibromatosis), or a history of underlying malignancy in combination with the clinical presentation, might raise the suspicion for a specific entity that it is better evaluated with one or the other imaging tool. The extent of the tumor is usually unpredictable based on clinical examination alone and therefore, review of the images while the patient is still on the scanner with immediate modification of the imaging protocol represents the most optimal work-up of patients with skull base lesions. Despite most optimal image quality, a significant number of patients will end up with both – CT and MR – studies for narrowing of the differential diagnostic considerations and better delineation of the tumor margins (Figs. 13.13, 13.17, 13.19 and 13.20).

Although no specific protocol is given here, the following issues need to be considered or followed to facilitate the most optimal quality of a cross-sec- tional study: CT and MRI should be obtained as a high resolution study with a small field of view (8– 14 cm) that is coned down to the area of interest, such as the central skull base or temporal bone. The images should be performed with contiguous slices. It is recommended that CT images are acquired with a slice thickness of 1 mm or less for optimal display of the bony details of the skull base in bone algorithm. In the era of interactive capabilities of the picture archive communication system (PACS), such thin sections are also desirable in soft tissue algorithms to allow high quality multi-planar reformations that facilitate better delineation of the lesion from adjacent anatomical structures. In contrast, such thin slices are not feasible nor desirable with MR, as the signal to noise would be too low and/or the imaging time too long. Therefore, MR images should be obtained with a slice thickness of 3 mm (Casselman 2005). When a large lesion is present, the slice thickness might need to be increased to 4 mm to be able to cover the entire tumor in a reasonable time frame.

High resolution fast spin-echo T2and standard T1-weighted sequences, without and with intravenous contrast administration, should be performed in axial and coronal planes (Casselman 2005). Some tumors also benefit from sagittal imaging such as chordomas. The standard T1-weighted images

are most sensitive for bony invasion while the T2weighted images are very helpful in delineation of the outer tumor margins (Figs. 13.13a,b and 13.22a,b). Post-contrast enhancement T1-weighted images are most useful for evaluation of involvement of adjacent soft tissues (Fig. 13.22a,b). Utilization of fat suppression techniques in the skull base is controversial as it is hampered by loss of most of the anatomical landmarks, and is predisposed for susceptibility artifacts. In addition, the suppression of fat makes the typically high signal difference between tumor and adjacent fatty bone marrow disappear, and the detectability of a lesion on the contrast enhanced, fat suppressed images will depend upon the degree of intra-lesional contrast accumulation (Casselman 2005).Therefore, it is advisable to obtain standard non-contrasted T1weighted images without fat suppression in at least one imaging plane (preferentially the axial plane) that would supplement the fat suppressed images. Additional imaging sequences such as heavily T2weighted high resolution volumetric acquisitions or MR angiography/venography might also be considered for further evaluation of the cisternal portions of the cranial nerves or of the intracranial vasculature, respectively. It is also recommended to perform larger field of view images (20–22 cm) that cover the entire head in non-enhanced T1 and fluid attenuation inversion recovery (FLAIR) weighing that could serve as an overview for optimal positioning of the subsequently acquired high resolution images; these would allow the detection of additional lesions as well as possible complications related to the skull base mass such as infarction at the same time. Additional DWI through the entire head might not only further characterize the age of potential infarction, but also narrow the differential diagnosis in some instances.

13.7 Radiologist’s Role

Pre-treatment, the radiologist’s main role is the delineation of the extent of the skull base lesion, including delineation of possible involvement of the neurovascular structures (Casselman 2005). Narrowing of the differential diagnostic considerations is certainly desirable but not as critical as the tumor extent. In addition, the radiologist might offer biopsy of a lesion when a soft tissue component is present below the skull base and accessible for biopsy with CT guidance. CT guided biopsy of the bony skull base itself is difficult due to the high frequency of traversing nerves

and vessels. Central skull base lesions can often be reached by placement of a guiding needle through the mandibular notch or through the buccal space along the anterior margin of the mandibular ramus.

During the treatment phase, the interventional radiologist might perform intra-arterial embolization of a vascular tumor prior to surgical resection or treat a petrous segment internal carotid aneurysm with balloon/coil occlusion.

Post-treatment imaging is critical as surgical resections are often subtotal in extent and radiation therapy may leave residual masses of unknown activity. To facilitate the radiologist’s ability to detect recurrent tumor as soon as possible, it is advisable to obtain a baseline CT and/or MR study approximately 6 weeks following treatment, and to perform the subsequent follow-up examinations with the same imaging protocol and technique.

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