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Introduction
Chordoma is a rare and slow-growing mesenchymal tumor originating from embryonic notochord remnants; the age-adjusted incidence rate is 0.08/100,000; it accounts for 1-4% of all bone malignancies [1,2] . Chordoma preferentially occurs in the axial skeleton; occurrence is most common in the sacrum (50-60%), the spheno-occipital region (25-30%), and cervical (10%) and thoracolumbar vertebrae (5%) [3,4] . Given the poor sensitivity of chordoma toward conventional radiotherapy [5,6] and chemotherapy [7,8] , the mainstay of chordoma treatment is radical surgical excision [9] . However, incomplete resection can render patients vulnerable to relapse [10,11] ; metastases may occur in 5-40% of patients [11-15] .
Despite the advances in diagnostic imaging, surgical techniques, and adjuvant oncological therapy, the prognosis of chordoma is not well known. It has been suggested that the main cause of local recurrence and subsequent death in chordoma is likely insufficient tumor resection [8,11,13] . To date, prognostic factors for chordoma include older age, location in the upper cervical spine, prior resections, dedifferentiated pathology grade, greater extent of tumor invasion, and molecular alterations [11,16-35] . However, predicting survival in patients with chordoma remains challenging, as there are significant variations in the methodology and quality of such studies. Thus far, no study has attempted to review the complete range of relevant prognostic factors in chordoma.
Given the current therapeutic challenge, a better understanding of the factors, especially new predictive markers, that influence spinal chordoma prognosis may be helpful for guiding treatment planning to prolong survival. Our objective was to attempt to define new predictive factors affecting recurrence and survival and to investigate whether it was possible to perform a meta-analysis with the goal of establishing pooled estimates of the effect of specific prognostic factors.
Materials and methods
Data collection and study selection
We performed a comprehensive review of the MEDLINE and Embase databases to identify eligible studies from the inception of the literature up to December 29, 2014. The keyword combinations used were ("chordoma" or "chordomas") and ("spine" or "spinal" or "sacral" or "sacrococcygeal" or "thoracic" or "lumbar" or "cervical") and ("prognosis" or "prognostic factor" or "survival" or "recurrence" or "relapse" or "mortality"). Additional limitations were not applied to these search terms because they would have excluded many valuable studies in which patients with spinal...