Esophageal cancer is currently ranked 7^th and 6^th worldwide in incidence and mortality respectively (1). Esophageal cancer has two major histologic types: esophageal adenocarcinoma (EAC) and esophageal squamous cell carcinoma (ESCC) (2), which account for 90% of patients in China (3). Esophageal squamous cell carcinoma has a poor prognosis, with overall survival of about 20% at 5 years (4). Surgical ESCC patients have a 40%-59% 5-year survival (4, 5).

Current treatment of esophageal cancer is tailored to TNM stage in order to ensure the best clinical outcomes (6). However, the prognosis is heterogeneous even in ESCC patients of identical stage (7). Evaluation and integration of additional significant prognostic factors, such as serum tumor marker levels, may yield a more accurate estimate of ESCC prognosis (8).

Tumor markers are substances released by cancer cells or produced by other cells of the body in response to a malignant tumor or benign condition (9). Tumor markers allow convenient non-invasive assessment of prognosis. Associations between certain perioperative serum tumor markers and prognosis in certain cancers were consistently confirmed. Thus, such markers are used frequently in clinical practice as for example, CEA (Carcinoembryonic antigen) in colon tumors (10, 11), CA19-9 (Carbohydrate antigen 19-9) in pancreatic cancer (12) and a combination of AFP (Alpha-fetoprotein) and HCG (Human chorionic gonadotropin) in testicular germ-cell tumors (13). Regarding current evidence on relationships between tumor markers and prognosis of ESCC, no consensus was reached. Some studies have shown that elevated tumor markers portend a poorer prognosis (14–17), while other markers have shown no significant relationships with prognosis (18–20). The limitations of most previous studies have largely restricted their clinical application in stratification of prognosis. Of note, almost all prior studies of ESCC have focused on preoperative tumor markers and lacked real-world postoperative evaluation. Moreover, there were limitations due to a series of flaws in study design including 1) small sample size, 2) inadequate control for confounding factors, and 3) short-term follow-up.

In this study, based on a large real-world clinical cohort with up to 7 years of follow-up, we aimed to systematically evaluate associations of common tumor markers and overall survival of ESCC patients undergoing radical resection, both preoperatively and postoperatively, and to provide compelling evidence to promote proper clinical application of tumor markers in ESCC surgical patients.

The TNM staging system has been accepted as the most important prognostic predictor in ESCC patients. Nevertheless, the prognosis of ESCC still may be heterogeneous among ESCC patients with identical stage tumors (7, 8). Tumor markers with independent prognostic significance may be valuable as adjuncts for the TNM staging system. To date, tumor markers have been routinely used to help evaluate prognosis in clinical practice of certain cancers (23). However, due to lack of compelling evidence regarding the clinical significance of these markers, the pattern of current tumor marker use for ESCC is largely dependent on the personal experience of the clinicians. In an attempt to more exactly predict patient outcomes and promote the clinical utility of tumor markers in ESCC, we for the first time, systematically evaluated the associations of pre- and post-operative seropositivity of eight tumor markers and overall survival in a real-world setting based on the 2789 resectable ESCC patients with up to 7 years of follow-up. This demonstrates that postoperative SCC and CEA are independent prognostic factors for survival and may be used to inform clinical decisions.

SCC is a subfraction of the tumor antigen TA-4, which was originally purified from the human uterine cervix by Kato and Torigoe (24). An amino acid homology search for this molecule showed SCC is a member of the serine protease inhibitor family (25), which serves to modulate angiogenesis and support tumor growth and progression (26). Previous studies have shown that SCC plays a role in the prevention of TNF--induced cell death (27). Our results corroborate the correlation of postoperative serum SCC with overall survival and may be of use in distinguishing high-risk groups in ESCC patients. Few studies have focused on the prognostic value of postoperative SCC in ESCC. Despite the small sample size, findings of H Shimada et al. were similar to ours (17). Evaluation of other cancers has also demonstrated that postoperative SCC has a significant role in determining prognosis. For example, in cervical squamous cell carcinoma, elevated posttreatment serum SCC has been considered to be a risk factor for cancer recurrence after complete remission (28), and for survival using SCC cutoffs ranging from 1.5 to 2.0 ng/mL (29).

CEA was first isolated from human colon cancer tissue in 1965 by Gold and Freedman (30, 31). The gene sequencing suggests that CEA may act as an adhesion molecule, and CEA does seem to play a role in invasion and metastasis (32). Although the prognostic significance of postoperative CEA has not previously been evaluated in ESCC, studies of colon cancer have repeatedly demonstrated that decreasing postoperative serum CEA has independent predictive value for better overall survival (11, 33, 34).

In contrast with postoperative seropositivity, preoperative seropositivity of any of the eight targeted tumor markers showed no significant relationship with patients’ survival. Notably, although univariable analysis showed preoperative SCC has a significant relationship with prognosis, the multivariable analysis yielded only negative results, in agreement with a number of previous studies (19, 20, 35, 36). There have also been findings to the contrary (14, 16, 17), but the majority of these studies had methodological issues such as inadequate adjustment for confounding factors which may have caused apparent associations of preoperative SCC concentration and prognosis which were in fact spurious. Most previous studies have shown that the preoperative level of CEA is not a significant independent predictor of prognosis (17–19, 37). Although Yang et al. (20) reported that preoperative CEA may be a prognostic indicator for ESCC, it is very likely their findings were confounded as only the log-rank test was employed instead of multivariable modeling analysis.

The discrepancy between pre- and post-operative tumor markers’ prognostic significance may be explained partially by tumor burden. It has been proposed that the clearance rate of tumor markers after treatment may have prognostic significance (38). If a tumor is completely extirpated by surgery, there is no source of tumor marker production and ideal serum tumor markers, which reflect the total amount of cancer in the body would be negative (39). Positive postoperative results for serum tumor markers may reflect the presence of occult residual diseases and have potential to bring about subsequent biologic disorders. As seen in colon cancer, elevated postoperative CEA rather than elevated preoperative CEA normalized after resection, and this was an indicator of poor prognosis (11). With regard to prognosis for ESCC, the postoperative serum tumor markers seem to provide more tumor burden-related information than the preoperative markers.

To evaluate the temporal trend and determine the optimal time window for prognostic analysis, we plotted serum concentration-time curves for SCC and CEA based on population-level data. Serum levels of SCC and CEA declined after tumor resection, which was consistent with the individual disappearance curves of tumor markers as seen elsewhere (40). Multiple studies have reported a logarithmic decrease in the disappearance curves of tumor markers after tumor resection (38, 40, 41). For SCC and CEA, decreases in concentration curves (0-14 days) mirroring the clearance of serum tumor markers (41) may reflect a decrease in cancer burden. Based on the temporal trends, our study performed prognostic analysis using a stable period (14-180 days) and excluding a decreasing time interval (0-14 days), to ensure the stability of the association between the postoperative serum tumor markers and overall survival.

The limited evidence regarding prognostic relevance of tumor markers poses challenges for applying tumor markers in routine clinical examination for ESCC patients. Tumor markers have not been implemented in routine clinical management even in some tertiary hospitals in China. In this study, owing to the availability of large-scale, real-world clinical EMR data, we 1) had relatively adequate statistical power to assess targeted markers both pre- and post-operatively (post-hoc test: >95% power for SCC and CEA); 2) and we were able to adjust up to eleven conventional confounding factors which ensured the establishment of an independent association between serum tumor markers and prognosis in ESCC patients. Consequently, the compelling evidence provided by our study may maximize the likelihood of the routine use of tumor markers in clinical practice. Also, our study provides implications for patient management. First, early intervention becomes paramount for patients with postoperative seropositivity of SCC and CEA, allowing clinicians to schedule more frequent follow-ups or initiate other therapies. Additionally, the postoperative serum level of SCC and CEA can be instrumental in risk assessment, helping to stratify patients for graded surveillance, and ensuring high-risk patients receive the necessary support and services. Moreover, our study adopted the clinically recommended cutoffs for SCC (1.5 ng/mL) and CEA (5 ng/mL) to define seropositivity, which may further facilitate their clinical usage.

Our study has limitations. This is a historical cohort study that relied exclusively on single-institutional data. Validation in other populations and geographic areas is needed. Despite the large sample size of the whole cohort, some subgroups of the eight targeted tumor markers were still relatively small due to their infrequent utilization in clinical practice, and this may have limited the statistical power in subgroup analysis.

In summary, postoperative serum SCC and CEA are independent factors for prognosis in ESCC surgical patients. We recommend routine use of SCC and CEA to inform clinical decisions directly. Furthermore, they are also candidates for use as predictors in statistical models that predict individualized survival for ESCC surgical patients (8).

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by the Institutional Review Board of the Peking University School of Oncology, China (2018KT68). The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent for participation was not required from the participants or the participants’ legal guardians/next of kin in accordance with the national legislation and institutional requirements.

YiH: Data curation, Formal Analysis, Methodology, Software, Visualization, Writing – original draft. FL: Data curation, Formal Analysis, Methodology, Visualization, Writing – review & editing. RX: Resources, Writing – review & editing. FZ: Resources, Writing – review & editing. WY: Data curation, Methodology, Writing – review & editing. YuH: Data curation, Methodology, Writing – review & editing. ZL: Methodology, Formal Analysis, Writing – review & editing. BH: Data curation, Writing – review & editing. LL: Methodology, Formal Analysis, Writing – review & editing. LZ: Data curation, Writing – review & editing. ML: Methodology, Writing – review & editing. YP: Methodology, Writing – review & editing. YL: Methodology, Writing – review & editing. ZH: Conceptualization, Methodology, Resources, Supervision, Writing – review & editing. YK: Conceptualization, Supervision, Writing – review & editing, Methodology.