Lung Cancer 158 (2021) 9–14

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Lung Cancer
journal homepage: www.elsevier.com/locate/lungcan

The impact of pathological lymph node metastasis with lymphatic invasion
on the survival of patients with clinically node-negative non-small cell lungcancer: A multicenter study
Tomo Sato a, Yoshihisa Shimada a, *, Takahiro Mimae b, Yasuhiro Tsutani b, Yoshihiro Miyata b,
Hiroyuki Ito c, Haruhiko Nakayama c, Morihito Okada b, Norihiko Ikeda aa

Department of Thoracic Surgery, Tokyo Medical University, 6-7-1 Nishishinjuku, Shinjuku-ku, Tokyo, 160-0023, Japan
Department of Surgical Oncology, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima, Hiroshima, 734-0037, Japanc
Department of Thoracic Surgery, Kanagawa Cancer Center, 2-3-2 Nakano, Asahi-ward, Yokohama, Kanagawa, 241-8515, Japanb

A R T I C L E I N F O

A B S T R A C T

Keywords:
Lymphatic invasion
Lymph node metastasis
Recurrence
Non-small cell lung cancer
Surgery

Objectives: Lymphatic vessel invasion (Ly) plays a crucial role in pathological lymph node metastasis (pN), andwe consider pN + Ly + disease to indicate a high affinity for the lymphatic system. This study evaluated theoutcomes of patients with clinically node-negative (N0) non-small cell lung cancer (NSCLC) who presented with
pN + with Ly+.
Materials and Methods: This retrospective study evaluated 1775 patients with clinically N0 stage I–III NSCLC who
underwent R0 anatomical resection and systematic lymph node dissection at three institutions between January2010 and December 2017. Patients were classified into four groups according to their pN and Ly statuses.
Univariable and multivariable analyses were performed to identify factors associated with poor recurrence-freesurvival (RFS) and pN + Ly+.
Results: Kaplan-Meier curves revealed that the 5-year RFS rates were 90.8 % for pN–Ly– patients, 55.6 % for
pN–Ly + patients, 63.4 % for pN + Ly– patients, and 41.3 % for pN + Ly + patients. Distant and lymph noderecurrences were more common in the pN + Ly + group, relative to in the pN–Ly– and pN–Ly + groups (bothp < 0.001). Multivariable analyses revealed that pN and Ly statuses were independently associated with RFS,
while the solid tumor size and maximum standardized uptake value were independently associated with
pN + Ly + status. The proportion of pN + Ly + disease was 17.2 % in patients with a solid-part size of > 1.80 cm
and a SUVmax of > 3.55.
Conclusion: pN and Ly statuses were independent prognostic factors in patients with clinically N0 stage I–III
NSCLC. Diseases presenting with pN + with Ly + were associated with increased rates of distant and lymph noderecurrence.

1. Introduction
Lung cancer is the most common cause of cancer-related mortality
worldwide [1]. However, recent advances in early detection and ther­apeutic modalities have failed to improve the cure rate significantly. It is
likely to be related to the fact that even early-stage lung cancer candirectly invade nearby tissues and lead to metastasis via the circulatory
and lymphatic systems. Conventionally, clinically suspected lymph nodemetastasis is evaluated as having the enlarged lymph nodes on chest

computed tomography (CT) and the lesions with the greater maximum
standardized uptake values (SUVmax) on F18-fluorodeoxy
glucose-positron emission tomography/CT (FDG-PET/CT). However,the sensitivity is far from optional [2,3]. Therefore, pathologicalconfirmation of lymph node metastasis is additionally made by endo­
bronchial ultrasound-guided transbronchial needle aspiration biopsiesof suspected lymph nodes for further clinical diagnosis. Even though a
growing number of patients are diagnosed with clinical stage I non-small
cell lung cancer (NSCLC), they have an approximately 15 % risk ofAbbreviations: NSCLC, Non-small cell lung cancer; TNM, Tumor node and metastasis; ND, Node dissection; CT, Computed tomography; FDG-PET, Fluorodeox­
yglucose-positron emission tomography; pN, Pathological lymph node metastasis; Ly, Lymphatic vessel invasion; OS, Overall survival; RFS, Recurrence-freesurvival.
* Corresponding author.
E-mail address: zenkyu@za3.so-net.ne.jp (Y. Shimada).
https://doi.org/10.1016/j.lungcan.2021.05.029
Received 20 March 2021; Received in revised form 7 May 2021; Accepted 24 May 2021
Available online 29 May 2021
0169-5002/© 2021 Elsevier B.V. All rights reserved.

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Lung Cancer 158 (2021) 9–14

pathological lymph node metastasis after curative lung resection [4–6].
The lymphatic system consists of lymph nodes and lymphatic vessels.
Pathological lymph node status (pN) is an important prognostic factor
for patients with resected NSCLC, with 5-year survival rates of 49 % for
pN1 disease, 36 % for pN2 disease, and 20 % for pN3 disease [7]. The
presence of tumor cells inside the lymphatic vessels is known as
lymphatic vessel invasion (Ly), which increases the risk of
micro-metastasis and plays a crucial role in developing lymph nodemetastasis [8–11]. Furthermore, lymphatic vessels can provide a routefor tumor cell dissemination even in the absence of detected Ly or lymph
node metastasis [12].
Several studies have demonstrated that Ly is an independent poor
prognostic factor for NSCLC, regardless of nodal involvement, although
Ly is an optional descriptor scored as LX, L0, and L1 but not consideredin the tumor component of the tumor, node, and metastasis (TNM)
staging system [8–11,13]. Therefore, further studies are needed toclarify the prognostic relevance of Ly status. The present study aimed toevaluate whether pN and Ly statuses were associated with outcomesamong patients who underwent surgical resection for clinically N0
NSCLC.

before the evaluation. The PET/CT images were acquired using a Dis­
covery ST scanner (GE Healthcare, Little Chalfont, UK) or a Biographscanner (Siemens Healthcare, Erlangen, Germany). Low-dose unen­
hanced CT images for attenuation correction and localization of PETidentified lesions were acquired from the head to the pelvic floorusing a standard protocol. Immediately after the CT, PET scanning of theidentical axial field of view was performed for 2–4 min per table posi­tion, based on the patient’s condition and scanner performance. Varia­
tions in SUVmaxs, which radiologists judged at each institution, wereminimized using an anthropomorphic body phantom (NEMA
NU2− 2001; Data Spectrum Corp, Hillsborough, NC, USA). A calibration
factor was created by dividing the actual SUV by the mean SUV in thephantom background, and the calibrated SUVmax value was used tominimize inconsistencies between the institutions.
2.4. Pathological classification
All dissected lymph nodes were microscopically evaluated to identify
any metastatic infiltration. The invasive component’s size was measured
using the pathological slides. Blood vessel invasion and visceral pleuralinvasion were evaluated using hematoxylin and eosin staining andelastica van Gieson staining. The Ly status was evaluated using hema­
toxylin and eosin staining and lymphatic vessel staining (D2− 40) whennecessary. The results for pN and Ly were used to classify patients into
four groups (pN–Ly–, pN + Ly–, pN–Ly+, and pN + Ly+). Histologicalsubtypes were determined according to the World Health Organization
classification system.

2. Materials and methods
2.1. Study population
A total of 3125 patients had undergone complete surgical resectionof lung cancer at three institutions between January 2010 and December
2017. Complete surgical resection was defined based on the Interna­
tional Association for the Study of Lung Cancer (IASLC) Staging Com­mittee criteria [14]. All patients had undergone preoperative CT and
FDG-PET/CT for routine lung cancer staging, which was determinedaccording to the 8th edition of the TNM classification of malignant tu­
mors. The exclusion criteria were patients who were pathologicallydiagnosed with small cell lung cancer, patients with clinical stage 0 or IV
disease, patients with clinical N1–3 disease, patients who underwent
wedge resection, and patients who underwent ND0–1 lymph nodedissection. Thus, the present study evaluated 1775 patients with clini­cally N0 stage I–III NSCLC who underwent R0 anatomical resection(segmentectomy or lobectomy) and systematic lymph node dissection(ND2). The CONSORT flowchart is shown in Supplement Fig. 1. The
retrospective study protocol was approved by each center’s institutional
review board, which waived the requirement for informed consent.

2.5. Statistical analysis
Overall survival (OS) was measured from the date of surgery to thedate of death because of any cause or the last day on which the patient
was known to be alive. Recurrence-free survival (RFS) was measured
from the date of surgery to the first date of recurrence, death because of
any cause, or the last day on which the patient was known to be alive.
The Kaplan-Meier method was used to create OS and RFS curves, whichwere compared using the log-rank test. Univariable and multivariable
analyses were performed to identify factors that were associated withunfavorable OS and RFS outcomes. The univariable analyses were per­formed using Pearson’s chi-squared test for categorical data and using
Student’s t-test or one-way analysis of variance for continuous data. A
backward stepwise selection method was used to build the binominal
logistic regression model. The complementary log-log plot model wasused for the proportional hazards assumption, and variables with athreshold of p < 0.15 were adopted for the backward stepwise model
selection procedure. All tests were two-sided, and differences wereconsidered statistically significant at p-values of <0.05. All analyses
were performed using SPSS statistical software (version 26.0; IBM Corp.,
Armonk, NY, USA).

2.2. Patient follow-up
Patients were examined at 6-month intervals for the first two years
and at 1-year intervals thereafter. Follow-up evaluation included aphysical examination, chest radiography, blood examination, and CT
scan of the chest. Further evaluations, including CT scans of theabdomen, brain MRI, and bone scintigraphy, were performed when
symptoms or signs of recurrence were detected. PET/CT had also been
performed when appropriate. Recurrence was diagnosed by physicalexamination and diagnostic imaging of the lesions. The diagnosis was
histologically confirmed when clinically feasible. The date of recurrence
was defined as the date of histological confirmation or the date ofidentification based on clinic-radiological findings by a physician.

3. Result
The patients’ characteristics are shown in Table 1. The medianfollow-up time for survivors was 31.2 months (range: 0.5–94.4 months).
The histological types were adenocarcinoma for 1456 patients (82.0 %)and non-adenocarcinoma for 319 patients (18.0 %). The pathologicalstages were classified as stage 0–I for 1410 patients (79 %), stage II for
226 patients (13 %), and stage III for 139 patients (8%). The pN and Ly
statuses were pN–Ly– for 1285 patients (72.4 %), pN + Ly– for 82 pa­
tients (4.6 %), pN–Ly + for 259 patients (14.6 %), and pN + Ly + for 149
patients (8.4 %). The Kaplan-Meier curves revealed that the 5-year OS
rates were 93.3 % for pN–Ly–, 84.8 % for pN + Ly–, 82.2 % for pN–Ly+,and 68.7 % for pN + Ly+ (pN-Ly- vs. pN + Ly-, p = .004; pN-Ly- vs. pN-

2.3. Radiographic assessment of the primary tumor
All patients had undergone preoperative CT and FDG-PET/CT.
Tumor sizes were measured for the entire tumor and the solid tumor
component on chest CT. Patients were instructed to fast for ≥ four hbefore intravenous injection of FDG (74–370 MBq) and rested for one h

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Lung Cancer 158 (2021) 9–14

adenocarcinoma were 80.5 % for pN–Ly–, 72.2 % for pN + Ly–, 44.7
% for pN–Ly+, and 34.2 for pN + Ly+ (pN-Ly- vs. pN-Ly+, p < .001;
pN + Ly- vs. pN + Ly+, p = .026; pN-Ly + vs. pN + Ly+, p = .009;
Fig. 1F).
Univariable and multivariable analyses were performed to identify
factors that were associated with poor RFS among patients with clinicalstage I–III NSCLC (Table 2). Older age (p = .001), larger solid-part size
(p < .001), increased tumor SUVmax (p < .001), pT status (p = .001), pN
status (p < .001), Ly status (p = .001), and histology (p < .001) wereindependently associated with poor RFS.
Given that the presence of both pN and Ly (pN + Ly+) was associatedwith the poorest prognosis, we aimed to identify related clinicopatho­
logical factors (Table 3). Multivariable analysis of all patients showedthat older age (p = .002), male (p = .003), smoker (p = .008), larger
solid-part size (p < .001), and increased tumor SUVmax (p < .001) wereindependently associated with the presence pN + Ly+. Among patientswith adenocarcinoma, pN + Ly + was independently associated witholder age (p = .006), male sex (p = .002), smoking status (p = .008),
advanced clinical stage (p = .023), larger solid-part size (p < .001), andincreased tumor SUVmax (p < .001; Supplement Table 1), whereaspN + Ly + was only independently associated with an increased tumor
SUVmax (p = .019) in patients with non-adenocarcinoma (Supplement
Table 2). As tumor SUVmax and solid-part size were the two most sig­
nificant factors associated with pN + Ly + on the multivariable analysis,
we performed receiver operating characteristic curve analyses to eval­uate their prognostic values (Supplement Fig. 2). The area under the
curve (AUC) values and the optimal cutoff values relevant to
pN + Ly + among patients with adenocarcinoma were 0.620 and1.80 cm for solid-part size and 0.721 and 3.55 for tumor SUVmax. The
AUC values and the cutoff values in patients with adenocarcinoma were0.765 and 3.12 for tumor SUVmax and 0.689 and 2.09 cm for soli-partsize, whereas those with non-adenocarcinoma was 0.608 and 9.42 fortumor SUVmax (data now shown). The combination of SUVmax andsolid-part size was associated with pN + Ly + more accurately than theuse of the parameters of either modality alone, and pN + Ly + was
observed for 17.2 % of patients with a solid-part size of > 1.80 cm and atumor SUVmax of > 3.55 (Table 4). pN + Ly + was observed in 17.0 % of
adenocarcinoma patients with a solid-part size of > 2.09 cm and a tumor
SUVmax of > 3.12 (Supplement Table 3), whereas that was shown in
13.2 % of non-adenocarcinoma patients with a tumor SUVmax of > 9.42
(Supplement Table 4).
Fig. 2showed the frequency of postoperative recurrences and recur­
rence patterns. Distant recurrence was more common in thepN + Ly + group than the pN–Ly– and pN–Ly + groups (both p < .001;
Fig. 2A). Furthermore, lymph node recurrence was more frequently
observed in the pN + Ly + group than the pN–Ly– and pN–Ly + groups(both p < .001; Fig. 2B). Among patients with adenocarcinoma, distantrecurrence was more frequently shown in the pN + Ly + group than thepN–Ly– and pN–Ly + groups (both p < .001; Fig. 2C), and lymph noderecurrence was also more common in the pN + Ly + group than thepN–Ly– and pN–Ly + groups (both p < .001; Fig. 2D). The
pN + Ly + group also had the highest rates of distant recurrence(Fig. 2E) and lymph node recurrence in patients with nonadenocarcinoma (Fig. 2F).

Table 1
Patient characteristics.
Variables

Number of patients (%)

Age (mean ± SD)
Sex
Male
FemaleSmoking status
SmokerNever smoker
Surgical procedure
Segmentectomy
Lobectomy
Pneumonectomy
Whole-tumor size, cm (mean ± SD)
Solid-part size, cm (mean ± SD)
SUVmax on tumor
Clinical T factor
T1mi
T1a
T1b
T1c
T2a
T2b
T3
T4
Pathological T factor
Tis
T1mi
T1a
T1b
T1c
T2a
T2b
T3
T4
Histology
Adenocarcinoma
Non-adenocarcinoma
Pathological lymph node metastasis (pN)
pN0
pN1
pN2
Lymphatic invasion (Ly)
Positive (L1)
Negative (L0)Classification according to pN and Ly status
pN-LypN + LypN-Ly+
pN + Ly+

67 ± 9972 (55.8)
803 (45.2)
1017 (57.3)
758 (42.7)
247 (13.9)
1526 (86.1)
2 (0.1)2.7 ± 1.4
2.2 ± 1.5
4.9 ± 5.3
89 (5.0)213 (12.0)
621 (35.0)443 (25.0)
221 (12.4)
85 (4.8)
74 (4.2)
29 (1.6)
8 (0.4)15 (0.8)562 (31.7)
440 (24.8)
24 (1.4)
542 (30.5)
57 (3.2)
113 (6.4)
14 (0.8)1456 (82.0)
319 (18.0)
1544 (87.0)
123 (6.9)
108 (6.1)
1367 (77.0)
408 (23.0)
1285 (72.4)
82 (4.6)
259 (14.6)
149 (8.4)

SD, standard deviation; SUVmax, the maximum standardized uptake value.

Ly+, p < .001; pN + Ly- vs. pN + Ly+, p = .040; pN-Ly + vs. pN + Ly+,p = .003; Fig. 1A). The 5-year RFS rates were 90.8 % for pN–Ly–, 55.6 %for pN + Ly–, 63.4 % for pN–Ly+, and 41.3 % for pN + Ly+ (pN-Ly- vs.
pN + Ly-, p < .001; pN-Ly- vs. pN-Ly+, p < .001; pN + Ly- vs. pN + Ly+,p = .017; pN-Ly + vs. pN + Ly+, p < .001; Fig. 1B).
There was the substantial heterogeneity in the patient characteristics
according to histology. Therefore, survival analysis and binomial lo­
gistic regression analysis were separately performed for patients withadenocarcinoma and non-adenocarcinoma. The 5-year OS rates among
patients with adenocarcinoma were 94.7 % for pN–Ly–, 87.1 % for
pN + Ly–, 87.2 % for pN–Ly+, and 73.1 % for pN + Ly+ (pN-Ly- vs.
pN + Ly-, p = .043; pN-Ly- vs. pN-Ly+, p < .001; pN-Ly + vs. pN + Ly+,p = .002; Fig. 1C). The 5-year RFS rates among patients with adeno­carcinoma were 92.5 % for pN–Ly–, 50.0 % for pN + Ly–, 70.4 % for
pN–Ly+, and 43.2 % for pN + Ly+ (pN-Ly- vs. pN + Ly-, p < .001; pNLy- vs. pN-Ly+, p < .001; pN-Ly + vs. pN + Ly+, p < .001; Fig. 1D).
The 5-year OS rates among patients with non-adenocarcinoma were
84.9 % for pN–Ly–, 81.7 % for pN + Ly–, 68.9 % for pN–Ly+, and 50.7 %for pN + Ly+ (pN-Ly- vs. pN-Ly+, p = .031; pN-Ly- vs. pN + Ly+,p = .001; Fig. 1E). The 5-year RFS rates among patients with non-

4. Discussion
The present study revealed several important findings. First, among
patients with clinically N0 NSCLC, Ly was observed in 20.8 % of ade­
nocarcinomas and 32.6 % of non-adenocarcinomas. Second, in thesecases, pN + Ly (8.0 %) was more common than pN + Ly– (3.8 %) inpatients with adenocarcinoma, although only a slight difference wasobserved among patients with non-adenocarcinoma (10.0 % vs. 8.5 %).
Third, pN and Ly were independent prognostic factors, andpN + Ly + was associated with even poorer survival and recurrence
rates. Among patients with adenocarcinoma, pN + Ly + was associated11

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Lung Cancer 158 (2021) 9–14

Fig. 1. Overall survival and disease-free survival among patients with clinical N0 stage I–III non-small cell lung cancer according to pathological findings regarding
lymph node metastasis and lymphatic vessel invasion. (A) Overall survival and (B) disease-free survival among all patients. (C) Overall survival and (D) disease-freesurvival among patients with adenocarcinoma. (E) Overall survival and (F) disease-free survival among patients with non-adenocarcinoma.
pN, pathological nodal involvement; Ly, lymphatic vessel invasion.
Table 2
Univariable and multivariable analysis for RFS in patients with clinical stage I-III
NSCLC.
Variables
Univariable analysis
Age
Sex (male vs. female)
Smoking status (present vs. absent)
Procedure (lobectomy vs. segmentectomy)
Solid-part size
SUVmax on tumor
Clinical stage (II-III vs. I)
pT (T2-4 vs. Tis-T1)
pN (positive vs. negative)
Ly (positive vs. negative)
Histology (non-adenocarcinoma vs.
adenocarcinoma)
Multivariable analysis
Age
Solid-part size
SUVmax on tumor
pT
pN
Ly
Histology

Hazard ratio(95 % CI)p value

1.03 (1.02–1.05)
1.98 (1.51–2.60)
1.89 (1.43–2.49)
2.46 (1.43–4.22)
1.32(1.24− 1.40)
1.09(1.08− 1.11)
3.52 (2.61–4.72)
4.50 (3.37–6.02)
5.83 (4.51–7.52)
4.05 (3.13–5.21)
2.35 (1.79–3.09)< .001< .001< .001.001< .0011.03 (1.00–1.04)
1.35 (1.37–1.62)
1.05 (1.03–1.09)
1.74 (1.25–2.42)
2.69 (1.94–3.72)
1.68 (1.21–2.33)
2.52 (1.70–3.73).001< .001< .001.001< .001.002< .001

Table 3
Factors associated with lymph node metastasis and lymphatic invasion(pN + Ly+).
Variables
Univariate analysis
Age
Sex (male vs. female)
Smoking status (present vs. absent)< .001

Procedure (lobectomy vs. segmentectomy)< .001< .001< .001< .001< .001

Clinical stage (II-III vs. I)
Solid-part size
SUVmax on tumor
Histology (non-adenocarcinoma vs.
adenocarcinoma)
Multivariate analysis
Age
Sex
Smoking status

RFS, recurrence-free survival; NSCLC, non-small cell lung cancer; CI, confidence
interval; SUVmax, the maximum standardized uptake values; pT, pathological T
factor; pN, pathological lymph node metastasis Ly, lymphatic invasion.

Solid-part size
SUVmax on tumor

with rates of > 40 % for overall recurrence and > 20 % for distantrecurrence. Among patients with non-adenocarcinoma, pN + Ly + was
associated with rates of > 50 % for overall recurrence, > 30 % for distantrecurrence, and > 20 % for lymph node recurrence. Fourth,pN + Ly + was associated with solid-part size, tumor SUVmax, age, sex,
and smoking status. Fifth, the combination of SUVmax and solid-partOdds ratio(95 % CI)p value

1.02(1.00–1.04)
1.60(1.13–2.27)
1.12(0.79–1.57)
2.67(1.34–5.32)
1.72(1.08–2.75)
1.21(1.10–1.34)
1.09(1.07–1.12)
1.28(0.85–1.93)

.024

1.03
(1.01–1.04)
1.97(1.25–3.10)
1.86(1.18–2.94)
1.21(1.10–1.34)
1.10(1.07–1.13)

.002

.009
.530
.005
.022
<.001
<.001
.245

.003
.008
< .001< .001pN, pathological lymph node metastasis Ly, lymphatic invasion; CI, confidence
interval; SUVmax, the maximum standardized uptake values.

size was associated with pN + Ly + more accurately than the parame­ters of either modality alone. pN + Ly + was observed for 17.2 % ofpatients with a solid-part size of > 1.80 cm and a tumor SUVmax of >
3.55.
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Lung Cancer 158 (2021) 9–14

factors that promote lymphangiogenesis, such as vascular endothelial
growth factor C, which can induce metastatic spread [12,17–21].
Furthermore, poor outcomes are associated with high concentrations ofvascular endothelial growth factor C, increased lymphatic vasculardensity, and lymph node metastasis [12,19,21]. However, lymphatic
vessels also provide a therapeutic target for modulating the immune
response to cancer and restricting metastasis [16,22], as immune cells
from the peripheral tissues can navigate to the regional lymph nodes via
the lymphatic network to stimulate the immune response [17,22]. The
present study revealed that pN + Ly + was associated with significantly
increased nodal and distant recurrence rates relative to the other pNLy
groups. Thus, pN + Ly + patients may benefit from a further refinementof their adjuvant treatment and intensive follow-up even though theadministration of adjuvant therapies was not associated with RFS inpatients with pN + Ly + in the current study (data not shown). Intact
and functional tumor lymphatic systems may be useful for deliveringanti-cancer treatment, including immune checkpoint inhibitors, andmolecular targets that block lymphatic vessel-mediated metastasis
might be a promising strategy for treating these tumors.
The prognostic value of Ly reportedly varies between lung adeno­carcinoma and squamous cell carcinoma [23,24]. Kinoshita et al. havereported that Ly was an independent prognostic factor for adenocarci­
noma but not for squamous cell carcinomas [23]. However, Masuda
et al. have reported that moderate and severe Ly independently pre­dicted an unfavorable prognosis for lung squamous cell carcinoma [24].
They demonstrated two histological patterns in lung squamous cell
carcinoma according to the morphological findings of tumor infiltration
to surrounding stromal tissue, and lung squamous cell carcinoma withtumor infiltration was significantly associated with venous invasion,
scirrhous stromal type, and poorer postoperative survival [24]. Thatmight be a potential reason why patients with Ly + non-adenocarcinoma
had a poorer prognosis than Ly + adenocarcinoma. The present study
revealed that Ly predicted a poor prognosis, regardless of histology, andthat pN + Ly + was associated with a higher likelihood of postoperative
metastasis among patients with non-adenocarcinoma. As there should
be significant differences in the biological behaviors of these two tumor
types, further studies are needed to clarify the roles of the lymphatic
system in both tumor types and the potential ability to target lymphatic
vessels as part of a combination treatment strategy.
The present study revealed that pN + Ly + was significantly

Table 4
Predictive performance of significant factors for patients with lymph nodemetastasis and lymphatic invasion (pN + Ly+).
Variables

No. ofpatients

No. of patients with
pN + Ly+ (%)

Solid-part size < 1.80 cm + SUVmax< 3.55
Solid-part size > 1.80 cm
SUVmax > 3.55
Solid-part size > 1.80 cm + SUVmax> 3.55665

13 (2.0)

738
779
407

93 (12.5)
113 (14.5)
70 (17.2)

pN, pathological lymph node metastasis Ly, lymphatic invasion; SUVmax, themaximum standardized uptake values.

Previous studies have indicated that Ly is associated with anincreased risk of lymph node involvement and unfavorable outcomes inmetastasis and long-term survival [8–11,13,15]. Mimae et al. have re­ported that 6.7 % of patients with clinical T1N0M0 adenocarcinoma had
lymph node metastases and that a high Ly + rate (68 %) was observed
among pN + patients [9]. They also demonstrated that Ly only inde­
pendently predicted RFS among pN + patients and that pN + Ly + pa­
tients had a poorer prognosis than pN + Ly– patients [9], which agrees
with our results. The lymphatic capillary vessels have a thin basement
membrane and no supporting muscle that connects the collecting vessels
and lymph nodes where the lymph is filtered [16,17]. Furthermore,
lymphatic vessels near the primary tumor serve as an escape route formetastatic cancer cells, which can travel to the regional lymph nodes,
and Ly accompanied most pN + disease. However, the pathologicaldiagnosis fails to identify Ly in approximately 30 % of cases (in previous
studies and the present study), although this does not definitively indi­cate that there is no Ly [9]. This finding is because slide-based patho­
logical evaluations can be limited by the sampling methodology and theinability to visualize tissue structures in their native three-dimensional
context. Nevertheless, pN + Ly + disease likely indicates substantial
Ly + and pN + involvement. The possible reason that thepN + Ly + population demonstrated a poorer prognosis than othergroups in the current study is involved in the molecular mechanism of
the relationship between tumor lymphatic flows and metastatic capa­
bility. Tumor lymphatic vessels have complex and contradictory func­
tions during cancer progression [16]. For example, tumor cells produce

Fig. 2. Recurrence and recurrence patterns according to lymph node metastasis and lymphatic vessel invasion. (A) The proportions of patients with recurrence
according to recurrence patterns and (B) the presence of lymph node metastasis. (C) The proportions of patients with recurrence of adenocarcinoma according torecurrence patterns and (D) the presence of lymph node metastasis. (E) The proportions of patients with recurrence of non-adenocarcinoma according to recurrence
patterns and (F) the presence of lymph node metastasis.
LN, lymph node; pN, pathological nodal involvement; Ly, lymphatic vessel invasion.
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associated with a solid-part size and tumor SUVmax in all the patientsand those with adenocarcinoma patients, and with tumor SUVmax inpatients with non-adenocarcinoma. To the best of our knowledge, this is
the first study to identify preoperative factors that predicted
pN + Ly + in clinically N0 cases of lung adenocarcinoma and nonadenocarcinoma. In this setting, the combination of PET/CT and highresolution CT provides excellent ability to identify pathological inva­
sive factors, including Ly, and predict prognosis among patients withearly-stage NSCLC [25,26]. Therefore, further studies are needed toclarify parameters that can predict a high affinity for the lymphatic
system and create prognostic subgroups of patients who might benefit
from intensive adjuvant chemotherapy after systematic lymph nodedissection.
This study has several limitations. First, we only retrospectively
selected patients with histologically confirmed NSCLC after surgicalresection. Second, the multicenter design suggests that there is a possi­
bility of inter-center differences in the pathological diagnostic criteria.
Third, the cutoff values for the tumor SUVmax were based on a cali­
bration factor and are only likely applicable to the three participating
institutions.
In conclusion, the present study revealed that pN and Ly statuses
were important prognostic factors, regardless of histology, and thatpN + Ly + was associated with increased rates of distant and lymph noderecurrence among patients with clinically N0 NSCLC. Preoperativeevaluations using high-resolution CT and PET/CT may be useful for
predicting pN + Ly+, which may help guide the selection of surgicalstrategy and intensive adjuvant chemotherapy.

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Data availability
All relevant data are included in the manuscript and the associatedfiles.
Funding
This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.
CRediT authorship contribution statement
Tomo Sato: Data curation, Formal analysis, Writing - original draft.
Yoshihisa Shimada: Conceptualization, Methodology, Validation,
Writing - review & editing. Takahiro Mimae: Supervision. Yasuhiro
Tsutani: Supervision. Yoshihiro Miyata: Supervision. Hiroyuki Ito:
Supervision. Haruhiko Nakayama: Supervision. Morihito Okada:Supervision. Norihiko Ikeda: Supervision.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
The authors thank Editage for English language editing and Mami
Murakami for assistance with the statistical analyses.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.lungcan.2021.05.029.

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