Chondrosarcoma of the Skull Base
Both chordomas and chondrosarcomas are rare, slow growing tumors that can arise in the skull base. Chordomas arise from embryonic notochordal remnants along the neuraxis while chondrosarcomas derive from transformed cells that produce cartilage. Historically, chordomas have been divided into three histopathologic subtypes: typical, chondroid and dedifferentiated. Chondroid chordomas can resemble low-grade chondrosarcoma but have a better prognosis than typical chordoma. Dedifferentiated chordomas have a more aggressive profile. Chondrosarcomas, on the other hand, are classified based on the World Health Organization (WHO) grading system (grade 1, 2 and 3). For patients with chondrosarcoma, metastases occur in about 10 percent of cases. Patients with chordoma have a low metastases rate, but very high local recurrence rate.
Clinical presentation depends on location and size of the tumor. Encasement of critical vessels and cranial nerves by a tumor involving the skull base may result in headaches, visual and hearing loss, difficulty with balance, facial numbness and other cranial neuropathies. Nasal congestion and rhinorrhea can be a sign of skull base erosion.
Chordomas and chondrosarcomas are typically heterogeneously bright on T2-weighted MRI images, which can aid with diagnosis. While MRI is superior to CT in delineating the extent of the lesion because it offers anatomic detail and tissue contrast, CT scan is effective for evaluating cortical bone and calcification. Chondrosarcomas typically originate laterally at the petroclival junction, but there are no pathognomonic imaging findings that distinguish chordoma from chondrosarcoma. Biopsy and histologic studies are necessary to confirm the diagnosis, and distinguish chordomas from chondrosarcomas.
Retrospective series support a combined modality approach using maximal surgical resection and radiation therapy. Although chordomas and chondrosarcomas are slow growing, they are locally invasive tumors and there is a high incidence of local recurrence that can result in death due to uncontrolled local disease. Local recurrence has been shown to be significantly associated with an increased risk of metastasis and tumor-related death. Complete resection is the mainstay of therapy for chordomas and chondrosarcomas of the skull base, but is often not feasible because of anatomic constraints to surgical access and proximity to critical neural structures. Postoperative radiotherapy is generally recommended for skull base tumors to reduce risk of local recurrence, given that a wide surgical resection cannot be achieved. Radiation delivery techniques with particles (protons or carbon ions) or photons (stereotactic radiosurgery [SRS] and intensity modulated radiation therapy [IMRT]) have allowed for higher doses of radiation to be delivered to the tumor while sparing surrounding structures.