Chapter 30

Skull Base Tumors

Anitha Raghunath and Jade S. Schiffman

Abstract Ocular manifestations are not infrequent in the presence of skull base tumors because of the crucial visual and oculomotor pathways traversing the skull base. Because familiarity with the intricate skull base anatomy is imperative for accurately diagnosing and effectively managing skull base tumors, this chapter provides a description of the anatomy of the skull base and a discussion of imaging techniques. Numerous types of tumors and their neuro-ophthalmologic correlations are also discussed, including esthesioneuroblastomas, chordomas, craniopharyngiomas, meningiomas, sinonasal tumors, schwannomas, pituitary tumors, myelomas, paragangliomas, and metastases.

30.1 Introduction

Skull base tumors account for 25.2% of cranial neoplasms [1]. Ocular manifestations are common in the presence of skull base tumors because of the crucial visual and oculomotor pathways traversing the skull base. A primary tumor of the skull base can directly spread to the orbit or cause cranial neuropathies by compressing or infiltrating cranial nerves along their course. Visual sensory symptoms can be due to involvement of the visual apparatus in the orbit, optic canal, chiasm, or retrochiasmal pathway. Visual motor symptoms can be due to involvement of the orbit, superior orbital fissure, or cavernous sinus and/or subarachnoid involvement of cranial nerves III, IV, or VI. The skull base is also a frequent site of direct spread of periorbital tumors (e.g., tumors of sinonasal structures) through bony walls and foramina. Additionally, sinonasal tumors and masticator space tumors can be associated with indirect perineural spread along the trigeminal and facial nerve branches, which introduce these tumors from the sinuses, often through skull base foramen into the intracranial aspect of the skull base. Familiarity with the intricate skull base

A. Raghunath (B)

Section of Ophthalmology, Department of Head and Neck Surgery, The University of Texas M.D. Anderson Cancer Center, Houston, TX, USA

e-mail: anitha raghunath@gmail.com

Oncology Series 6, DOI 10.1007/978-1-4419-0374-7_30,

C Springer Science+Business Media, LLC 2011

anatomy and interconnections with knowledge of the nuances of radiologic imaging are helpful for accurate diagnosis and multidisciplinary management of skull base tumors.

30.2 Anatomy of the Skull Base

The skull base is the floor of the cranial cavity on which the brain rests and is important in terms of the various foramina through which the nerves, vessels, and medulla–spinal cord junction pass. The skull base can be divided into anterior, middle, and posterior cranial fossae.

The anterior fossa is formed by the frontal bone, ethmoid bone, and body and lesser wing of the sphenoid bone. The cribriform plate (part of the ethmoid bone) allows passage of olfactory fila and ethmoidal vessels. The planum sphenoidale forms the posterior boundary of the anterior fossa.

The middle cranial fossa is formed mainly by the sphenoid bone and the temporal bone. The body of the sphenoid bone in the anteromedial aspect houses the optic canal and, laterally, the superior orbital fissures, which are formed between the lesser and greater wings of the sphenoid bone. The optic canal transmits the optic nerve, while the superior orbital fissure transmits cranial nerves III and IV; the ophthalmic division of cranial nerve V (V1); cranial nerve VI; and the superior ophthalmic vein. The central portion of the middle fossa contains the sella turcica, which houses the pituitary gland and stalk; the parasellar region includes the cavernous sinuses, which contain the internal carotid artery and cranial nerves III and IV, the V1 and V2 (maxillary division) branches of cranial nerve V, and cranial nerve VI. Postganglionic sympathetic fibers traveling on the internal carotid artery transfer to V1 within the cavernous sinus before entering the orbit. Inferomedial to the superior orbital fissure is the foramen rotundum, which transmits V2 extracranially. Posterolateral to the foramen rotundum is the foramen ovale, which transmits the mandibular division (V3) of the fifth cranial nerve.

The clivus forms the anterior portion of the posterior cranial fossa. The clivus is composed of the sphenoid bone and the occipital bone. The sphenoid sinus anteriorly and clivus posteriorly forms the sloped roof of the nasopharynx which lies below these structures.

The posterior cranial fossa contains the midbrain, pons, medulla, and cerebellum, which are important structures that contain the cranial nerve nuclei and their interconnections.

Some extracranial structures are extremely important in that disease within these locations can result in intracranial spread. The infratemporal fossa is an extracranial space that extends from the inferior orbital fissure anteriorly to the jugular foramen posteriorly. The pterygopalatine (also known as sphenopalatine) fossa is a deep recess bound anteriorly by the maxilla, medially by the palatine bone, and posteriorly by the pterygoid process. The lateral aspect of the pterygopalatine fossa connects with the pterygomaxillary fissure. Superior to the pterygopalatine fossa

is the opening into the orbital apex through the inferior orbital fissure. This fossa contains the pterygopalatine ganglion, which is a major junction for the V2 division passing from the foramen rotundum. The vidian canal also connects to the fossa inferomedial to the foramen rotundum and transmits the vidian nerve. Lateral to the pterygopalatine ganglion is the masticator space that contains V3.

30.3 Imaging and Diagnosis of Skull Base Tumors

The skull base is best evaluated by neuroimaging techniques. Computed tomography (CT) and magnetic resonance imaging (MRI) have complementary roles in evaluation of skull base tumors.

As the skull base is an undulating surface of limited thickness oriented in the axial plane, CT scans of the skull base must always include sections in at least two different planes and slices approximately 3 mm thick [2]. Use of iodinated contrast agent is necessary in most cases to delineate tumor margins and adjacent vascular structures. CT seems well suited for studying the bony skull base and the effect of the tumor on adjacent bone, which helps characterize a tumor’s aggressive or benign nature [2].

MRI is useful in the study of intracranial tumors and intracranial spread and the detection of dural, leptomeningeal, and cranial nerve involvement. MRI can also discriminate between secretions and tumor in sinonasal cavities as well as depict bone marrow invasion [2–8]. T1-weighted images are best used for the depiction of bone marrow invasion. As bone marrow normally has high-intensity signal due to its fat content, invasion of the marrow is seen as low to intermediate signal intensity. To discriminate between tumor and retained secretions, it is best to use a combination of T1-weighted, T2-weighted, and contrast-enhanced T1-weighted sequences, as secretions do not enhance. For depicting meningeal invasion and perineural spread of the disease, gadolinium enhancement is essential [7–9]. Gadolinium-enhanced, fat-suppressed images are routinely used to detect enhancing tumors close to fatcontaining spaces, such as the orbit, neurovascular channels and openings. This technique is also useful for the skull-based structures of the clivus and petrous bone where marrow with fat exists. Gadolinium-enhanced skull base imaging must be used with caution [10] because failure of fat suppression near bone–air or bone–fat interfaces may mimic tumor enhancement [11].

Positron emission tomography using fluorodeoxyglucose has proven useful in follow-up to differentiate between posttreatment changes and recurrence but not as useful in the primary evaluation of tumors.

Orbital invasion occurs most commonly through the lamina papyracea medially. If the tumor has not penetrated the periorbita, a smooth convex protrusion of the wall can be seen with fat separating this line from the medial rectus muscle, which allows sparing of the orbital contents during tumor resection. Orbital invasion may also occur when lesions in the maxillary sinus or pterygopalatine ganglion invade