INTRACRANIAL CHORDOMA

Duc Tuan Tran Vo

Overview

• Chordomas are relatively rare (0.51 cs/106) malignant tumors that arise from embryonic remnants of the primitive notochord, a primitive cell line around which the skull base and the vertebral column develop. Remnants of the notochord usually remain in or close to the midline, entrapped within bone Chordomas lie in bone (extradural)

• Chordomas are considered to be low-grade neoplasms, locally invasive, but they rarely metastasize

• Chordomas account for 1% of intracranial tumors and 4% of all primary bone tumors

Epidemics

• Chordomas were relatively evenly distributed along the cranial (32%), spinal (32.8%), and sacral (29.2%) portions of the axial skeleton

• They may occur at any age but are usually seen in adults, with a peak prevalence in the 4th decade of life.

• Chordomas have a 2:1 male predilection and affect whites more than blacks.

• Intracranial chordomas constitute one-third of all chordomas and usually occur in the vicinity of the clivus (spheno-occipital bones), may arise unilaterally from the petrous apex.

Three-dimensional CT scan demonstrates the sites of origin ofintracranial chordomas: the upper (yellow), middle (red), and lower(green) clivus

Sagittal T1-weighted MR images demonstrate involvement of the upper, middle, and lower clivus.

Clinical features

• Generally, chordomas grow slowly and produce symptoms insidiously.

• Symptoms of intracranial chordomas vary with lesion location and proximity to critical structures, reflecting the specific sites of extension from the clivus (ie, the sellar, parasellar, and retroclival areas and, occasionally, the sphenoid sinus).

• The most common initial complaint is diplopia related to cranial nerve palsy and headache. Among cranial nerves, the abducent nerve is the most commonly affected. Headache is usually reported in an occipital or retro-orbital location

Characteristics

• Although intracranial chordomas are generally slow growing, their intimate relation to critical structures and extremely high local recurrence rate have often resulted in high mortality rates in the past. However, recent advances in skull base surgery and radiation therapy now provide the opportunity for cure.

• The excellent imaging capabilities of magnetic resonance (MR) imaging and computed tomography (CT) allow precise delineation of the tumors with respect to volume and relation to adjacent neural structures, thereby helping achieve this cure.

• Thin-section axial and coronal unenhanced and contrast material–enhanced images are usually obtained for assessment.

• The classic appearance of intracranial chordoma at high-resolution CT is that of a centrally located, well-circumscribed, expansive soft-tissue mass that arises from the clivus with associated extensive lytic bone destruction. The bulk of the tumor is usually hyperattenuating relative to the adjacent neural axis. Intratumoral calcifications appear irregular at CT and are usually thought to represent sequestra from bone destruction rather than dystrophic calcifications in the tumor itself.

• Sagittal reformatted CT scan reveals bone sequestra at the distal end of a lytic clival lesion (arrows).

• Axial CT scan of the skull base demonstrates the lesion with a clival origin and extension to the prepontine cistern with typical trabecular entrapment

• There is moderate to marked enhancement following administration of iodinated contrast material. Solitary or multiple low-attenuation areas are sometimes seen within the soft-tissue mass and probably represent the myxoid and gelatinous material seen at gross examination.

MR Imaging

• MR imaging is the single best modality for radiologic evaluation of intracranial chordomas.

• MR is considerably superior to CT in the delineation of lesion extent because it provides excellent tissue contrast and exquisite anatomic detail

• The multiplanar capability of MR imaging is also helpful in this regard. Sagittal images are generally the most valuable in defining the posterior margin of the tumor, showing the relation between the tumor and brainstem, and depicting nasopharyngeal extension of the tumor

• Sagittal imaging is also useful in disclosing transdural transgression by a tumor, an important factor in surgical planning.

• Coronal images, on the other hand, are helpful in detecting tumor extension into the cavernous sinus and depicting the position of the optic chiasm and tract

• MR imaging is deficient only in the evaluation of calcification and cortical bone. Osseous destruction is implied by replacement of the signal void of cortical bone with the soft-tissue signal intensity of tumor.

• Use of contrast-enhanced imaging can solve this problem

T1W

• On conventional spin-echo T1-weighted MR images, intracranial chordoma has intermediate to low signal intensity and is easily recognized within the high signal intensity of the fat of the clivus.

• Small foci of hyperintensity can sometimes be visualized in the tumor on T1-weighted images, a finding that represents intratumoral hemorrhage or a mucus pool. The presence of hemorrhagic foci can be confirmed with gradient-echo imaging that is susceptible to blood.

(a) Axial T1-weighted MR image shows a small, hypointense mass in the right side of the clivus (arrow). (b) Sagittal T1-weighted MR image obtained in a different patient shows a large, hypointense soft-tissue mass that arises from the distal clivus with anterior extension into the nasopharynx (arrows) and extradural extension into the posterior fossa (arrowhead)

Sagittal T1-weighted MR image shows a retroclival mass (arrows) that has a hyperintense rim and projects posteriorly, a finding that represents highly proteinaceous material or blood products.

T2W

• Classic intracranial chordoma has high signal intensity on T2-weighted images, a finding that likely reflects the high fluid content of vacuolated cellular components.

• The intratumoral areas of calcification, hemorrhage, and a highly proteinaceous mucus pool usually demonstrate heterogeneous hypointensity at T2-weighted imaging.

• Low-signal-intensity septations that separate high-signal-intensity lobules are commonly seen corresponding to the multilobulated gross morphologic features of the tumor.

• Also, T2-weighted imaging is excellent for differentiating tumor from adjacent neural structures

• The majority of intracranial chordomas demonstrate moderate to marked enhancement following contrast material injection. Occasionally, the enhancement is slight or even absent. Such a finding likely represents necrosis and a large amount of mucinous material in the tumor. The enhancement pattern of the tumor sometimes has a “honeycomb” appearance created by intratumoral areas of low signal intensity.

• Fat suppression is useful for differentiating enhanced tumor margins from adjacent bright fatty bone marrow. In addition, small intraclival chordomas can be better demarcated with this technique.

• Sze et al reported that because a watery, gelatinous matrix is replaced by cartilaginous foci, chondroid chordomas have shorter T1 and T2 values than do typical chordomas. Therefore, chondroid chordomas may not be as bright as typical chordomas on T2-weighted MR images. This finding is an important prognostic factor due to the significantly better survival rate of patients with chondroid chordoma

MR Angiography and Conventional Angiography• A clear advantage of MR imaging is its capacity to demonstrate patent

major vessels as flow voids. The internal carotid and basilar arteries and their anatomic relationship to tumors are well visualized in most intracranial chordomas. Tumoral displacement or partial encasement of intracranial arteries is common, being visualized in up to 79% of intracranial chordomas. Despite the high frequency of intracranial arterial involvement, arterial narrowing is rare in intracranial chordomas, a finding that reflects the fact that these tumors are generally soft and easily dissected from adjacent vessels. Therefore, MR angiography allows better evaluation of vascular encasement and obviates cerebral angiography,which does not allow detection of encasement without luminal narrowing or occlusion. Venous involvement or occlusion is also readily visualized at MR venography.

• Angiographic evaluation of intracranial chordomas is nonspecific. Abnormal tumor vascularity or staining is rare. Angiographic evaluation is reserved for cases in which there is significant displacement, encasement, or narrowing of the internal carotid or vertebral artery at MR angiography. Cerebral angiography can better demonstrate the degree of luminal narrowing or occlusion and the extent of collateral circulation. Temporary balloon occlusion of the internal carotid artery is frequently used to determine whether patients are at risk for neurologic injury during surgery due to permanent vessel occlusion.

Tumor Spread

• The classic midline clival chordoma can spread anteriorly, laterally, posteriorly, inferiorly, and superiorly, thereby affecting the sellar area, petrous apex–middle cranial fossa, prepontine cistern, foramen magnum–nasopharynx, and chiasm–third ventricle, respectively.

• Usually more than one of these areas is involved.

• Anterior tumor extension can involve the sphenoid sinus and, less commonly, the posterior ethmoid sinus.

• Anteroinferior extension can affect the nasopharynx and parapharyngeal space.

• Posteroinferior extension leads to involvement of the jugular fossa and foramen magnum, with erosion of the atlas and other cervical vertebrae. Intracranial chordomas may arise in the sellar and parasellar areas.

• Lateral extension of these tumors can invade the middle cranial fossa, whereas posterior extension can affect the petrous apex. Intracranial chordomas can grow into the basal cisterns with compression of the brainstem. In addition, intracranial chordomas commonly encroach on the anterior visual pathway and on the cranial nerves in the prepontine cistern and cavernous sinus, resulting in visual and cranial nerve abnormalities.

Treatment

• Surgical removal is a very effective treatment for intracranial chordomas. Longer survival rates have been associated with more extensive tumor removal

• Residual tumors can be successfully treated with radiation therapy

• The recurrence-free 5-year survival rate for patients with skull base chordoma who undergo combined treatment with surgery and radiation therapy is 60%–70%. Surgery plus radiation therapy remains the most effective treatment

Recurrence and Metastasis

• Local recurrence of intracranial chordomas is still common regardless of the mode of therapy

• MR imaging is the modality of choice for postsurgical follow-up and detection of recurrence. Parenchymal signal intensity changes are frequently observed at MR imaging performed after surgery or radiation therapy, especially in the temporal lobe and visual pathways. Striking hyperintensity at T2-weighted MR imaging is helpful in suggesting tumor recurrence rather than postoperative changes. Local recurrence is more common following subtotal or partial tumor resection.

• Tumor recurrence can occur along the surgical pathway but is uncommon; it was described in only 5% of cases in one large series. Distant metastasis is rare, although in one study it was observed in 7%–14% of intracranial chordomas as pulmonary, liver, bone, or lymph node involvement.

Differential Diagnosis

• Chondrosarcomas are the lesions most often confused with intracranial chordomas. Unlike intracranial chordomas, which have a midline skull base location, the majority of chondrosarcomas arise along the petro-occipital fissure. However, chondrosarcomas can sometimes have a midline location, making differentiation between a chondrosarcoma and an intracranial chordoma difficult. Also, the two tumors have similar signal intensity on T1- and T2-weighted MR images. Therefore, linear, globular, or arclike calcifications when present in chondrosarcomas can help distinguish them from intracranial chordoma.

• Clival meningiomas have a dural attachment and do not have the appearance of a destructive bone lesion. Instead, they cause bone sclerosis and demonstrate homogeneous enhancement. They also have a characteristic angiographic appearance.

• Nasopharyngeal malignancies usually extend more anteriorly and have associated head and neck lymphadenopathy. Plasmocytoma and lymphoma occasionally involve the skull base and cause lytic bone destruction. If centrally located, these tumors can mimic intracranial chordomas.

• Craniopharyngiomas, besides demonstrating a relatively characteristic signal intensity, are located more anteriorly and superiorly in midline than are intracranial chordomas.

• Because skull base metastases are relatively infrequent in the absence of a primary neoplasm, they should be viewed as an unlikely differential diagnosis. Additionally, the extraosseous tumor component of metastases is usually small relative to the intracranial chordoma.

• Other differential diagnoses, although rare, include aggressivepituitary adenoma, histiocytosis X, dermoid and epidermoid cysts, trigeminal neuroma, and fibrous dysplasia

Summary

• Intracranial chordomas are rare midline tumors of clival origin. • MR imaging and CT are the imaging modalities of choice for diagnosis,

treatment, and follow-up. • Intracranial chordomas are often visualized as soft-tissue masses that

originate from the clivus with extensive lytic bone destruction. • They commonly displace or partially encase intracranial arteries, but

arterial narrowing is rare. • Classic findings in intracranial chordomas include intermediate to low

signal intensity on T1-weighted MR images and very high signal intensity on T2-weighted images. Enhancement is marked and often heterogeneous.

• Treatment that consists of radical surgical resection followed by proton beam radiation therapy achieves the best results.