br In the present study the clinical information of patients
In the present study, the clinical information of patients with chordoma of the skull base, vertebral column, and pelvis between 1973 and 2013 were collected from the Surveillance, Epidemiology and End Results (SEER) dataset and analyzed. SEER is a US population-based cancer database that contains approximately 28% of the overall US population  and collects clinical information of tumor patients in 18 registries in
the US. The current study aimed to develop validated prognostic nomograms to predict OS and CSS of patients with chordoma.
Materials and Methods
Patient eligibility and variables
The patient information was collected from the SEER database. The SEER*Stat software (Version 8.3.5; NCI, Bethesda, USA) was applied to the patient information acquired from the SEER database.
The inclusion criteria in the present study were as follows:
(2) Positive histological confirmation of chordoma;
(3) site limited to the skull base, vertebral column, and pelvis;
(4) confirmation of histologic type of chordoma;
(5) known cause of death and survival months after diagnosis.
The exclusion criteria in the present study were as follows:
(1) unknown use of radiotherapy, chemotherapy or surgery;
(2) unknown surgical stage;
(3) unknown tumor size
The clinicopathological features including patient age, gender, histology, surgical stage, tumor size, use of surgery, use of radiation, use of chemotherapy, and survival time were incorporated in the present study. The cutoff value of tumor size and age at diagnosis were calculated via X-tile software (Yale University, New Haven, Connecticut, USA). X-tile software was initially developed to determine the best cutoff values for variables in breast malignancy . The optimal cutoff values of chordoma tumor size in the current study were identified as 2.9 and 10.0 cm (Figure 1). The optimal age cutoff values of patients with chordoma were 38.54 and 66 years. The surgical stage in patients with chordoma was further categorized as localized, regional, and distant according to the American Joint Committee on Cancer (AJCC) staging system for bone sarcomas. Tumors confined to the periosteum were defined as localized tumors. And, a tumor that extended beyond the periosteum but without distant JQ 1 was defined as a regional tumor. Surgical resection was assigned to those who underwent surgical resection. However, the type of surgical resection such
as wide, marginal, and intralesional resection could not be obtained from the SEER
database. Radiation was divided into those treated with radiation and those who did
not undergo radiation. Nuances such as radiation type and fractionation could not be
acquired from the SEER database. Chemotherapy was dichotomized into those who
received it and those who did not.
All the chordoma patients identified according to aforementioned inclusion and exclusion criteria (n=558) were randomly divided into the training cohort (n = 372) and the validation cohort (n = 186) to construct and validate the prognostic nomograms. A chi-square test was applied to compare the clinical characteristics between the training and validation cohorts.
Continuous variables and categorical variables were presented as the number of patients with the respective percentages. The cutoff value of tumor size and age at diagnosis were calculated via X-tile software (Yale University, New Haven, Connecticut, USA) based on OS (Figure 1). The prognostic factors (age, gender, primary site, tumor size, histology, surgical stage, use of surgery, use of chemotherapy and use of radiation) were further evaluated via univariate and multivariate Cox proportional hazards regression analyses. Furthermore, hazard ratios and corresponding 95% confidence intervals (CIs) of variables were calculated. The two primary endpoints of the study were OS and CSS. Survival time was calculated from the date of disease diagnosis to the date of death from any disease cause (OS) and death from chordoma (CSS). Missing data were excluded from the present study. Nomograms for 3- and 5-year OS and 3- and 5-year CSS were constructed according to the analysis results of univariate and multivariate Cox proportional hazards regression analyses. And, both internal and external validations of the prognostic nomogram based on the training cohort and validation cohort were performed in the present study. Harrell's concordance index (C-index), a useful evaluation value similar to area under curve of receiver operating characteristic curve , was applied to evaluate the performances of the prognostic nomograms. The C-index ranges from 0.5 to 1.0, with 0.5 indicating the total chance and 1.0 indicating a perfect match . Consistency between the predicted survival and the observed survival were assessed via calibration curves of the nomograms. The chi-square test and univariate and multivariate Cox analyses were performed via SPSS 22.0 (SPSS Inc, Chicago, IL, USA). Rms package in R (version 3.3.1) was applied to construct