In this study, we developed a convenient prognostic nomogram based on five independent prognostic variables (ECOG PS, presence/absence of liver metastasis, serum LDH, serum CRP, and serum CA19-9) which could accurately predict survival in patients with unresectable PC undergoing treatment with GnP or FFX. Currently, the AJCC TNM staging system is the most widely used prognostic tool for patients with cancer, including pancreatic cancer. However, this staging system has a few limitations in regard to the analysis of survival. Importantly, it focuses only on tumor characteristics, while the importance also of patient-related factors in determining the disease outcomes in cancer patients has come to be increasingly recognized in recent years [17] Thus, we were prompted to develop a more accurate prognostic tool, and the nomogram that we have developed is an inexpensive tool based on easily determined variables, including both patient and tumor characteristics; it is expected to be a helpful tool for clinicians engaged in the treatment of unresectable PC patients.
ECOG PS is recognized as one of the most important prognostic factors in patients with a variety of cancers [18, 19], and as in the present study, several previous studies have also reported ECOG PS as an independent prognostic factor in patients with PC [20, 21]. We demonstrated herein that the patient prognosis became poorer as the ECOG PS score increased.
Presence of liver metastasis has been reported as an important predictor of survival in patients with various cancers [4, 22], and the MPACT trial showed that the presence of liver metastasis is an important predictor of survival also in patients with PC [4]. Among the distant metastases, including those to the liver, lung and peritoneum, it is unclear why only the presence of liver metastasis was associated with a poor prognosis in our study. Liver metastasis is associated with activation of hepatic stellate cells, which are key components of the hepatic tumor microenvironment and can acquire chemoresistance [23, 24]. Another possible explanation is that patients with liver metastasis could eventually develop jaundice or hepatic coma with increase in the number of metastatic tumors, which would make it difficult to continue with effective systemic chemotherapy, and potentially result in a fatal outcome.
An elevated serum LDH level in PC patients has been recognized as an indicator of tumor aggressiveness, tumor burden, and poor outcome [25], and has also been associated with chemoresistance to several anticancer-drugs, including paclitaxel and gemcitabine [26]. These phenomena might be explained by tumor hypoxia, which promotes the growth of immature and highly permeable blood vessels that drive the abnormal growth and metastatic behavior of PC and facilitate the passage of tumor cells into the circulation [27]. Actually, serum level of LDH significantly increases in hypoxic condition, and serves as an indirect marker of tumor hypoxia [25]. For these reasons, the results of our study, consistent with previous reports, also suggested that an elevated serum LDH level might be associated with a poor prognosis [25, 28].
An elevated serum CRP level has also been demonstrated to be an independent prognostic factor in patients with various types of cancers [29]. Proinflammatory cytokines, such as interleukin-6 (IL-6), interleukin-1, and tumor necrosis factor-alpha, are secreted by monocytes or macrophages under inflammatory conditions and cancer [30]. Serum concentrations of IL-6 and CRP are known to be positively correlated with each other, and recent evidence suggests that IL-6 also affects the rate of cancer progression [31]. Furthermore, there is also evidence to suggest that these inflammatory cytokines play important roles in the genesis of cancer-associated cachexia, which shortens the survival time in patients with advanced PC [32, 33].
Serum CA19-9 is the only biomarker that the National Comprehensive Cancer Network guidelines for PC suggest is useful as a prognostic marker in patients receiving chemotherapy [34]. One prospective study has reported the possible usefulness of serum CA19-9 as a prognostic biomarker in patients with advanced PC [35], and another prospective study showed that a decrease of the serum CA19-9 level during chemotherapy is predictive of a longer survival time in patients with advanced PC [36].
Our prognostic nomogram was created based on the above theoretical background of the above-mentioned independent prognostic factors. Then, we verified the nomogram by determining the values of the C-index and t-AUC, and constructing a calibration plot and Kaplan-Meier curves for the three risk categories. The values of the C-index of the nomogram for 6-, 12-, and 18-month survival were all more than 0.7, indicating a good match between the predicted and actual survival. Calibration and validation using the bootstrapping method also indicated satisfactory performance of the nomogram. In addition, TNP can also be useful for predicting the survival, and Kaplan-Meier curves constructed using tertiles of the TNP showed clear separations among the survival curves. Moreover, our nomogram provided better predictive performance for OS as compared to the AJCC TNM staging system using t-AUC. Notably, our nomogram was not only based on the real-world data of patients treated with GnP or FFX, but also constructed using conventional variables which can easily be obtained at any medical institution in daily practice. Compared with previous nomogram in pancreatic cancer, our nomogram was created using larger cohort of 14 institutions, which could improve the accuracy of the model. In addition, our nomogram can predict prognosis not only at 6-month but also at 12- and 18-month [37]. Thus, this nomogram can be helpful to clinicians for making appropriate clinical decisions in daily practice. Furthermore, another benefit of this prognostic nomogram includes the possibility of selecting patients who are fit for clinical trials.
This study had several limitations. Firstly, it was a non-randomized, retrospective study, which could introduce selection bias, with a smaller number of patients as compared to previous studies [38]. Thus, we were unable to include several patient data, such as weight loss, quality of life, and screening status before the diagnosis, which were not fully documented in the hospital records. The second limitation was that the study lacked cross-validation so as that it would be difficult to generalize our results to other cohorts. However, we developed the nomogram using a spatiotemporally heterogeneous population recruited from multiple centers, which could contribute to improving the validity of this model. Finally, some patients were only clinically diagnosed as having PC, without histological confirmation. These indicate that some patients in the real-world situation have no choice, but to receive systemic chemotherapy without histological evidence for various reasons, including those related to the patients themselves and/or to the facilities that they seek treatment at.
In conclusion, our prognostic nomogram is a convenient and inexpensive tool for accurate prediction of the prognosis in patients with unresectable PC undergoing treatment with GnP or FFX, and will help clinicians in selecting appropriate therapeutic strategies for individualized management.