Background {#Sec1}
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There are approximately one million new cases of gastric cancer (GC) each year worldwide, and half of them occur in Eastern Asia, including China, Japan, and South Korea \[[@CR1]\]. Despite advances in early screening and comprehensive treatment of GC, it remains the third most common cause of cancer-related death in the world \[[@CR2]\]. For advanced GC, a consensus has been reached of radical gastrectomy with D2 lymphadenectomy \[[@CR3]\]. However, there is still controversy over the number of retrieved lymph nodes (RLNs) for accurate pathological staging.

Several studies have reported that RLN count was positively correlated with better overall survival in GC, even in lymph node-negative GC \[[@CR4]--[@CR7]\]. An RLN count of ≥ 16 has been recommended by the 8th edition TNM classification for GC to guarantee the accurate pN stage \[[@CR8]\]. Moreover, Okajima et al. suggested an optimal RLN count of ≥ 25 for nodal staging \[[@CR9]\]. Recently, by stratum analysis of 7620 patients, Deng et al. proposed an optimal RLN count of ≥ 16 for lymph node-negative GC and \> 30 for lymph node-positive GC \[[@CR10]\]. These above studies are all conducted by comparing the RLN count with long-term survival. But little is known about the relationship between the RLN count and short-term complications after radical surgery.

Postoperative complications of GC pose a significant impact on the length of postoperative stay and hospital charges, which further affect the quality of life \[[@CR11]\]. Therefore, investigating the relationship between RLN count and postoperative short-term complications would provide more comprehensive evidence for selecting the appropriate RLN count.

Methods {#Sec2}
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Patients {#Sec3}
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A total of 1487 consecutive GC patients between January 2016 and December 2018 at Nanjing Drum Tower Hospital were retrospectively reviewed. All patients underwent curative (R0) gastrectomy and were histologically confirmed. The exclusion criteria were as follows: (1) multivisceral resection, (2) patients accepting preoperative radiotherapy or chemotherapy, (3) patients with previous stomach surgery, and (4) patients with incomplete clinical data. This study was approved by the Ethics Committee of Nanjing Drum Tower Hospital.

Data collection {#Sec4}
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Data for preoperative characteristics, intraoperative index, and postoperative features were extracted. Preoperative characteristics included age, gender, body mass index (BMI), comorbidities, and laboratory data. The intraoperative index involved the American Society of Anesthesiologists (ASA) grade, surgical approach, type of resection, operation time, and blood loss. Postoperative features included depth of tumor invasion, tumor site, retrieved lymph node count, lymph node metastasis, lymph node ratio (LNR), log odds of positive lymph nodes (LODDS), pTNM stage, Lauren subtype, short-term complications, postoperative stay, and total hospital charges. LNR was defined as the ratio of positive to retrieved lymph nodes. LODDS was calculated by log \[(positive lymph nodes + 0.5)/(total lymph nodes − positive lymph nodes + 0.5)\] \[[@CR12]\]. The postoperative short-term complications occurring in the hospital or within 30 days were collected. All complications were evaluated according to the Clavien-Dindo classification system \[[@CR13]\].

Statistical analysis {#Sec5}
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Statistical analyses were conducted by SPSS 19.0 (Chicago, IL, USA). Continuous variables were shown as means ± SD. Student's *t* test was applied for normally distributed data; Mann-Whitney *U* test was applied for non-normally distributed data. Categorical variable data were presented as numbers and analyzed using the chi-squared test or the Fisher exact test. Univariate and multivariate analyses were performed to analyze the risk factors associated with the postoperative complications or retrieved lymph node count. The optimal cutoff values of LNR and LODDS were determined by receiver operating characteristic (ROC) analysis. All statistical tests were conducted two-sided, and statistical differences were termed as *P* value \< 0.05.

Results {#Sec6}
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Patient characteristics {#Sec7}
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The background characteristics of the patients enrolled in this study were presented in Table [1](#Tab1){ref-type="table"}. There were 1487 GC patients in all, including 1089 (73.2%) men and 398 (26.8%) women. The median age was 60 years with a range from 21 to 96 years. A total of 1411 (94.9%) patients underwent open gastrectomy while 76 (5.1%) underwent laparoscopic surgery. The type of resection was distal gastrectomy in 617 (41.5%) patients, proximal gastrectomy in 163 (11.0%), and total gastrectomy in 707 (47.5%). The mean operation time was 232 min, and the mean intraoperative blood loss was 221 ml. Pathological results were stage I/II/III/IV in 506/368/597/16 patients, respectively. The mean RLN count was 25.1 (range, 2--84), and 864 (58.1%) patients were tested with lymph node metastasis. Overall, postoperative short-term complications occurred in 435 (29.3%) patients. The mean postoperative stay was 12 days, and the mean total hospital charges were 7.5 × 10^4^¥. Table 1Demographic and clinical features of patientsCharacteristics*N* = 1487Age (years)60.4 ± 17.3Gender (*n*) Male1089 Female398BMI (kg/m^2^)23.0 ± 3.5Preoperative comorbidities (*n*) Previous abdominal surgery209 Diabetes mellitus131 Hypertension488Preoperative laboratory data Serum albumin (g/L)39.4 ± 3.3 CRP (g/L)6.0 ± 12.4ASA ≥ 3884Mode of surgical approach (*n*) Laparoscopic76 Open1411Type of resection (*n*) Distal gastrectomy617 Proximal gastrectomy163 Total gastrectomy707Operation time (min)232.3 ± 61.8Blood loss (ml)221.8 ± 204.5Tumor invasion T1--2631 T3--4856Tumor site Cardia/fundus452 Body381 Pylorus/antrum654RLN count25.1 ± 9.1Lymph node metastasis Positive864 Negative623LNR0.17 ± 0.24LODDS− 0.96 ± 0.75pTNM stage I/II/III/IV506/368/597/16Lauren subtype Intestinal620 Diffuse428 Mixed401 Unknown38Postoperative complications Positive435 Negative1052Postoperative stay (days)12.0 ± 8.1Total hospital charges (10^4^¥)7.5 ± 3.5*BMI* body mass index, *CRP* C-reactive protein, *ASA* American Society of Anesthesiologists, *RLNs* retrieved lymph nodes, *LNR* lymph node ratio, *LODDS* log odds of positive lymph nodes

Association between perioperative characteristics and postoperative complications {#Sec8}
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As presented in Table [2](#Tab2){ref-type="table"}, univariate and multivariate analyses indicated that postoperative short-term complications were significantly correlated with age, gender, level of preoperative serum albumin, and operation time. Stratified analyses by type of resection revealed that complications occurred frequently in proximal gastrectomy compared with total gastrectomy, while there was no significant difference between distal gastrectomy and total gastrectomy. No significant association was observed between RLN count and overall postoperative complications. Table 2Univariate and multivariate analyses of characteristics associated with postoperative complicationsCharacteristicsUnivariateMultivariateOR95% CI*P*OR95% CI*P*Age ≥ 701.5811.232--2.029\< 0.0011.5781.219--2.0440.001Gender Male0.7650.597--0.9790.0330.7100.551--0.9160.008 FemaleBMI (kg/m^2^)0.9880.956--1.0200.449Preoperative comorbidities Previous abdominal surgery0.9960.722--1.3740.982 Diabetes mellitus1.1560.787--1.7000.460 Hypertension1.1280.891--1.4280.317Preoperative laboratory data Serum albumin \< 35 g/L1.6601.162--2.3720.0051.5441.068--2.2320.021 CRP ≥ 10 g/L1.3150.892--1.9380.167ASA ≥ 31.0470.834--1.3150.693Mode of surgical approach Laparoscopic0.6840.394--1.1880.178 OpenType of resection0.0670.025 Total gastrectomyReferenceReference Distal gastrectomy1.1830.932--1.5030.1671.2420.972--1.5880.083 Proximal gastrectomy1.5031.047--2.1570.0271.6131.117--2.3290.011Operation time1.0021.001--1.0040.0091.0031.001--1.0050.002Blood loss1.0001.000--1.0010.094Tumor site0.780 Cardia/fundusReference Body1.0540.811--1.3700.693 Pylorus/antrum0.9470.716--1.2520.701Tumor invasion (T3--4)1.2160.968--1.5270.093RLNs0.9910.979--1.0040.165Lymph node metastasis1.0440.832--1.3100.707LNR \> 0.051.2130.969--1.5170.091LODDS \> − 1.11.2190.975--1.5250.083pTNM stage (≥ III)1.0360.826--1.3000.757Lauren subtype0.952 IntestinalReference Diffuse0.8660.427--1.7540.689 Mixed0.9040.442--1.8480.782 Unknown0.9250.452--1.8940.832

Impact of RLN count on postoperative complications {#Sec9}
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Of the 1487 patients, 435 (29.3%) developed complications: 74% (323 of 435) encountered a single complication, and 26% (112 of 435) encountered multiple complications. The details of patients with short-term complications based on the Clavien-Dindo classification are 15.5% for grade I, 9.2% for grade II, 4.0% for grade III, 0.3% for grade IV, and 0.2% for grade V. The rate of major complications (CDC grade ≥ III) was 4.5%. The median RLN count in this study was 24. So, we divided all patients into two groups based on the median RLN count. Univariate analyses showed no significant difference between RLN count and postoperative complications (both overall and stratified by CDC grade) (Table [3](#Tab3){ref-type="table"}). Table 3Univariate analyses of postoperative complications associated with RLN countCharacteristicsAllRLN count*P* value\< 25≥ 25Overall (*n*)4352481870.062Grade I (*n*)231132990.198 Fever \> 37.5 °C1448559 Emesis1568373 Pain301812 Abdominopelvic collection110 Pleural effusion440Grade II (*n*)13778590.366 Blood transfusions603822 Early postoperative bowel obstruction211 Gastroparesis251411 Liver function abnormalities110 Wound infection853 Pneumonia271512 Intra-abdominal infections20128 Urinary tract infection404 Enteritis312 Bacteremia1477Grade III (*n*)5932270.878 Anastomotic leakage23149 Lymphatic leakage835 Pancreatic fistula202 Biliary fistula101 Bleeding853 Abdominopelvic collection110 Pleural effusion954 Intra-abdominal abscess211 Wound disruption330 Delayed wound healing431 Gastroparesis101 Early postoperative bowel obstruction101 Splenic necrosis101Grade IV (*n*)5410.452 Heart failure110 Kidney failure110 Brain infarction101 MODS220Grade V (*n*)3211.000Grade ≥ III (*n*)6738290.562*RLNs* retrieved lymph nodes, *MODS* multiple organ dysfunction syndrome

Factors associated with RLN count {#Sec10}
---------------------------------

We further explored the potential factors associated with RLN count. Univariate analyses revealed that preoperative serum albumin, type of resection, tumor invasion, lymph node metastasis, and pTNM stage were associated with RLN count (*P* \< 0.05; Table [4](#Tab4){ref-type="table"}). Stratification by type of resection showed that RLN count in either distal gastrectomy or proximal gastrectomy was significantly lower than that in total gastrectomy. Multivariate analyses further indicated that type of resection, tumor invasion, and lymph node metastasis were still significantly associated with RLN count (*P* \< 0.05; Table [4](#Tab4){ref-type="table"}). Table 4Univariate and multivariate analyses of factors associated with RLN count ≥ 25CharacteristicsUnivariateMultivariateOR95% CI*P*OR95% CI*P*Age ≥ 700.8730.689--1.1050.259Gender Male1.0430.828--1.3120.723 FemaleBMI (kg/m^2^)0.9720.944--1.0020.064Preoperative comorbidities Previous abdominal surgery1.0300.768--1.3800.844 Diabetes mellitus0.9600.670--1.3750.822 Hypertension0.8520.685--1.0590.148Preoperative laboratory data Serum albumin \< 35 g/L1.4841.048--2.1020.026 CRP ≥ 10 g/L1.1950.827--1.7260.343ASA ≥ 30.8920.725--1.0980.282Mode of surgical approach Laparoscopic1.2820.808--2.0360.292 OpenType of resection\< 0.001\< 0.001 Total gastrectomyReferenceReference Distal gastrectomy0.6490.522--0.807\< 0.0010.7160.572--0.8960.004 Proximal gastrectomy0.3340.231--0.485\< 0.0010.3570.245--0.519\< 0.001Operation time1.0011.000--1.0030.086Blood loss1.0001.000--1.0010.482Tumor site0.304 Cardia/fundusReference Body0.9030.709--1.1480.404 Pylorus/antrum1.1190.869--1.4420.382Tumor invasion (T3--4)1.6131.310--1.987\< 0.0011.2991.010--1.6700.042Lymph node metastasis1.5851.286--1.952\< 0.0011.3041.018--1.6690.035pTNM stage (≥ III)1.5551.263--1.914\< 0.001Lauren subtype0.082 IntestinalReference Diffuse1.0400.536--2.0190.908 Mixed1.3880.709--2.7160.339 Unknown1.3280.677--2.6030.409*BMI* body mass index, *CRP* C-reactive protein, *ASA* American Society of Anesthesiologists, *RLNs* retrieved lymph nodes, *OR* odds ratio, *CI* confidence interval

Discussion {#Sec11}
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Nodal involvement significantly affected the prognosis of GC patients because it is the major root of tumor relapse after surgery \[[@CR14], [@CR15]\]. Thus, standardized lymph node dissection is the basic requirement for curative (R0) gastrectomy. Curative gastrectomy with D2 lymphadenectomy has been considered as the standard fashion for decades in Eastern Asia, especially in Japan \[[@CR16], [@CR17]\]. This procedure has been gradually accepted by Western countries in recent years \[[@CR18], [@CR19]\]. As for the RLN count, the 8th edition TNM classification for GC recommended dissecting at least 16 lymph nodes. Moreover, emerging evidence revealed the positive correlations between RLN count and overall survival of GC patients \[[@CR4], [@CR5], [@CR20]\]. By comparing RLN count to survival time, Okajima et al. suggested an optimal RLN count of ≥ 25 \[[@CR9]\]; Deng et al. proposed an optimal RLN count of ≥ 16 for lymph node-negative GC and \> 30 for lymph node-positive GC by stratum analysis of 7620 patients \[[@CR10]\]; Sano et al. reported that RLN count preferably achieved 30 or more by a multicenter study enrolling 25,411 patients \[[@CR20]\]. Additionally, LNR and LODDS were also reported to be associated with GC prognosis \[[@CR21]--[@CR23]\]. These above studies mainly focused on the relationship between RLN count and long-term prognosis. However, little is known about its effects on postoperative short-term complications.

In this study, we concentrated on the association between RLN count and short-term prognosis. Univariate analyses showed no significant difference between RLN count and postoperative complications (both overall and stratified by CDC grade). Therefore, more lymph nodes were encouraged to be dissected from the perspective of short-term prognosis.

Although curative gastrectomy with D2 lymphadenectomy is considered a pivotal strategy for advanced GC, there are international and institutional differences in the number of RLN count \[[@CR24], [@CR25]\]. Various factors were reported to influence the RLN count, including the confidence and enthusiasm of doctors (both surgeons and pathologists), surgical situation, and innate lymph node count in each patient \[[@CR7], [@CR9]\]. In our study, we concluded that RLN count was related to the type of resection, tumor invasion, and lymph node metastasis. Of note, RLN count was positively correlated with the lymph node metastasis rate, which underlined the importance of RLN count for accurate staging.

Actually, for a thorough pathological examination, RLNs should be individually divided from a complete tissue sample after surgery. Owing to much time and effort was required during this procedure, it has not been widely implemented clinically. Therefore, the examined lymph node count by pathologists might be lower than the dissected lymph node count. Multiple attempts have been conducted to improve the detection rate of lymph nodes \[[@CR26]--[@CR28]\]. Li et al. elucidated that the mean number of RLNs could be significantly elevated by injecting carbon nanoparticles before surgery compared with controls (38.33 vs 28.27) \[[@CR26]\]. Markl and colleagues reported a twofold lymph node pick up rate utilizing methylene blue staining than unstained groups (35 vs 17) \[[@CR27]\]. Several dye materials were also used to increase the number of lymph nodes dissected during surgery, such as fluorescent indocyanine green (ICG) and 5-aminolevulinic acid (5-ALA) \[[@CR29], [@CR30]\].

We acknowledge that this study had some potential limitations. First, it was a retrospective, single-center study, so the results might be flawed because of residual confounding factors. Second, the RLN count was closely related to the quality of surgeons and pathologists. The perioperative variables might differ in different doctors. Therefore, multicenter studies are needed to confirm our results.

Conclusions {#Sec12}
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In conclusion, the current study demonstrated that RLNs\\ count was not associated with postoperative short-term complications following gastrectomy of GC. Therefore, our analysis encouraged more lymph nodes to be dissected for accurate pathologic staging.

BMI

:   Body mass index

CRP

:   C-reactive protein

ASA

:   American Society of Anesthesiologists

RLNs

:   Retrieved lymph nodes

LNR

:   Lymph node ratio

LODDS

:   Log odds of positive lymph nodes

**Publisher's Note**

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Feng Sun, Song Liu and Peng Song contributed equally to this work.

The authors gratefully acknowledge all the investigators for their contributions to the trial.

FS worked on the study design, collected the data, and drafted the manuscript. SL contributed to the study design and data collection. PS was involved in the data collection and extraction. CZ helped collect the data. WG was involved in the study design and data extraction. MW revised the manuscript. All authors have read and approved the final manuscript.

There is no funding supporting this work.

Access to the data and the calculation method can be obtained from the authors by email (medsunfeng\@163.com).

This retrospective study was approved by the ethics committee of Nanjing Drum Tower Hospital, Medical School of Nanjing University. Due to the retrospective nature, the requirement for informed consent was waived by the IRBs from Nanjing Drum Tower Hospital, Medical School of Nanjing University.

Not applicable.

The authors declare that they have no competing interests.
