To evaluate the prognostic value of the number of retrieved lymph nodes (LNs) and other prognostic factors for patients with distal cholangiocarcinomas, and to determine the optimal retrieved LNs cut-off number.

The Surveillance, Epidemiology and End Results database was used to screen for patients with distal cholangiocarcinoma. Patients with different numbers of retrieved LNs were divided into three groups by the X-tile program. X-tile from Yale University is a useful tool for outcome-based cut-point optimization. The Kaplan-Meier method and Cox regression analysis were utilized for survival analysis.

A total of 449 patients with distal cholangiocarcinoma met the inclusion criteria. The Kaplan-Meier survival analysis for all patients and for N1 patients revealed no significant differences among patients with different retrieved LN counts in terms of overall and cancer-specific survival. In patients with node-negative distal cholangiocarcinoma, patients with four to nine retrieved LNs had a significantly better overall (P = 0.026) and cancer-specific survival (P = 0.039) than others. In the subsequent multivariate analysis, the number of retrieved LNs was evaluated to be independently associated with survival. Additionally, patients with four to nine retrieved LNs had a significantly lower overall mortality risk [hazard ratio (HR) = 0.39; 95% confidence interval (CI): 0.20-0.74] and cancer cause-specific mortality risk (HR = 0.32; 95%CI: 0.15-0.66) than other patients. Additionally, stratified survival analyses showed persistently better overall and cancer-specific survival when retrieving four to nine LNs in patients with any T stage of tumor, a tumor between 20 and 50 mm in diameter, or a poorly differentiated or undifferentiated tumor, and in patients who were ≤ 70-years-old.

The number of retrieved LNs was an important independent prognostic factor for patients with node-negative distal cholangiocarcinoma. Additionally, patients with four to nine retrieved LNs had better overall and cancer-specific survival rates than others, but the reason and mechanism were unclear. This conclusion should be validated in future studies.

Cholangiocarcinoma constitutes approximately 15% of hepatobiliary tumors and 3% of gastrointestinal tumors[1]. According to its anatomic location, cholangiocarcinoma is classified as intrahepatic, perihilar or distal malignancy. Distal cholangiocarcinoma comprise approximately 30% of all cholangiocarcinoma; it is a relatively uncommon disease. The only optimal treatment for distal cholangiocarcinoma is surgical resection, as a result of the insensitivity of cholangiocarcinoma to radiation and chemotherapy[2]. Additionally, complete tumor resection of distal cholangiocarcinoma always relies on pancreaticoduodenectomy, which is a complicated operation with high morbidity and mortality[3]. Hence, the postoperative prognosis of patients with distal cholangiocarcinoma has attracted great interest in several studies[4,5]. Lymph node (LN) status was determined to be a strong predictor for the prognosis of patients with distal cholangiocarcinoma[6]. Patients without LN metastasis had a better prognosis than those with LN involvement. Thus, an adequate number of retrieved LNs is vital to distinguish N0 patients from N1 ones. The appropriate cut-off of retrieved LNs counts should be determined.

Currently, the number of LNs that should be retrieved is still under debate. Several studies evaluated the prognostic value of retrieved LN counts and tried to determine the benchmark number of examined LNs[6-9]. Nevertheless, most of them were designed retrospectively with a small sample size, and cases that met their inclusion criteria comprised both perihilar and distal cholangiocarcinomas. The differences in biological and pathological features, as well as surgical strategies and prognoses, between perihilar and distal cholangiocarcinomas lead to a different influence of retrieving LN counts on survival.

The American Joint Committee on Cancer (AJCC) staging system suggested a different appropriate number of retrieved LNs for perihilar and distal cholangiocarcinomas. For distal cholangiocarcinoma, the number that AJCC suggested was 12. However, this suggestion lacks verification because the retrieved LN counts in most previous studies did not reach 12. Additionally, in the study of Kawai et al[10], patients with more than 12 retrieved LNs only had a moderately better survival rate than patients with a smaller number of retrieved LNs in a univariate analysis, not a multivariate analysis. A subgroup study of Kiriyama et al[11], using a cohort of N0 patients, found that patients with more than 10 retrieved LNs had a better survival. This subgroup analysis was based on a small sample size with a univariate analysis, and the cancers of the involved cases were all stages I and II. Therefore, the appropriate cut-off number of retrieved LNs is still unconfirmed.

Our study was performed to evaluate the interactions between the number of retrieved LNs and the prognosis of patients with distal cholangiocarcinoma; additionally, this study determined the appropriate retrieved LN cut-off number. To obtain a larger sample size, the Surveillance, Epidemiology and End Results (SEER) database was used for the selection of patients with at least one retrieved LN.

Nodal status is a well-studied indicator for the prognosis of patients with distal cholangiocarcinoma. In addition to the stage of LNs, the prognostic value of positive node counts and lymph node ratios has been evaluated in several studies[6,10,11]. While the prognostic value of the number of retrieved LNs is still under debate, the optimal cut-off number of retrieved LNs is also unconfirmed. Several studies of other diseases revealed the difference in retrieved LNs’ influence on survival between N0 and N1 patients[8,14,15]. Nevertheless, for N0 patients, more LNs retrieved significantly improved survival. Therefore, studies of distal cholangiocarcinoma on retrieved LN counts should be performed in a cohort of N0 patients for whom the prognostic value of retrieved LN counts has never been systematically studied.

Our study screened 449 patients with distal cholangiocarcinoma in a population-based database; a total of 226 patients with node-negative distal cholangiocarcinoma were among them. Retrieved LN counts did not show its prognostic value in the whole cohort and N1 patients. However, in patients with node-negative distal cholangiocarcinoma, patients with four to nine retrieved LNs were determined to have a significantly better prognosis than patients with ≤ 3 retrieved LNs in terms of overall and cancer-specific survivals. Additionally, tumor size, grade and T stage of tumor were evaluated to be independent risk factors of cancer-specific survival. Therefore, retrieving at least four LNs would be optimal for patients with node-negative distal cholangiocarcinoma.

More retrieved LNs could promote the accuracy of LNs staging to avoid the under-staging effect, thus to improve survival of patients with distal cholangiocarcinoma. Studies on cholangiocarcinoma demonstrated there were micrometastases in approximately 5% of LNs which were diagnosed as negative nodes[16]. The more LNs that were resected and retrieved meant less micrometastases were left, therefore the survival of patients with more retrieved LNs counts could be improved. Additionally, more retrieved LNs represented adequate surgical, pathological and institutional care[17]. What’s more, anatomic studies determined that more resected LNs could improve the underlying tumor-host interactions and reset the immunological balance to improve survival[18].

The AJCC system suggested the optimal number of retrieved LNs of patients with distal cholangiocarcinoma should be 12. Whereas study of Sasaki et al[6] demonstrated there was no difference between patients with ≥ 12 retrieved LNs and < 12 retrieved LNs in terms of overall survival. A subgroup analysis in the study of Kiriyama et al[11] revealed that patients with stage I or II tumors who had more than 10 LNs retrieved had a better survival rate than others. While 83.2% of patients in their study retrieved more than 12 LNs, the number of patients with < 10 retrieved LNs was very small (n = 22), and their results were based on a univariate analysis. Therefore, selection bias should be kept in mind when interpreting their results. The fact that retrieving more than 10 or 12 LNs is an indicator of better prognosis is still disputable.

The present study denoted that retrieving more than nine LNs did not indicate a better prognosis in patients with node-negative distal cholangiocarcinoma, but an increase in terms of all-cause mortality risk and cancer cause-specific mortality risk was observed compared with retrieving four to nine LNs. For patients with distal cholangiocarcinoma, retrieving too many LNs did not obtain better outcomes. This result was contrary to the prevailing dogma that a better prognosis was always associated with higher retrieved LN counts.

There were several hypotheses for the reason why more retrieved LNs represented a worse prognosis. Necrosis represented an aggressive biology of tumor and a decreased survival rate; it had a close association with LN hyperplasia. LN hyperplasia always resulted in increases in the size and number of detectable LNs; therefore, more retrieved LNs (detectable LNs) were related to a worse prognosis[19-21]. The other hypothesis was that there might be a difference at the molecular level between tumors with more and less detectable LNs. Tumors with more detectable LNs, i.e. more retrieved LNs, might belong to another subset of distal cholangiocarcinoma that acts biologically more aggressively[19]. Additionally, routine histologic techniques for retrieving LNs may ignore the micrometastases, leading to the under-staging of LNs. Hence, without the application of immunohistochemical techniques that was determined to increase the detection rate of micrometastases, more retrieved LNs could not promote the accuracy of LN staging or improve patient survival. And, we wanted to know if more retrieved LNs reflected extended lymphadenectomy that may result in increasing postoperative complications. However, because of the limitation of the SEER database, we could not compare the background data in the > 9 retrieved LNs group with that in four to nine retrieved LN groups.

There were several limitations in our study. First, although a population-based database was utilized to screen patients, the total number of patients involved in our study was still not large enough compared with congener studies for other diseases. Second, information for adjuvant chemotherapy and radiotherapy in survival analysis did not contain the details of the protocols, and the SEER database did not provide data. Third, disease-free survival could not be calculated because of the lack of information about local recurrence in the SEER. Fourth, patients who received preoperative radiation were excluded. There might be patients who received radiation in some other centers that were not recorded in the SEER, so the down-staging effect of radiation could not be entirely ruled out. Fifth, the number of lymph nodes retrieved may depend upon the type of surgical procedures. And, the lymph nodes distant from the lesion (for example, nodes from Whipple’s procedure) may not have the same predicting value as these from local or limited resection specimen. However, the detailed operation methods were unknown because of the limitation of the SEER, i.e. we could not know how many patients underwent pancreatoduodenectomy or segmental bile duct resection. Sixth, the number of lymph nodes retrieved may rely on lymph node dissection skill in each individual institution (grossing by resident vs practicing pathologists or pathologist assistant). But, the SEER database only provided the information of the region where the patients were from, and the classes of hospital were unknown. Therefore, such institution bias should be taken into consideration when interpreting our results. Seventh, we could only compare survival among the groups of patients with four to nine lymph nodes and others based upon pathological stage and not clinical stage due to the limitation of SEER (the SEER database only provided the information of pathological stage for patients with resectable distal cholangiocarcinomas). Eighth, the AJCC staging system utilized in the present study was the 6^th edition, which was not the most commonly used one nowadays (due to limitations of SEER). Finally, we could not get the data referring to the surgical margin status in SEER; surgical margin status was an important prognostic factor in patients with resected distal cholangiocarcinoma.

In conclusion, the number of retrieved LNs did not show its prognostic value in the whole group of patients (a mixture of N0 and N1 patients) and N1 patients. However, the number of retrieved LNs was an independent prognostic factor of overall and cancer-specific survival for patients with node-negative distal cholangiocarcinoma. Patients with four to nine retrieved LNs had better overall and cancer-specific survival rates than others, but the reason and mechanism were unclear. The future studies should consider more operation- and adjuvant therapy-related parameters into their analysis to evaluate our results.