Chondrosarcomas of the base of the skull are rare compared with other skull base tumors. Still, they are an important differential diagnosis as surgical resection and management are affected by the preoperative diagnosis.
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Epidemiology
Chondrosarcomas of the base of the skull make up only a small fraction of all chondrosarcomas (head and neck chondrosarcomas in one series make up only 7%). They are an even smaller proportion of intracranial neoplasms (making up only ~0.2% of all intracranial neoplasms in one series).
Risk factors
The vast majority of chondrosarcomas of the base of the skull are sporadic, however, some predisposing conditions are reported and include:
previous trauma/fractures
Clinical presentation
Patients usually present due to mass effect, either on the adjacent brain, brainstem, cranial nerves or (if extension inferiorly) structures of the superior neck.
Pathology
They are thought to arise from embryonal crest cells (remember that the base of the skull forms via endochondral ossification). Chondrosarcoma is composed of a cartilaginous matrix with chondrocytes inside the lacunae. Direct bone formation by tumoral cells is usually not seen6. Tumoral chondrocytes arranged in a diffuse or lobular pattern, depending on grade:
grade I: less cellularity, lobular growth pattern and rarely binucleated lacunae
grade II: cellularity is more than grade 1 and more diffuse growth pattern
grade III: high cellularity, pleomorphic nuclei with high mitotic count and diffuse growth pattern
Invasion of bone trabeculae by tumoral cells is one the most important diagnostic clue 7.
Location
petro-occipital synchondrosis (most common)
sphenoethmoidal junction
sella turcica
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other rare sites that have been reported (presumably from metaplasia) include:
The majority of chondrosarcomas of the base of the skull are located off the midline (82% in one series), a helpful sign compared to chordomas which are usually midline.
Local extension is common, extending intracranially, into the cavernous sinuses, paranasal sinuses and soft tissues beneath the base of the skull.
Radiographic features
Chondrosarcomas of the base of the skull follow the same general imaging characteristics of chondrosarcomas elsewhere – see generic chondrosarcoma article. Importantly CT and MRI are complementary, the former exquisitely delineating the relationship to the skull base and showing calcification within the mass, whereas the latter giving important information on signal intensity and relationship to neural structures.
Plain radiograph
Only of historical interest, skull x-rays were important in diagnosing these lesions and demonstrated lytic lesions in 50% and calcifications in approximately 60%. As such the differentiation of chondrosarcomas from other skull base tumors was very difficult before cross-sectional imaging.
CT
CT with thin tri-planar bone algorithm images is important in confirming bony involvement and demonstrating calcification of the tumor, often in characteristic rings and arcs.
MRI
T1: low signal
T2: high signal
SWI/GRE: calcifications show low signal
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T1 C+ (Gd)
usually heterogeneous enhancement
fat saturation should be employed to better delineate inferior component
Treatment and prognosis
Chondrosarcomas are relatively slow growing but locally aggressive. Local resection is often the treatment of choice. Radiotherapy may sometimes be employed although sensitivity is thought to be minimal. Metastatic spread is uncommon.
Differential diagnosis
Imaging differential considerations include lesions of the petrous apex. One should also consider specifically:
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usually midline projecting posteriorly
lower ADC value than chondrosarcoma5
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usually not high T2 signal
calcification pattern usually not chondroid
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calcification not as common
more destructive
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calcification uncommon
epicenter in the nasopharynx
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calcification uncommon
centered on pituitary fossa
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also high T2 signal but usually homogeneous
no calcification
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