Radiologic Findings and Differential Diagnosis of Chordomas at the Skull Base

Skull base chordomas deserve an extensive investigation and meticulous evaluation by an experienced radiologist in order to provide the most detailed information that is necessary for an optimal surgery in this complex anatomical location.¹ Radiology has the objective of presenting a diagnosis by indicating the origin of the tumor, defining its main radiologic features, determining the anatomical relationship of the tumor to the adjacent vital neurovascular structures, as well as enabling differential diagnosis from various skull base lesions. During the posttreatment period, the goal is to document the extent of the surgery, to guide adjuvant treatment, and in the long term, to differentiate between recurrence and postsurgical tissue changes. X-ray radiography, computed tomography (CT), magnetic resonance imaging (MRI), and angiography are performed in order to fulfill the above objectives, all of which have complementary roles in the evaluation of chordomas.

9.2 Radiologic Features of Skull Base Chordomas

9.2.1 Location and Extent

Although they can originate at any location in the skull base, most chordomas occur in the midline and involve the basisphenoid and the basiocciput clivus² ( ▶ Fig. 9.1). Chordomas are also encountered in the petrous apex, at or near the petro-occipital junction³ ( ▶ Fig. 9.1 a). More precisely, 34% of chordomas are found in the clivus, 29% in the spheno-occipital synchondrosis, 12% in the sella, and 5% in the sphenoid bone region.⁴ Extension beyond the skull base structures is common ( ▶ Fig. 9.1 i). Tumor extension into the cavernous sinuses, ethmoid air cells, orbit, sphenoid sinus, sella turcica, or suprasellar cistern is commonly encountered due to the anatomical proximity of the above structures to the clivus and the local aggressive nature of the tumor ( ▶ Fig. 9.1 b,h,i). Extension to the anterior fossa is uncommon but is possible in extensive tumors. Nasopharynx extension is critical; this may necessitate differential diagnosis from nasopharyngeal carcinomas because nasopharyngeal carcinomas are more common and require a different form of therapy. Large chordomas can occupy the prestyloid and/or the poststyloid parapharyngeal space, and the infratemporal fossa ( ▶ Fig. 9.1 i). It is not uncommon for chordomas to extend into the prevertebral space, the hypoglossal canals, and the jugular foramens.

Fig. 9.1 Although most chordomas occur in the midline and involve the basisphenoid and the clivus, they can originate at any location in the skull base such as the petrous bone and the parasellar region (a–c). The anatomical structures most commonly involved are the middle clivus, followed by the superior and inferior clivus (d–f) However, involvement of the entire basisphenoid and basiocciput till the cervical spine can be seen (g). The extension to the cavernous sinus, sphenoid sinus, and sella is not uncommon (h, i). Although rare, the extension to the infratemporal fossa can occur (i). Pure or mainly intradural chordoma is very exceptional (j).

In rare cases, chordomas may present as intadural soft tissue masses adjacent to the clivus without any evidence of bone involvement⁵^,⁶^,⁷ ( ▶ Fig. 9.1 j). Such tumors may originate from the middle clivus ( ▶ Fig. 9.1 e) or caudal ( ▶ Fig. 9.1 f) or cranial ( ▶ Fig. 9.1 d) margin of the clivus with some symmetric or asymmetric lateral extension. Chordomas in children, which commonly originate from the caudal margin of the clivus, may reach easily the cervical spinal canal ( ▶ Fig. 9.1 g). However, the anatomical structures most commonly involved in adults are the midclival regions.⁷

It is well known that chordomas are locally invasive tumors and can displace, or encase, the vital neighboring structures.⁷ Often, the skull base arteries are seen running through the tumor in their usual course and normal caliber. However, the displacement of the arteries by the mass is not unusual. According to some authors, none of the arteries displaced or encased by chordomas had resultant luminal narrowing.³ Although the above statement may hold true for many cases, luminal narrowing of arteries is not an exception. Displacement of the Meckel’s cave by the chordoma is also a possibility.

Hydrocephalus has been detected as a result of the chordoma displacing the brainstem, which then causes narrowing of the fourth ventricle and/or aqueducts cerebri ( ▶ Fig. 9.1 f). Although chordomas typically grow posteriorly, extension to the cavernous sinuses is a common finding ( ▶ Fig. 9.1 b).

9.2.2 Internal Structural and Bone Involvement

Chordomas are heterogeneous destructive and invasive soft tissue masses that can contain a variety of tissue components, including calcifications, sequestered bone fragments, hemorrhage, and proteinaceous mucus⁸^,⁹^,¹⁰^,¹¹ ( ▶ Fig. 9.2). Cysts with or without proteinaceous content, hemorrhage, bone sequestra, or dystrophic calcifications are common findings. A typical and very common feature for a skull base chordoma is bone destruction of the central skull base bones, which may be partially or totally destroyed with possible extension to the nearby bony structures. The ends of the destructed bone appear to be spiculated and irregular. The bone seems to be “moth eaten” and may contain some foci of mineralization or sequestered bone fragments where the bone has disappeared. The pattern of mineralization consists mostly of a few scattered punctate densities. Occasionally, an extensive fine, homogeneous calcification may be observed, whereas dense calcification is rare. The center of bone destruction may be interpreted as the origin of the tumor and most often corresponds to one side of the spheno-occipital synchondrosis. Considering the very small chordomas, the bone destruction is initially observed at one end of the spheno-occipital synchondrosis and extends along the same bone. The transection of the spheno-occipital synchondrosis occurs as the tumor attains larger dimensions. However, if bone destruction of the skull base is not depicted, the diagnosis of chordoma should be questioned.

Fig. 9.2 Heterogeneous internal structure of chordomas and various tissue components are demonstrated on T2-weighted images (a–d) and T1-weighted images (e–h). Cysts with (e) or without (c) proteinaceous content, calcifications (h), and hemorrhage (d) with some heterogeneity are common. Cases with a homogeneous soft tissue as the main feature can be seen on standard T2-weighted image (a). If thin-section high-resolution imaging at the same slice location is applied, usually some heterogeneity with thin hypointensities, consistent with calcifications and/or hemorrhage, can be demonstrated (b).

9.2.3 Shape and Margins

Generally, chordomas are round or lobulated soft tissue masses with distinct margins. Lobulation is more obvious in larger tumors. The tumor usually appears to be encapsulated. Its borders are well defined by MRI except at sites of bone invasion where margins become irregular and indistinct. Chordomas that extend to the adjacent skull base cisterns will exhibit a regular, well-defined soft tissue border. Sclerotic rim or new bone formation adjacent to a chordoma is almost never seen. The regular tumor margin may be occasionally disrupted where the tumor transects the dura mater. Occasionally, chordomas may produce an opacification of the dura mater, known as “dural tail,” which will then necessitate differentiation from a meningioma.

9.3 Radiologic Work-up

9.3.1 X-ray Radiography

The middle fossa bones are well displayed on the lateral skull X-ray and provide a quick diagnosis of bone destruction and/or mineralization by chordoma. Conventional radiographs can show erosion and sequestra in the skull base osseous structures with enlargement of foramina and/or fissures and extension to the paranasal sinuses. However, the information obtained by conventional radiographs is usually limited, because tumor margins cannot be easily delineated and the extent of the tumor appreciated. The conventional X-ray tomography is of historical interest only in the era of MRI and CT; its application is inappropriate due to the use of very high radiation doses and the cumbersome technique.

9.3.2 Computed Tomography

CT is advised for all cases suspect of chordoma in order to depict the bone destruction and the bone sequestra that are typical of these tumors ( ▶ Fig. 9.3). The scan, which is performed using 2- to 3-mm thin axial slices across the skull base without contrast injection, is sufficient for the above purpose. Three-dimensional (3D) CT is a more sophisticated reconstruction of the CT images, although impressive with the images showing the bone destruction, evaluation is more complicated due to the superimposition of the structures on these images. CT with contrast agent injection is indicated to delineate the margins of the tumor in case MRI is not available. CT angiography can display the relation of the displaced arteries to the destructed bone, or the encasement of the arteries by the calcifications contained in the tumor.

Fig. 9.3 Bone destruction and the extent of the bone involvement are easily interpreted from CT (a–f). (a) Only bone destruction without calcification or sequestrum at the right lateral part of the clivus extending to the petrous apex. (b) A homogeneous hyperattenuating midline skull base mass is seen on CT on soft tissue windowing (c). (c) A mixture of hypo- and hyperattenuating soft tissues with tiny sequestrum within the destructed clivus. A midline clival destructive mass with a sequestrum is demonstrated (d). (e) The ends of the destructed bone are spiculated and irregular with a “moth eaten” pattern. (f) The lytic bone lesion has a smooth border.

CT satisfactorily displays the nature and extent of bone involvement in chordomas. Moreover, lysis of skull base foramina can be accurately demonstrated on CT images. CT shows the chordoma as a hyperattenuating mass compared with the adjacent neural tissue, with moderate to marked contrast enhancement. Low-attenuating areas are occasionally seen within the mass, representing gelatinous material seen on gross examination.⁹.

9.3.3 Magnetic Resonance Imaging

Although MRI and CT are usually both required in the radiologic evaluation of chordomas, MRI is superior to CT in delineating the lesion extent, the tumor margins, and the histologic contents of the tumor and demarcating neighboring neurovascular structures. Moreover, MRI is of utmost importance in planning a radical resection and contouring for radiotherapy. MRI is also superior in the postsurgical and/or postradiation follow-up and enables a differentiation between recurrence and posttreatment changes.

Coronal, sagittal, and axial images with high in-plane and through-plane spatial resolution (2 to 3 mm section thickness) should be acquired to obtain complex anatomical details, identifying the exact location and extent of the tumor and delineating the adjacent cranial nerves and vascular structures. High-resolution images are crucial not only for delineating adjacent vital neurovascular structures, but also for revealing the true internal signal characteristics to enable proper differential diagnosis ( ▶ Fig. 9.2 a, b). Sagittal plane is the most valuable for surgical planning in defining the posterior margin of the tumor and showing the relation between the tumor and the brainstem. It can depict nasopharyngeal or oropharyngeal extension of the tumor and can demonstrate transdural invasion. Coronal and axial images are both useful to evaluate extension into the cavernous sinus. Coronal images are useful to depict the position of the optic chiasm and optic nerves.

Sites of hemorrhage, mucinous collections, foci of mineralization, and sequestered bone fragments account for the typically heterogeneous signal characteristics of chordomas on all MRI sequences ( ▶ Fig. 9.2). Although small foci of hyperintensity due to hemorrhage, proteinaceous cystic fluid, or calcification can be found, chordoma is seen usually as intermediate to low signal intensity and easily recognized within the high signal intensity of the clivus on T1-weighted images without fat saturation ( ▶ Fig. 9.4). On the other hand, chordoma has usually high signal intensity on T2-weighted images due to its high fluid content ( ▶ Fig. 9.4). But calcification or sequestration, hemorrhage, septations, and highly proteinaceous fluid content can cause intratumoral areas of heterogeneous hypointensity on T2-weighted images. Whorls of hypointensity on T2 images are a remarkable finding in tumors containing dystrophic mineralization and/or hemorrhage. If T2 hypointensity relative to gray matter cannot be assigned to calcifications and hemorrhage with the help of gradient echo (GRE) T2* and CT, a poorly differentiated chordoma subtype can be considered in the differential diagnosis. T2-weighted fat saturation images should be supplied for tumor border evaluation at sites of bone marrow invasion where margins can be irregular and indistinct.

Fig. 9.4 The signal intensity patterns of chordomas on T2- and T1-weighted images vary considerably. Chordomas can appear as (a) very hyperintense, (b) mildly hyperintense, (c) moderately hyperintense, and (d) hypointense relative to the brainstem on T2-weighted images. They may be seen on T1-weighted images also as (e) moderately hypointense (f), mildly hypointense (g), isointense with mild heterogeneity and hyperintense (h) relative to the brainstem.