3.1 Introduction

Management of skull base neoplasms requires a team whose members are dedicated and specifically experienced at dealing with these unique tumors. The radiologist is no exception, for imaging plays a vital role in the diagnosis and posttreatment evaluation of skull base tumors. Although radiologists have traditionally concerned themselves with preoperative diagnosis, such a goal is not attainable in every case. Thus the goal of imaging known or suspected skull base tumors is several-fold. First, some patients who are suspected, but not known, to have a skull base tumor (based on pain, cranial neuropathy, etc.) require imaging simply to establish or exclude such a diagnosis. For those who have known skull base tumors, the role of imaging is to establish the full extent and location of the abnormality, outline areas of possible spread and secondary effects on adjacent structures, exclude nodal disease, and, finally, suggest a possible histologic diagnosis.1 In some cases, distinguishing tumor from benign disease entities such as skull base osteomyelitis and other inflammations is a critical role of imaging. Obviously the radiologist can suggest a diagnosis when possible, and in fact for many lesions the imaging is quite characteristic.1 ^,​ 2

3.2 Selection of Imaging Modality

For lesions of the sinonasal cavity and skull base, unlike at some anatomical sites in the head and neck, MR and CT are complementary. At the University of Texas MD Anderson Cancer Center, virtually all patients who have such lesions will be imaged using both modalities prior to therapy, and many will have both modalities following therapy, at least early on.

The advantages of CT include its ability to detect calcification and bone—important in lesions that either destroy bone or produce some characteristic bony or calcific change. The latter include the classic sunburst periosteal new bone formation in osteosarcoma, the mineralized chondroid matrix of chondrosarcoma, and the hyperostotic reaction typical of certain types of meningioma. CT may also have value, through the multiplanar reconstruction capability of multidetector-row technology, for providing very high-quality images in virtually any plane of section. Furthermore, CT angiography has value, in cases of certain hypervascular lesions, such as glomus tumors or juvenile angiofibroma, for establishing that a lesion is indeed hypervascular and then for assisting in surgical planning.

The advantages of MR include its ability to distinguish tumor within a paranasal sinus from obstructed secretions, something not always possible with CT. MR is better able to detect small soft tissue tumor components, particularly those near bony surfaces for which the enhancement may be inconspicuous on CT, such as with intracranial spread of a sinonasal malignancy. MR is also more accurate in detecting tumor extension through neural foramina and canals, whether by direct or perineural mechanisms.

3.3 Anterior Cranial Base

The anterior cranial base comprises the orbital and ethmoid roofs and the cribriform plates. Tumors seldom primarily arise within these bony structures—most generally originate intracranially and extend inferiorly through the skull base (e.g., meningioma; Fig. 3.1) or by extending intracranially from an origin in the upper nasoethmoid or frontal sinus region.3 The latter are typically sinonasal malignancies (Fig. 3.2; Fig. 3.3).

Anterior cranial fossa meningiomas often arise at the olfactory groove or tuberculum sella (Fig. 3.1).4 These benign dural tumors may extend inferiorly, growing directly through the ethmoid roof (fovea ethmoidalis) and cribriform plate into the nasal cavity and/or ethmoid sinuses. In such cases, it is generally evident radiographically that the bulk (or so-called “epicenter”) of the tumor is intracranial. In addition, characteristic imaging features of meningioma—such as relatively homogeneous isointensity on T1-weighted images; iso- or slightly hyperintense signal on T2-weighted images; and bright homogeneous enhancement following gadolinium-based IV contrast administration, often with a so-called dural tail of enhancement—make the diagnosis of meningioma straightforward in most cases (Fig. 3.1).4

Sinonasal malignancies may arise in the upper nasoethmoid region. Typical histologic tumor types include olfactory neuroblastoma (esthesioneuroblastoma), sinonasal undifferentiated carcinoma, squamous cell carcinoma, and neuroendocrine carcinoma.3 Other tumor types are less common. Although the imaging characteristics of these lesions are relatively nonspecific, imaging is important in evaluating for intracranial spread, which will often have implications for surgical therapy. CT can often make this determination, but MR is better in this regard (Fig. 3.2; Fig. 3.3). On CT, sinonasal malignancies generally enhance to a mild or moderate degree. When they involve bone, the pattern is usually one of destruction; sclerosis of bone is uncommon. With MR, sinonasal malignancies are typically close to muscle or brain in signal on T1- and T2-weighted sequences and enhance to varying degrees with contrast administration. Obstructed sinonasal secretions are usually low-signal on T1 and high-signal on T2. If chronic or inspissated, sinus mucosal secretions may become hyperintense (high-signal) on T1-weighted images (Fig. 3.3).3 Review of all sequences generally allows determination of whether sinus is involved with tumor or merely obstructed. It is important to always image the neck in cases of sinonasal malignancy, for such lesions may present with, or recur as, nodal disease.

3.4 Central Skull Base

The central skull base (CSB) includes primarily the sphenoid bone and its various parts as well as adjacent structures such as the cavernous sinus, sella turcica, and parasellar region. A very large variety of tumors may afflict the CSB.1 ^,​ 2 ^,​ 3 As with the anterior cranial base, lesions may arise intracranially and secondarily involve the sphenoid bone. Alternatively, lesions may arise primarily within the sphenoid bone, or arise inferiorly or anteriorly, and secondarily affect it by upward or posterior spread.

The most common intracranially arising lesion to affect the CSB is meningioma.4 Various aspects of the CSB may be involved, including the planum sphenoidale and tuberculum sella, the anterior clinoid, the greater sphenoid wing, the parasellar region and cavernous sinus, and the petroclival region (Fig. 3.4; Fig. 3.5). In most of these locations, the lesion is merely dural in location; while there may be some reactive bony sclerosis or hyperostosis, the bone is uninvolved.4 One exception is hyperostosing en plaque meningioma of the greater sphenoid wing, which is associated with a very striking sclerotic response, or hyperostosis, because meningioma cells actually involve the bone.4 ^,​ 5 This bony involvement has a very characteristic radiographic appearance, including intraorbital extension (Fig. 3.6).1 ^,​ 4 ^,​ 5

Although most meningiomas, other than the hyperostosing sphenoid wing type, remain confined to the dural space, some meningiomas may invade the CSB in an aggressive, almost malignant manner.1 ^,​ 4 The imaging for such invasion can be dramatic, and if the diagnosis of meningioma is unknown, the radiologic diagnosis may be more difficult (Fig. 3.7). Such meningiomas may achieve massive size and may extend directly through the bone, or through neural foramina, into extracranial spaces such as the pterygopalatine fossa, orbit, or masticator space. Such lesions may be grossly destructive of bone. It should be kept in mind, for any location, that dural metastases may be radiographically indistinguishable from meningioma. In a known cancer patient, a lesion that resembles a meningioma on the first imaging study may be a dural metastasis; the finding of growth on serial imaging should raise that possibility (Fig. 3.8).6 ^,​ 7 Finally, another lesion that can be radiographically confused with meningioma is the rare, highly vascular malignancy hemangiopericytoma. These also have the potential to be highly destructive of the cranial base (Fig. 3.9).

Another common benign CSB lesion is the pituitary adenoma.1 ^,​ 8 ^,​ 9 ^,​ 10 ^,​ 11 Some macroadenomas (those greater than 1 cm) may become giant adenomas (greater than 5 cm) and/or invade the CSB. Such cases may present a diagnostic challenge to the radiologist. There is no imaging feature with which to specifically differentiate giant pituitary adenoma from other CSB masses, except that an adenoma should encompass the pituitary gland and may demonstrate cavernous sinus invasion without perineural spread (PNS). Accordingly, the radiologist should consider pituitary adenoma when confronted with a large, destructive CSB mass and should check hormone levels, especially prolactin (Fig. 3.10).

One relatively uncommon benign tumor that arises near, and that may secondarily affect, the CSB is the juvenile angiofibroma. These highly vascular tumors typically present with epistaxis in teenage males and have very characteristic imaging features.1 ^,​ 12 They arise in or near the sphenopalatine foramen, the opening between the pterygopalatine fossa and the nasal cavity. Tumor components are often seen in the nasal cavity and nasopharynx. These aggressive lesions often invade the clivus and sphenoid sinus. Because of the hypervascularity, juvenile angiofibromas enhance brightly on CT. On MR, they have small foci of low signal, so-called flow voids, that represent rapidly flowing arterial supply to the tumor (Fig. 3.11). Vascular studies such as catheter angiography, CT angiography, and MR angiography can provide a surgical road map or facilitate preoperative embolization (Fig. 3.11e).

One final intracranial (and sometimes also extracranial) lesion affecting the CSB is peripheral nerve sheath tumor arising from the trigeminal nerve. Histologically such tumors can be schwannoma or neurofibroma, but the imaging appearance of each is similar to that of the other.2 ^,​ 6 These usually benign lesions can arise from the main trigeminal trunk and extend along any of its branches or may arise within one of its three divisions. Such lesions often cause widening of foramina or other benign bony remodeling (as opposed to destruction) of the CSB (Fig. 3.12).

Among primarily extracranial malignancies that may affect the CSB, probably the most common is nasopharyngeal carcinoma (NPC).1 ^,​ 8 ^,​ 13 ^,​ 14 Owing to its location immediately inferior to the clivus, upward spread of NPC very commonly involves the CSB. If the nasopharyngeal component is relatively small (Fig. 3.13), the radiologist must bear in mind the possibility of NPC. As opposed to some malignancies that tend to destroy bone—and even though NPC may do exactly this—NPC often has a tendency to infiltrate in a nondestructive manner.15 For this reason, MR imaging is more sensitive than CT for detecting such involvement (Fig. 3.14). Of course, NPC can also cause skull base destruction, which CT also plays a role in detecting.15 Because MR is also more sensitive for detecting intracranial spread via perineural and direct mechanisms, we prefer to use MR as the main imaging modality in cases of NPC.16

Another type of extracranial malignancy that may secondarily affect the CSB is head and neck malignancy with PNS of tumor.17 ^,​ 18 ^,​ 19 A thorough description is beyond the scope of this chapter. The most common manner in which the CSB is affected is in a patient who has a known, or sometimes an unknown, malignancy of the face (most commonly in a V2 distribution, such as the cheek; Fig. 3.15) or a lesion of the lower lip or palate (or, less commonly, of other mucosal surfaces). Common tumor types include squamous cell carcinoma, adenoid cystic carcinoma, and desmoplastic melanoma.18 Such lesions may spread proximally, toward the central nervous system, along the branch of the trigeminal nerve that innervates the primary site. For the cheek and palate, this would be the infraorbital (Fig. 3.15) or palatine nerves (Fig. 3.16), respectively, with tumor spread to the pterygopalatine fossa.

From here, tumor can spread posteriorly along the main trunk of the maxillary nerve, through foramen rotundum, and into the cavernous sinus or further posteriorly into Meckel’s cave or even onto the main trigeminal trunk.18 For lesions of the lower lip or gingiva, tumor can access the inferior alveolar branch of V3 and then spread upward onto the main trunk of the mandibular nerve (Fig. 3.17), through foramen ovale, and into Meckel’s cave. Though perhaps best considered in the temporal bone section, PNS along the facial nerve may arise from primary or secondary parotid gland tumors (Fig. 3.18).

Recognition of PNS is a critical function of the radiologist, because failure to recognize it—unfortunately a common occurrence—can have a very adverse effect on treatment outcome. Characteristically, PNS occurs in the form of, or at the time of, tumor recurrence (Fig. 3.15), not necessarily at the time of diagnosis or initial therapy. In other cases, unfortunately, PNS is not recognized until late in the patient’s course, as a progression of disease that went undiagnosed and untreated at clinical presentation.

Metastases are the most common malignancies that arise directly in the skull base.3 Such lesions may be lytic or blastic, depending on the primary malignancy; both are common (Fig. 3.19). Their imaging is generally straightforward, except that small lesions may be subtle.

Primary CSB malignancies include chordoma and chondrosarcoma. Chordomas are notochord-derived malignancies that occur in the sacrococcygeal region (50%), spine (15%), and clivus (35%).1 ^,​ 3 They may also occasionally occur off midline. Most chordomas occur in mid- to late adulthood. Radiographically, they appear as expansile lytic lesions, with internal areas of fragmented bone that are most readily seen on CT (Fig. 3.20). On MR they have characteristic T2 signal hyperintensity within the soft tissue mass component. Some degree of extracranial and/or intracranial extension is typical, the latter of which often results in brainstem compression.

Chondrosarcomas may also arise in the CSB.1 ^,​ 3 These cartilage-derived malignancies tend to occur in proximity to the various synchondroses. Common locations include the petroclival fissure and the upper nasal septum/vomer region (Fig. 3.21; Fig. 3.22; Fig. 3.23). Chondrosarcoma may also involve the greater sphenoid wing more laterally, often with masticator space components (Fig. 3.23). On imaging, many, though not all, will have a mineralized chondroid matrix seen in chondroid tumors anywhere—calcifications in an “arcs and rings” pattern (Fig. 3.21; Fig. 3.23). On MR, although signal intensity varies, marked heterogeneity is often seen, particularly on postcontrast T1-weighted images (Fig. 3.22). A lesion in the appropriate location and having typical imaging features should suggest the possibility of chondrosarcoma.