Imaging Cranial Base Chordoma and Chondrosarcoma

C H A P T E R

7 Imaging Cranial Base Chordoma and Chondrosarcoma Louis Golden, Arjun Pendharkar, Nancy Fischbein Stanford University School of Medicine, Stanford, CA, United States

O U T L I N E Imaging Technique

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Part III: Potentially Confounding Lesions

Part I: Chordoma

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Part II: Chondrosarcoma

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Further Reading

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.

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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 steadystate 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

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 m2), even if they are on dialysis. MR sequences should incl­ ude T1-weighted precontrast as well as T2-weighted,

Chordomas and Chondrosarcomas of the Skull Base and Spine, Second Edition http://dx.doi.org/10.1016/B978-0-12-804257-1.00007-4

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© 2018 Elsevier Inc. All rights reserved. Please note that the copyright for the original figures submitted by the contributors is owned by Contributors.

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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.1 The median age of diagnosis is 46 years, although chordoma can occur at any age.2 Men are affected about 1.6 times as often as women.2 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.3 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.4 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.

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Part I: Chordoma

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FIGURE 7.1  Typical computed tomographic (CT) features of skull base chordoma. An axial image in the soft tissue window (A) and the bone window (B) shows a midline mass (arrows) centered on the clivus, with low internal density reflecting the mucoid content of physaliphorous cells. (C) A sagittal CT image in the bone window from a different patient shows aggressive lytic erosion of the clivus (arrow).

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. Chordomas have a high rate of local recurrence after initial treatment because of the difficulty of completely

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FIGURE 7.2  Typical magnetic resonance features of skull base chordoma, in this case centered in the upper clivus. (A) The tumor (arrow) has low signal intensity on a sagittal T1-weighted image. (B) On an axial T2-weighted image, the chordoma (asterisks) displaces the basilar artery (B) to the right. The chordoma (asterisks) demonstrates mild diffusion restriction on diffusion-weighted imaging (C), which is confirmed on the apparent diffusion coefficient map (D).

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FIGURE 7.3  Chordoma enhancement patterns. Chordomas can demonstrate either homogeneous or heterogeneous enhancement. (A) A homogenously enhancing chordoma (asterisk) on an axial T1-weighted postcontrast image with fat suppression. (B, C) Another chordoma in a different patient that has stippled enhancement (arrows) on an early postcontrast three-dimensional fast spoiled gradient echo image (B) and more homogeneous enhancement on a later postcontrast spin echo image (C). This progressive enhancement pattern differs from brainstem glioma, a potential confounder, which typically demonstrates contrast washout over time.

resecting these often large and infiltrative tumors. Chordomas that are large or that extend to the craniocervical junction often require posterior occipitocervical fusion after resection to ensure stability (Fig. 7.9).

Hematogenous metastases from chordoma are uncommon but have been reported in the lymph nodes, lung, bones, and skin. Direct or “implant” metastases can develop along the surgical tract following tumor

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Part II: Chondrosarcoma

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excision, so it is important to know the pathway of surgical excision and scrutinize it carefully on follow-up imaging studies to evaluate for these lesions (Fig. 7.10).

PART II: CHONDROSARCOMA

FIGURE 7.4  Internal blood, a less common chordoma feature. Coronal T1-weighted image shows a chordoma with internal hyperintense blood (arrow), an uncommon finding.

FIGURE 7.5  Nonaggressive bone destruction, a less common chordoma feature. This small chordoma (arrow) on axial CT has a narrow margin/zone of transition between tumor and adjacent bone, a sign more commonly associated with nonaggressive entities.

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Primary chondrosarcomas arise de novo, whereas secondary chondrosarcomas arise in degenerated benign cartilaginous tumors such as enchondroma and osteochondroma. The overwhelming majority of chondrosarcomas at the skull base are primary tumors arising from primitive cartilaginous cells at the synchondroses. Chondrosarcomas account for 6% of skull base tumors and 0.15% of all intracranial tumors. Chondrosarcoma can occur at any age, but the mean age of presentation is 40 years. Men and women are affected approximately equally. The overall 5- and 10-year survivals are about 90% and 68%, respectively, markedly better than those with chordoma.5 The median overall survival is 22 years for patients treated with surgery compared with 13.5 years for patients with no treatment.6 Primary chondrosarcomas have been further categorized by the histologic subtype and can be defined as conventional intramedullary, clear cell, myxoid, mesenchymal, or dedifferentiated variants. The conventional intramedullary chondrosarcoma subtype accounts for the majority (about 85%–90%) of skull base chondrosarcomas and has the best-described imaging characteristics.7 One review of the literature revealed no case reports of clear cell or dedifferentiated subtypes occurring at the skull base.7 The myxoid subtype is most commonly encountered in an extraskeletal location, and fewer than 15 cases have been reported at the skull base.8 Mesenchymal subtypes account for most of the remaining skull base chondrosarcomas and are associated with higher mortality and poorer prognosis.7 (B)

FIGURE 7.6  Chordomas are not always centered on the clivus, the most common intracranial location. Sagittal T1-weighted (A) and axial T2-weighted (B) images of a chordoma centered on an unusually rostral position within the ethmoid sinus. The clivus is preserved. II.  DEMOGRAPHICS, PRESENTATION, AND DIAGNOSIS

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FIGURE 7.7  Sagittal T1-weighted (A), axial T2-weighted (B), and axial computed tomographic (C) images of an unusually exophytic chordoma with only subtle erosion of the posterior clivus (arrow). The chordoma’s prepontine cistern location makes it impossible to distinguish it from an epidermoid cyst on imaging alone. Tissue sampling was needed for definitive diagnosis. Brainstem glioma is another tumor to consider in this location. However, the lobulated margins, restricted diffusion (not shown), and low T1 signal of this mass do not support the diagnosis of glioma.

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FIGURE 7.8  Anaplastic chordoma, a less common subtype. Axial T2-weighted image (A), diffusion-weighted image (B), and an apparent diffusion coefficient map (C) show a mass (arrows) with marked restricted diffusion. The restricted diffusion of this chordoma has lower apparent diffusion map coefficient signal compared with the average chordoma, a finding suggesting increased cellularity.

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FIGURE 7.9  Preoperative considerations: craniocervical stability. A sagittal T1-weighted image (A) shows a large skull base chordoma (asterisk) extending to the craniocervical junction. A lateral radiograph following resection (B) shows a posterior occipitocervical fixation performed to ensure craniocervical stability.

Part II: Chondrosarcoma

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FIGURE 7.10  Postoperative considerations: direct spread of metastases. Image interpreters must ensure that no tumor deposits have seeded the surgical resection tract. A sagittal T1-weighted image (A) demonstrates a mass (arrows) with low T1 signal. On a T2-weighted image (B), the same mass (asterisk) has high T2 signal. Follow-up axial T2-weighted images (C,D) show two implants of recurrent tumor on the nasal septum and posterior nasal cavity near the right sphenopalatine foramen (arrows).

Chondrosarcomas have a predilection for arising from the petroclival synchondrosis and consequently are most often encountered off-midline, although some tumors arising from the sphenooccipital synchondrosis are centered in the midline. Similar to chordoma, clinical symptoms are frequently related to local extension with compression or infiltration of the adjacent cranial nerves and cavernous sinus. More inferiorly located lesions may involve cranial nerves 9 through 12, whereas more superior lesions often involve the abducens nerve as it crosses over the petrous apex adjacent to the petroclival synchondrosis. Commonly reported symptoms include diplopia (51%), headache (31%), decreased hearing, dizziness, and vestibular disturbance (21%).

On CT, chondrosarcomas characteristically erode the adjacent bone. About 50% of lesions show calcification of the cartilaginous matrix, typically in a curvilinear or “whorled” configuration (Fig. 7.11). On MR, chondrosarcomas often have low to intermediate signal intensity on T1-weighted images and very high signal intensity on T2-weighted images. Chondrosarcomas do not generally show restricted diffusion, and they have higher apparent diffusion coefficient values than chordomas (Fig. 7.12). Skull base chondrosarcomas can have either heterogeneous or homogeneous enhancement patterns. The solid component of the tumor typically demonstrates intense homogeneous enhancement after contrast administration, but there may also be a

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FIGURE 7.11  Typical computed tomographic (CT) features of skull base chondrosarcoma. (A) An axial CT image shows a mass centered on the left petroclival synchondrosis extending to the left middle cranial fossa. There is a narrow zone of transition at the tumor margin (arrow), which is not necessarily sclerotic. (B) A coronal CT image illustrates scattered high-density punctate mineralization (arrows) within low-density soft tissue.

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FIGURE 7.12  Typical imaging characteristics of skull base chondrosarcoma. (A) Axial computed tomographic image in the bone window shows a mass (arrow) at the left petroclival synchondrosis with sharply defined margins. The mass has high T2 signal on a coronal T2-weighted image (B). The mass (arrows) has high signal on both diffusion-weighted imaging (C) and the apparent diffusion coefficient map (D) due to a T2 shine-through effect rather than true restricted diffusion. This chondrosarcoma (arrows) demonstrates low signal on coronal T1-weighted precontrast images (E) and homogeneous enhancement on coronal T1-weighted postcontrast fat-suppressed images (F).

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Part III: Potentially Confounding Lesions

PART III: POTENTIALLY CONFOUNDING LESIONS

pattern of peripheral enhancement with internal lines and swirls of enhancement (Fig. 7.13). The presence of calcifications alters the internal signal characteristics of chondrosarcoma, and mineralized lesions often have lower signal intensity on T2-weighted images and more commonly show heterogeneous gadolinium enhancement (Table 7.1). (A)

Many other lesions occur in the clivus, and the differential diagnosis for chordoma or chondrosarcoma may include developmental or neoplastic lesions. Notochordal remnant lesions such as ecchordosis physaliphora, bone

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FIGURE 7.13  Typical magnetic resonance characteristics of skull base chondrosarcoma. Coronal T2-weighted (A), axial diffusion-weighted (B), apparent diffusion coefficient map (C), and axial T1-weighted pregadolinium (D) and postgadolinium fat-suppressed (E) images show a T1-hypointense, T2-hyperintense mass with heterogeneous enhancement and no diffusion restriction. Skull base chondrosarcoma can demonstrate either heterogeneous or homogeneous enhancement patterns. Heterogeneous gadolinium enhancement is often associated with mineralized lesions, although nonenhancing or less enhancing portions do not necessarily correlate to calcium particles.

TABLE 7.1  Typical Features of Chordoma Versus Chondrosarcoma of the Skull Base Feature

Chordoma

Chondrosarcoma

Location

Midline: sphenooccipital synchondrosis

Off-midline: petroclival fissure

Computed tomography

No internal calcifications, lytic expansile

50% “arcs and whorls” calcifications, lytic expansile

T1

Low to intermediate signal

Low signal

T2

High signal

High signal Mixed or low signal if many calcifications

Diffusion-weighted imaging

Mild to moderate diffusion restriction

No or minimal diffusion restriction

Postgadolinium enhancement

Homogeneous or heterogeneous, often mild; increases over time

Homogeneous or heterogeneous, often intense

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FIGURE 7.14  Fibrous dysplasia may mimic skull base tumors. Axial T2-weighted (A), sagittal T1-weighted precontrast (B), and axial T1-weighted postcontrast (C) images demonstrate a fibrous dysplasia lesion (arrow) that has high T2 signal and heterogeneous enhancement, findings similar to those of a chordoma. However, a sagittal computed tomographic (CT) image (D) reveals ground-glass internal matrix and bone expansion, findings typical of fibrous dysplasia. Chordoma, on the other hand, usually presents with lytic destruction on CT. Fibrous dysplasia does not always have a high T2 signal on magnetic resonance imaging. A different clival fibrous dysplasia in another patient demonstrates low T2 signal on an axial T2-weighted image (E), which is a more common imaging pattern for fibrous dysplasia. A sagittal CT image (F) of this fibrous dysplasia also demonstrates ground-glass internal matrix similar to that of the other patient with fibrous dysplasia illustrated in panels (A–D).

marrow lesions such as plasmacytoma or lymphoma, and bone metastases may be additional diagnostic considerations. Nonaggressive “do-not-touch” lesions such as fibrous dysplasia are frequently misinterpreted as chordoma when they involve the clivus, especially when only MR imaging is performed (Fig. 7.14). Unlike chordoma and chondrosarcoma, however, fibrous dysplasia expands the bone and contains ground-glass internal

matrix on CT. It also typically has low signal intensity on T2-weighted MR images. The behavior of notochordal remnants can range widely from malignancy such as chordoma to benign “do-not-touch” lesions such as ecchordosis physaliphora (Fig. 7.15). Imaging features can overlap significantly between the notochordal remnants, making diagnosis challenging. Both commonly have low signal

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FIGURE 7.15  Notochordal remnant spectrum. The behavior of notorchordal tissue can range widely from malignant such as chordoma to benign “do-not-touch” lesions such as ecchordosis physaliphora. Imaging features between these notochordal remnants can overlap significantly, confounding the diagnosis. Long-term stability, plaque-like morphology, sclerotic or nonaggressive margins, absence of gadolinium enhancement, and small size (less than 2 cm) are features more typical of less aggressive notochordal variants. Axial computed tomographic (A), sagittal T1 pregadolinium (B), sagittal T1 postgadolinium (C), and axial T2-weighted (D) images of ecchordosis physaliphora. This lesion (arrow) has low T1 and high T2 signal, no enhancement, sclerotic margins, and small size. Its imaging appearance did not change over several years.

on T1-weighted images and high signal on T2-weighted images. Unlike chordoma, however, ecchordosis physaliphora is often discovered incidentally when imaging is performed for other reasons, and patients are usually asymptomatic at initial diagnosis. These lesions are also small, often less than 2 cm in size, whereas the median size of chordoma is between 3.5 and 4 cm. Ecchordosis physaliphora is best evaluated on 1 mm or thinner section sequences, as it can be inconspicuous on thicker sections. In addition, ecchordosis physaliphora is most commonly described as intradural and nonenhancing, whereas chordomas are more commonly extradural and enhancing. On CT, the bone–ecchordosis interface may be sclerotic due to nonaggressive bone remodeling. A small bony stalk arising from the clivus is often

present.9 Ecchordosis physaliphora generally changes little, if at all, over the course of several years, whereas chordoma follows a more aggressive clinical course. Given the pathologic similarity between ecchordosis physaliphora and chordoma, distinguishing chordoma from ecchordosis physaliphora on imaging can be challenging if these classic features are not observed. Even if a lesion is thought to represent ecchordosis physaliphora, continuous follow-up is still critical to ensure that a chordoma is not overlooked. Plasmacytomas are monoclonal plasma cell masses with intermediate signal intensity on T2-weighted images due to a high nuclear–cytoplasmic ratio. They cause bone lysis with sharp margins and show moderate to avid homogeneous enhancement. Metastases can

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FIGURE 7.16  Plasmacytoma may mimic a skull base tumor. Sagittal T1-weighted (A) and axial T2-weighted (B) images show a clival plasmacytoma with low T1 and intermediate T2 signal. The mass extends along the posterior margin of the clivus into the prepontine cistern. It may be difficult to distinguish plasmacytoma from primary skull base tumor with imaging alone.

vary widely in appearance depending on the nature of the primary tumor. If there is a known history of cancer or there are bone metastases seen elsewhere on the image, then a clival metastasis is more likely than chordoma. Both plasmacytoma and metastases tend to be cellular with intermediate signal intensity on T2-weighted images (Fig. 7.16). Ultimately, if a clival lesion does not fit clearly into a “do-not-touch” category such as fibrous dysplasia, then tissue sampling will likely be required to make a specific diagnosis.

References 1. Koutourousiou M, Snyderman CH, Fernandez-Miranda J, Gardner PA. Skull base chordomas. Otolaryngol Clin North Am October 2011;44(5):1155–71. 2. Smoll NR, Gautschi OP, Radovanovic I, Schaller K, Weber DC. Incidence and relative survival of chordomas. Cancer 2013;119: 2029–37. 3. Chambers KJ, Lin DT, Meier J, Remenschneider A, Herr M, Gray ST. Incidence and survival patterns of cranial chordoma in the United States. Laryngoscope 2014;124:1097–102. 4. Erdem E, Angtuaco EC, Van Hemert R, Park JS, Al-Mefty O. Comprehensive review of intracranial chordoma. Radiographics 2003;23:995–1009.

5. Neelakantan A, Rana AK. Benign and malignant diseases of the clivus. Clin Radiol 2014;69:1295–303. 6. Jones PS, Aghi MK, Muzikansky A, Shih H, Barker FG, Curry WT. Outcomes and patterns of care in adult skull base chordomas from the Surveillance, Epidemiology, and End Results (SEER) database. J Clin Neurosci 2014;21:1490–6. 7. Bloch OG, Jian BJ, Yang I, Han SJ, Aranda D, Ahn BJ, Parsa AT. A systematic review of intracranial chordosarcoma and survival. J Clin Neurosci 2009;16(12):1547–51. 8. Arpino L, Capuano C, Gravina M, Franco A. Parasellar myxoid chondrosarcoma: a rare variant of cranial chondrosarcma. J Neurosurg Sci December 2011;55(4):387–9. 9. Mehnert F, Beschorner R, Küker W, et al. Retroclival ecchordosis physaliphora: MR imaging and review of the literature. AJNR Am J Neuroradiol 2004;25(10):1851–5.

Further Reading 1. Erdem E, Angtuaco EC, Van Hemert R, et al. Comprehensive review of intracranial chordoma. Radiographics 2003;23(4):995–1009. 2. Small JE. Ecchordosis physaliphora versus chordoma. In: Small JE, Schaefer PW, editors. Neuroradiology key differential diagnoses and clinical questions. Philadephia: Saunders Elsevier (imprint); 2012. p. 171–5. 3. Murphey MD, Walker EA, Wilson A, Kransdorf MJ, Temple HT, Gannon FH. Imaging of primary chondrosarcoma: radiologicpathologic correlation. Radiographics 2003;23:1245–78.

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