Patient Population
Patients with advanced and metastatic PC from January 2010 to December 2018 were identified from the Chinese People’s Liberation Army (PLA) General Hospital. Patients who underwent first-line chemotherapy were enrolled in this study. We would perform the follow-up evaluations every 6 months by telephone or medical records.
The inclusion criteria were as follows: (1) diagnosis confirmed by histopathology or cytological; (2) Karnofsky performance status (KPS)score ≥70; (3) without first-line chemotherapy previously; (3) finishing computed tomography (CT) evaluable before starting first-line chemotherapy;(4)collecting complete baseline information before first-line chemotherapy; (5) availability of evaluation with CT scans at 6-week (before the 3rd cycle chemotherapy); (6) 6-week laboratory factors obtained before the 3rd cycle chemotherapy; (7) having explicit terminal status.
323 patients were selected into the baseline model according to these criteria. And among these patients, 233 patients who had initial tumor responses to first-line chemotherapy were enrolled into the chemotherapy response-based model, because they had been evaluated with CT scans before the third cycle chemotherapy. We chose the time point of 6 weeks from the beginning of first-line chemotherapy because the chemotherapy regimens were given with 3-week cycle and the evaluation was accomplished before the start of 3rd cycle chemotherapy. In our study, there are 134 patients were treated with nab-paclitaxel plus S-1 regimen; it’s a clinical trial (NPSPAC) which was a single-arm, single-center and phase II trial in our hospital (https://clinicaltrials.gov/ number, NCT02124317).
Clinicopathological Variables
Demographic and clinicopathological variables were collected: gender, age, BMI, KPS, smoke, alcohol, diabetes, jaundice, ascites, metastatic sites, the total number of
metastasized organs, primary tumor location. The laboratory values were collected as following: white blood cell (WBC), platelet (PLT), neutrophil (N), albumin (Alb), lactate dehydrogenase (LDH), total bilirubin (TB), serum carcinoembryonic antigen (CEA), serum carbohydrate antigen 19-9 (CA 19-9). And 6-week serum albumin, 6-week lactate dehydrogenase (LDH), 6-week total bilirubin concentrations, 6-week carcinoembryonic antigen (CEA), 6-week carbohydrate antigen 19-9 (CA 19-9). Tumor lesion was assessed by CT scans before first cycle chemotherapy and after two cycles of chemotherapy. Chemotherapy efficacy was evaluated by Response Evaluation Criteria In Solid Tumors (RECIST version 1.0), and the patients were classified into progressive disease (PD) or non-progressive disease (non-PD), according to tumor responses. The changes in tumor markers levels at 6-week were defined as value measured at 6-week subtraction the baseline value and divided by the baseline value. For example, {change in LDH value at 6-week = ([LDH at week-6] -[LDH at baseline]) / (LDH at baseline)}. According to the value of the results, the patients were divided into two categories: the value with zero or more would be defined as no change and increase group and the value less than zero was defined as decrease group.
Statistical Analysis
Continuous predictors were expressed by medians with interquartile ranges (IQRs), and categorical predictors were described by numbers and proportions. Continuous
variables (i.e. age) were categorized into two groups according to their median levels. The correlation analysis of variables was evaluated by the correlation matrix. And multicollinearity diagnosis was performed by Variance inflation factor (VIF) using R version 3.6.3 software (car package in R, http://www.r-project.org/). The value of VIF >10 means strong collinearity problems between variables. Otherwise, if the value of VIF was from 0 to 10, it means inexistence of collinearity problems between variables. Overall survival (OS) was the defined as the time interval from the date of beginning of first cycle chemotherapy to the date of death. The OS was converted into binary variable based on the overall survival time of patients whether exceeds six months. The relationships between variables and 6-month survival were assessed by univariate logistic regression analysis. The variables associated with 6-month survival at a significant level would be incorporated in multivariate logistic regression analysis. We used the backward stepwise selection with the Akaike information criterion (AIC) to choose variables. Based on the multivariate analysis, selected variables with statistics significantly were enrolled into the nomograms to predict the probability of 6-month survival using rms package in R. According to the proportional, the nomogram turned regression coefficient in multivariate logistic regression into 0- to 100-point scale. The point of each independent variable could be added together and derive total points to predicted probabilities.
The predictive model performance was evaluated by concordance index (C index) and calibration. The C-index is equal to the area under the receiver operating characteristic (ROC) curve. The closer the value of C-index was to 1, the better separation of patients with different outcomes. Calibration with 1000 bootstrap samples to reduce the overfitting was estimated by calibration plot. In a perfect calibrated model,
the prediction curve could coincide with the 45-degree diagonal line. The accuracy of the predictive model was evaluated by receiver operating characteristic (ROC) curve. We used Hosmer–Lemeshow (H–L) x2 statistic to perform the model goodness-of-fit test, non-significant statistic means that the model fitted very well. The ROC analysis was performed by MedCalc15.2.2 with DeLong et al. method. And the net benefit of the model was assessed by decision curve analysis (DCA). We calculated total scores of patients predicted by the nomograms and used the X-tile version 3.6.1 (Yale University, CT, USA) to find the optimal cut-off value to stratify patients and performed Kaplan-Meier survival analysis (SPSS26.0). All statistical analyses were performed by R (version 3.6.3), SPSS26.0, X-tile (version 3.6.1) and MedCalc15.2.2. P<0.05 was considered statistically significant.