Figures 1, 2, 3, and 4: Axial and sagittal images demonstrate a destructive skull base lesion centered on the sphenoid sinus and the anterior clivus. There are multiple sites of osseous dehiscence including in the planum sphenoidale. There is a bulky soft tissue mass seen entering the nasal cavity and extending into the expected region of the nasopharynx.

Figure 5: Axial soft tissue window CT image demonstrates a subtle attenuation abnormality with osseous erosion suspicious for cavernous sinus involvement of the left. No significant hyperattenuation sites are seen within the soft tissue mass except for osseous fragments.

Figure 6: Sagittal T1-weighted image demonstrates a heterogeneous polypoid mass lesion involving the ethmoid and sphenoid sinuses and replacing the majority of the anterior clivus and eroding the floor of the anterior cranial fossa. There is no sellar expansion noted.

Figure 7: Axial T2-weighted image demonstrates a prominently hyperintense abnormality in the areas of soft tissue abnormality noted on the CT images (Figures 1-5). This soft tissue mass has a multiloculated appearance and is displacing the left internal carotid artery.

Intracranial chordoma.

Chordoma is a rare malignant primary bone tumor that arises from the embryonic remnants of the primitive notochord. The notochord is a column of cells extending the length of the embryo between the neural tube and gut precursors. It is 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 can occur anywhere along the course of the notochord but are most often found in the sacrococcygeal region (50-60%), spheno-occipital area (especially the clivus; 25-40%), and in the vertebral column (especially the cervical spine). Chordomas account for 1% of intracranial tumors and 4% of all primary bone tumors. They have a wide age of distribution with the average age of patients around 40 years. Chordomas have a 2:1 male predilection and affect whites more than blacks.

Histologically, chordomas can be divided into two variants: typical and chondroid. The chondroid subtype is more commonly seen in the skull base, constituting up to one third of cases in that region. Pathologically, chordoma is a soft gelatinous tumor which destroys bone, incorporates sequestra of bone within itself and may calcify. Necrosis and hemorrhage are common. Chordomas are usually locally invasive, but they can rarely metastasize (10%) and demonstrate leptomeningeal dissemination.

Intracranial chordomas most often originate in the midline near the spheno-occipital synchrondrosis and involve the clivus, sphenoid, and petrous bones. The classic midline clival intracranial chordoma can extend anteriorly, laterally, posteriorly, superiorly, and inferiorly such that the tumor can compress the brain stem and cerebellum, displace the cavernous sinus and sellar tissues with extension into the middle cranial fossa, and extend ventrally into the nasopharynx. Intracranial chordomas can occasionally (15%) arise unilaterally from the petrous apex. Other sites of origin include the sellar area, sphenoid sinus, and rarely the nasopharynx, maxilla, paranasal sinuses, or intradural region. The differential diagnosis for intracranial chordoma includes lesions that directly or secondarily invade and destroy the clivus, sphenoid, and/or petrous apex, such as, plasmacytoma, metastases, chondroma/chondrosarcoma, and invasion from nasopharyngeal malignancies. Occasionally, invasive pituitary adenoma, epidermoid, meningioma, or extensive glomus jugulare tumore might mimic chordoma, although these tend to be less destructive than chordoma.

Intracranial chordomas generally grow slowly and thus produce symptoms insidiously. The symptoms of intracranial chordomas vary with lesion location, proximity to critical structures, and reflect the areas of tumor extension. The most common initial symptoms are diplopia and headache. Diplopia is related to a cranial nerve palsy, with the abducent nerve most commonly affected. Headache is usually reported in an occipital or retro-orbital location. Other symptoms secondary to cranial nerve palsies include facial numbness, facial droop, dysphagia, and hoarseness. Large tumors can cause signs and symptoms related to brainstem compression such as ataxia.

Both CT and MR imaging are usually required for pretreatment evaluation of intracranial chordoma. High resolution CT with bone and soft tissue windows is sensitive fro detecting lesions of the skull base. Thin section axial and coronal unenhanced and contrast enhanced CT images are usually required for tumor assessment. On high resolution CT, the classic appearance of intracranial chordoma is a hyperattenuating, centrally located, well-circumscribed, expansile soft tissue mass that arises from the clivus and is associated with extensive lytic bone destruction. This tumor demonstrates moderate to marked enhancement on contrast enhanced images.

MR imaging is the best imaging modality for both pre- and post-treatment evaluation of intracranial chordoma because it allows for excellent delineation of lesion extent by providing exceptional tissue contrast and anatomic detail. Sagittal MR images are useful in demonstrating the posterior margin of the tumor, showing the relationship between the tumor and the brainstem, and demonstrating nasopharyngeal and transdural tumor extension. Coronal images are used to demonstrate tumor extension into the cavernous sinus. On T1-weighted MR images, intracranial chordomas demonstrate intermediate to low signal intensity and can be easily discriminated from the high signal intensity fat of the clivus. Areas of high signal intensity on T1 images can represent intratumoral hemorrhage or a mucus pool. Osseous destruction by the tumor is implied by the replacement of the signal void of cortical bone with the soft-tissue signal intensity of the tumor. On T2-weighted MR images, intracranial chordomas classically demonstrate very high signal intensity, a finding that likely reflects the high fluid content of vacuolated cellular components. Areas of heterogeneous hypointensities on T2-weighted images can represent tumoral calcification, hemorrhage, and a highly proteinaceous mucus pool. Intracranial chordomas demonstrate moderate to marked enhancement after contrast administration. Areas that demonstrate little or no enhancement most likely represent areas of necrosis and mucinous material within the tumor. Areas of enhancement mixed with areas of little to no enhancement sometimes give the tumor a “honeycomb” appearance. Fat suppression techniques can be used to help differentiate enhanced tumor margins from adjacent bright fatty bone marrow. MR imaging can also indicate the subtype of chordoma, with the chondroid types having shorter T1 and T2 values than typical chordomas. Thus, the chondroid subtype of chordoma may not appear as bright as the typical subtype of chordoma on T2-weighted MR images.

The natural history of chordoma is variable, and there is no histologic distinction between benign and malignant chordomas. However, the chondroid subtype of chordoma has a better prognosis than the typical subtype, 16 year life expectancy post-diagnosis versus 4 year life expectancy post-diagnosis respectively. Combination treatment with radical surgical resection and proton beam radiation therapy achieves the best results.

References:

1. Atlas, Scott W. Magnetic Resonance Imaging of the Brain and Spine, 3rd Ed. Philadelphia: Lippincott Williams & Wilkins, c2002. pp. 461-462.
2. Erdem E, Angtuaco EC, Van Hemert R, et al. Comprehensive Review of Intracranial Chordoma. RadioGraphics. 2003; 23: 995-1009.
3. St. Martin M. and S. C. Levine. Chordomas of the Skull Base: Manifestations and Management. Current Opinions in Otolaryngology & Head and Neck Surgery. 2003; 11: 324-327.