Chondrosarcoma of the skull base
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Chondrosarcomas of the base of the skull are rare compared with other skull base tumors but are an important differential diagnosis as surgical resection and management are affected by the preoperative diagnosis.
Chondrosarcomas of the base of the skull make up only a small fraction of all chondrosarcomas (head and neck chondrosarcomas in one series making up only 7%). They are an even smaller proportion of intracranial neoplasms (making up only ~ 0.2% of all intracranial neoplasms in one series).
The vast majority of chondrosarcomas of the base of the skull are sporadic, however, some predisposing conditions are reported, and include:
Patients usually present due to mass effect, either on adjacent brain, brainstem, cranial nerves or (if extension inferiorly) structures of the superior neck.
They are thought to arise from embryonal crest cells (remember that the base of skull forms via endochondral ossification).
petro-occipital synchondrosis (most common)
other rare sites that have been reported (presumably from metaplasia) include:
Chondrosarcomas of the base of 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 later giving important information of signal intensity and relationship to neural structures.
Only of historical interest, skull x-rays were important in the diagnosis of 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 prior to cross-sectional imaging.
CT with thin triplanar bone algorithm images is important in confirming bony involvement and demonstrating calcification of the tumor, often in characteristic rings and arcs.
T1: low signal
T2: high signal
SWI/GRE: calcifications show low signal
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.
Imaging differential considerations include lesions of the petrous apex. One should also consider specifically:
usually midline projecting posteriorly
lower ADC value than chondrosarcoma5
usually not high T2 signal
calcification pattern usually not chondroid
calcification not as common
epicenter in the nasopharynx
centered on pituitary fossa
also high T2 signal but usually homogeneous
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