Imaging Cranial Base Chordoma and Chondrosarcoma

Abstract

This image-rich chapter discusses the role that imaging plays in diagnosing primary chordoma and chondrosarcoma of the skull base. It describes the optimal cross-sectional imaging sequences to use, reviews the common symptoms and demographics of patients presenting for imaging evaluation, illustrates the distinguishing magnetic resonance and computed tomographic features for each tumor, and briefly presents the common pitfalls on imaging that can lead to diagnostic confusion.

Keywords

Chondrosarcoma, Chordoma, Skull base tumor

Outline
Imaging Technique 67
Part I: Chordoma 68
Part II: Chondrosarcoma 71
Part III: Potentially Confounding Lesions 75
References 78
Further Reading 78

Chordomas and chondrosarcomas are uncommon primary bone tumors with a predilection for the skull base. On computed tomographic (CT) and magnetic resonance (MR) examinations, they have well-described imaging features that help to distinguish them from each other and from other skull base lesions. Imaging also defines the tumor’s relationship to adjacent nerves, vessels, and bones, allowing for detailed surgical and radiation treatment planning. After treatment, imaging helps to evaluate for treatment-related complications and tumor recurrence.

Imaging Technique

MR is essential for determining the signal characteristics and soft tissue extent of chordoma and chondrosarcoma. An MR with gadolinium-based contrast agent should be performed if the patient has no absolute contraindication to MR imaging. Serum creatinine should be checked prior to gadolinium administration to calculate the glomerular filtration rate (GFR). Gadolinium is not generally administered to patients with a GFR less than 30 (mL/min/1.73 m ²), even if they are on dialysis. MR sequences should include T1-weighted precontrast as well as T2-weighted, diffusion-weighted, and T1-weighted postcontrast fat-suppressed images. In most cases, the information from these four sequences is sufficient to make the correct diagnosis. To obtain additional information about the internal characteristics and the extent of skull base tumors, many also perform additional sequences, including heavily T2-weighted steady-state imaging, gradient echo T2∗ imaging, and/or fluid-attenuated inversion recovery (FLAIR) imaging. Heavily T2-weighted steady-state sequences are called fast imaging employing steady-state acquisition (FIESTA) or constructive interference in steady state (CISS) depending on the manufacturer of the MR imaging machine. These steady-state sequences provide thin-section T2-weighted imaging of the basal cisterns and adjacent fluid-containing structures. They are relatively insensitive to cerebrospinal fluid motion and are helpful for evaluating small structures that traverse the basal cisterns such as blood vessels and cranial nerves. Diffusion-weighted imaging is also typically performed, not only to evaluate the diffusion characteristics of the tumor but also to assess for any complications such as posterior fossa infarction or other tissue injury. This sequence is especially important in the postoperative setting. Postgadolinium T1-weighted images are typically performed with fat suppression to help distinguish enhancing tumor from intrinsically bright fat, which is abundant in the marrow spaces of the skull base and the adjacent soft tissues. Fat-suppressed images do often suffer from artifacts related to the air, bone, and soft tissue interfaces that are common at the skull base, but these artifacts can be mitigated by careful attention to imaging technique.

CT may be the first imaging study performed in a patient who initially presents with nonspecific symptoms such as headache and who is found to have a skull base tumor. If CT is not the initial study obtained, then it should be performed to complement the MR, as CT provides essential information about the integrity of the bones of the skull base and can be very helpful in the differential diagnosis of an unknown lesion. On CT, bones should be evaluated with a sharp or bone kernel reconstruction using bone windows. Soft tissues should be evaluated with a soft tissue kernel reconstruction using soft tissue windows. If an MR examination using a gadolinium-based contrast agent has already been performed, then a CT examination without iodinated contrast is appropriate to evaluate the adjacent bony structures. If no MR has been performed or if no gadolinium-based contrast agent was administered, then CT images should be acquired with iodinated contrast to assess the enhancement characteristics of the skull base mass.

Part I: Chordoma

Chordomas represent 0.15% of all intracranial tumors, or about 1 case per 2,000,000 individuals per year. The median age of diagnosis is 46 years, although chordoma can occur at any age. Men are affected about 1.6 times as often as women. The overall survival for patients diagnosed with intracranial chordoma between 1995 and 2004 is reported as 81% at 5 years and about 63% at 10 years.

Chordomas arise from remnants of the notochord, a flexible rod-shaped structure composed of mesodermal cells that forms the principal longitudinal axis for the embryo in all vertebrates. The notochord extends from the Rathke pouch to the clivus, continuing inferiorly to the dens and through the center of the vertebral bodies. The primitive notochord is later surrounded by cartilaginous matrix. As this cartilage ossifies, the notochord is limited to the intervertebral regions where it evolves into the nucleus pulposis of the intervertebral discs. Notochordal remnants can thus occur anywhere along the neural axis from the skull base to the coccyx.

Chordomas arise in characteristic locations: 50% in the sacrum or coccyx, 35% in the clivus, and 15% in the spinal column. In particular, skull base chordomas are often centered on the midline clival sphenooccipital synchondrosis. Although chordomas grow slowly, they are locally invasive and will compress or infiltrate critical structures in any direction they spread. Lesions may be large at the time of initial diagnosis. Sixty percent of patients with clival chordomas present with a headache. Cranial neuropathy is common (94%) given the proximity of multiple cranial nerves to the central skull base. The sixth cranial nerve, which traverses Dorello’s canal close to the sphenooccipital synchondrosis, is the most commonly affected cranial nerve, and nerve compression may result in diplopia. Tumors may also grow superiorly and laterally to the cavernous sinus, where cranial nerves 3, 4, and 6 and/or the ophthalmic and maxillary divisions of the trigeminal nerve may be affected. Here, chordomas may also encase or compress the internal carotid artery. Tumors may extend laterally to Meckel’s cave, affecting the 5th cranial nerve, or posteriorly and laterally to the jugular foramen, affecting the 9th and 10th cranial nerves. Tumor extension to the basiocciput and hypoglossal canal may affect the 12th cranial nerve. Inferiorly, chordomas can extend to the foramen magnum and narrow the spinal canal. Chordomas can extend anteriorly to invade the prevertebral space, pterygopalatine fossa, nasopharynx, and/or paranasal sinuses. Finally, in up to 79% of cases, chordomas extend posteriorly into the prepontine cistern where they may displace or encase the vertebral or basilar arteries. Chordomas can narrow these arteries; however, this finding is not common, likely due to the malleable texture of the tumor cells. Larger tumors may impress, sometimes deeply, upon the pons.

An understanding of the cellular structure of chordoma helps explain its key imaging features on CT and MR. Chordomas are composed of sheets, nests, and cords of vacuolated, physaliphorous cells containing large amounts of glycogen and mucin. These cells are surrounded by a mucinous matrix, forming a multilobulated gelatinous tumor with delicate fibrovascular septa. The high mucin content of the tumor accounts for the typical high signal on T2-weighted images and low density on CT.

On CT, chordomas are well-circumscribed, expansile, usually midline masses that cause lytic bone destruction ( Fig. 7.1 ). The margin between the bone and tumor is not typically sclerotic and may be irregular or sharp. If calcified densities are present at the tumor–bone interface or within the tumor mass, they generally represent fragments of lysed bone rather than matrix mineralization. This absence of internal calcifications is a distinguishing feature from chondrosarcoma, since about 50% of chondrosarcomas show calcification of intratumoral chondroid matrix.

On MR, conventional or classic chordomas have intermediate to low signal on T1-weighted images and high signal on T2-weighted images ( Fig. 7.2 ). On heavily T2-weighted steady-state images such as CISS or FIESTA sequences, signal characteristics are variable, although usually lower in signal intensity than spin echo or fast spin echo T2-weighted images. On FLAIR images, chordomas frequently have isointense or intermediate signal. Variable enhancement after gadolinium administration is common ( Fig. 7.3 ). Most of the time, chordomas show mild to moderate homogeneous enhancement following gadolinium administration, but occasionally they demonstrate bright, homogeneous enhancement. If multiple postcontrast sequences are obtained over the course of 5–15 min, chordomas often show progressive enhancement over time, which is a helpful imaging feature. Mildly restricted diffusion is also typical of chordomas. On perfusion imaging, either endogenous arterial spin labeling or exogenous contrast-enhanced gadolinium-based techniques, chordomas do not usually show increased blood flow or blood volume.

Skull base chordomas may not show all of the classic MR and CT imaging characteristics. The internal signal ( Fig. 7.4 ), size ( Fig. 7.5 ), location, and degree of enhancement can vary considerably. Less commonly, chordomas can arise in the nasopharynx or paranasal sinuses, a more rostral location along the neuraxis ( Fig. 7.6 ). Alternatively, they may have only minimal attachment to and involvement of bone ( Fig. 7.7 ). In these cases the diagnosis is more difficult to make prior to tissue sampling. A more aggressive variant, anaplastic chordoma, shows intermediate rather than high signal intensity on T2-weighted images and moderate rather than mild restricted diffusion ( Fig. 7.8 ). These anaplastic subtypes are generally impossible to distinguish from lesions such as metastases or small round blue cell tumors on imaging alone.