Introduction

Lung cancer is the leading cause of cancer death worldwide and late diagnosis at an advanced stage is a fundamental obstacle to improving lung cancer outcomes. An obvious relationship between TNM stage and survival rate of patients has been shown in some studies [1]. Thus, accurate staging of non-small cell lung cancer (NSCLC) provides important prognostic information and determines the best treatment approach [2]. In particular, metastasis to N2 Lymph nodes is considered to be crucial for operability, while patients without lymph node metastases or only intrapulmonary or hilar lymph node can receive surgery [3]. Neoadjuvant chemotherapy with surgery or concurrent or sequential chemoradiotherapy are legitimate choices for patients with positive N2 lymph nodes [4].

Even though contrast enhanced CT has been the most common imaging modality for TNM staging, it has limitations in evaluating lymph node status because the prediction of positive lymph nodes on CT is based on size criteria alone [5]. ¹⁸F-fluorodeoxyglucose positron emission tomography (¹⁸F-FDG PET) is a functional imaging modality that is based on the increased glucose metabolism of malignant cells [6]. Since the introduction of integrated PET/CT, functional information and morphological information can be combined on TNM staging with ease. Although some previous studies have indicated that the integrated PET/CT are more effective for detecting HMLN metastasis, results regarding the extent of its benefits have been inconsistent [7]. Further, the incidence of occult lymph node metastasis in NSCLC patients showing negative uptake by FDG-PET/CT is 7-16% [8-10], and false positive findings from inflammatory or granulomatous lesions are still problematic on PET/CT.

The purpose of this study was to assess the diagnostic accuracy of integrated PET/CT in HMLN staging of NSCLC, and to evaluate the characteristics of false negative and false positive lymph nodes to improve specificity and sensitivity.

Materials and Methods

Results

Evaluation of HMLN status by pathological examination and PET/CT

The characteristics of the enrolled patients are shown in Table 1. Of these 219 patients, 175 (79.9%) had adenocarcinoma, 32 (14.6%) had squamous cell carcinoma, and 12 had other types including 5 adenosquamous carcinomas, 2 large cell carcinomas, 2 carcinoids, and 2 mucoepidermoid carcinomas, 1 hemangioendothelioma. Positive HMLN was found in 30.1% (66/219) of those patients. A total of 1173 hilar and mediastinal lymph stations (2959 lymph nodes) were dissected and 140 (11.9%) stations showed positive.

Characteristics	Distribution(n)
Sex
Male/Female	120/99
Age
Mean±SD/Range	60.7±11.1/26-86
Smoking status
Smoker/Never Smoker	106/113
History of lung disease or diabetes
Yes/Never	30/189
Location of the tumor
Central/Non-central	48/171
Lobar distribution of the tumor
RUL/RML/RLL/LUL/LLL*	82/15/37/52/33
Histopathological type
Adenocarcinoma/Squamous cell carcinoma/Other types	175/32/12
Pleural invasion
Yes/No	104/115
Pathological T stage
T1a/T1b/T2a/T2b/T3/T4	43/40/113/5/11/7
Lymph node metastasis
pN0/pN1/pN2	145/25/49
Pathological stage after surgery
Ⅰa/Ⅰb/Ⅱa/Ⅱb/Ⅲa/Ⅲb/Ⅳ	67/62/26/7/42/3/12
Tumor size (cm)
Median/Mean±SD/Range	2.6/2.8±1.4/0.5-11.0
SUVmax of primary tumor
Median/Mean±SD/Range	5.4/6.2±4.1/0-24.2

Table 1. Characteristics of patients and tumors (n=219).

*RUL: right upper lobar; RML: right middle lobar; RLL: right lower lobar; LUL: left upper lobar; LLL: left lower lobar

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All evaluating indicators of PET/CT examination were analyzed in Table 2. The sensitivity, specificity, PPV, NPV, and accuracy of PET/ CT in detecting HMLN metastases were 74.2% (49/66), 73.2% (112/153), 54.4% (49/90), 86.8% (112/129), and 73.5% (161/219). The ROC curve based on the SUVmax of lymph nodes is shown in Figure 1. The SUVmax of nodes had an AUC of 0.791 (95% CI 0.723-0.860) with a cut-off SUVmax of 2.34. If nodal uptake with SUVmax >2.34 were interpreted as positive, the sensitivity, specificity, PPV, NPV, and accuracy of PET/ CT were 80.3% (53/66), 70.6% (108/153), 54.4% (53/98), 86.8% (108/121), and 73.5% (161/219).

	SUVmax ≦ 2.5	SUVmax >2.5
Pathological negative	112 (TN)	41 (FP)
Pathological positive	17 (FN)	49 (TP)

Table 2. Histopathological findings and PET/CT results of HMLN.

TN, true negatives; FP, false positives; FN, false negatives; TP, true positives.

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Figure 1. ROC curve for SUVmax of HLMN.

The curve had an AUC of 0.791 (95% CI 0.723-0.860) with a cut-off SUVmax of 2.34.

https://doi.org/10.1371/journal.pone.0078552.g001

Discussion

The determination of HMLN metastasis is an important part of staging in patients with NSCLC. Stereotactic body radiation therapy (SBRT) is not recommended for small peripheral lung cancer patients with hilar lymph node involved (N1) [11]. When contralateral mediastinal lymph nodes (N3) or more than one ipsilateral mediastinal lymph nodes are involved, primary surgery is not recommended and such patients are commonly treated with chemotherapy and radiotherapy. In recent years, PET/CT is widely used for the staging of NSCLC [12], which is expected to be a very promising optimization tool for lymph nodal staging. In a meta-analysis [7], the pooled sensitivity and specificity on a per-patient analysis were 71.9% and 89.8%, respectively. It suggested PET/CT had more specificity but less sensitivity for lymph nodal staging. So we undertook the present study to improve the sensitivity and specificity of integrate PET/CT.

The results of our study can be summarized as follows: The SUVmax of HMLN is a predictor of malignancy. The ROC curve identified optimal cut-off SUVmax of 2.34 for HMLN metastasis in our cohort. The false negatives and false positives accounted for 13.2% (17/129) patients and 45.6% (41/90), respectively.

In previous study [13], as in this study, a traditional value for the SUVmax of 2.5 was chosen. HMLN with a value of 2.5 or greater was considered positive. Integrated PET/CT provides somewhat unsatisfied sensitivity (74.2%), specificity (73.2%), and reasonably low accuracy (73.5%) for nodal staging of NSCLC (on a per-patient basis). In order to maximize the accuracy of integrated PET/CT we generate ROC curve. The curve identifies the optimal cut-off SUVmax value that maximizes the sensitivity and specificity and consequently the accuracy. The value found was 2.34. When a SUVmax of 2.34 or greater is used to label any lymph node as positive on PET/CT and less than 2.34 as negative, the sensitivity of all patients is 80.3% at the expense of specificity. The accuracy in SUVmax of 2.34 is remarkably similar to the accuracy reported in SUVmax of 2.5. In ROC curve analysis, it is obvious that with increasing of cut-off value of SUVmax, the specificity become higher and higher at the expense of sensitivity. When SUVmax reach at value of 3.85 or greater, the specificity is at least 90.0%, while the sensitivity is only 47.0%.

Gerfolio [10,14] has shown the clinical importance and relevance of the SUVmax of a primary non-small cell lung tumor. It provides information on a tumor’s biologic aggressiveness, key pathologic features, and its potential to spread. This study illustrates the importance of the SUVmax of lymph nodes. The SUVmax of the primary tumor along with each lymph node that is hypermetabolic should be provided by nuclear radiologist in the text of every PET/CT report.

Seventeen of one hundred twenty nine patients had falsely negative nodes from integrated PET/CT, who had microscopic positive disease. 11 of 17 patients had N2 disease, and 8 of 11 patients had more than one N2 lymph node station involved. This finding was concordant with comparable previous studies. Al-Sarraf [8] reported the incidence of occult N2 disease in similar patients as 16%, while Cerfolio [15] reported a 14% incidence. When they limited the analysis to clinical stage i patients, the incidence decreased to 7% [9].

Risk factors for occult N2 disease reported in those previous studies were as follows: adenocarcinoma, tumors located in right upper lobe, larger tumor size, a high SUVmax of the primary tumor, centrally located tumors, positive N1 nodes on PET and poorly differentiated histology. A limitation of these studies was the lack of histological examination of involved lymph nodes. It is similar to previous studies that a high SUVmax (>4.0) of the primary tumor was also identified as risk factor for false negatives in the present study.

In our study, visceral pleural invasion (VPI) was identified as a risk factor for occult lymph node metastasis in both univariate and multivariate analysis. One possible reason [16] is that the visceral pleura is very rich in lymphatic vessels, with an intercommunicating “network” arranged over the lung surface and penetrating into the lung parenchyma to join the bronchial lymph vessels with drainage to various hilar lymph nodes. The larger lymphatic vessels in the visceral pleura have one-way valves which also direct flow toward the hilum of the lung. VPI was significantly associated with more extensive HMLN involvement. The present findings support the suggestion by Shimizu et al. that there is a possible VPI tumor cell pathway through the subpleural lymphatics, hilar lymph nodes, and into the mediastinal lymph nodes [17].

Interestingly, we found concurrent lung disease or diabetics and non-adenocarcinoma were also risk factors for occult nodal metastasis, although Kubota [18] insisted on that the accuracy of PET/CT were not affected as long as the blood glucose level in diabetic patients was controlled at less than 140mg/dL and Kanzaki [19] thought occult lymph node metastasis more likely to occur in patients of lung adenocarcinoma. Obviously, it showed a distinct trend in this study that there are five diabetic patients of 17 false negative cases. By the way, it is reported that lymph node metastases from adenocarcinoma were of normal size (the short-axis diameter≤ 1 cm) more frequently than those from squamous cell carcinoma [5]. Therefore, we speculate that patients with lymph node metastasis from squamous cell carcinoma tended to be excluded in Kazaki’s series and included in our series of patients.

PET/CT imaging is the most accurate imaging modality for lung cancer staging but faces the most serious challenge due to increased glycolytic activity of benign tumors and inflammatory tissue, in addition to that of malignant tumors. As expected, up to 45.5% patients (41/90) were confirmed as false positives with upstaging from N0 to N1 or N2 by PET/CT scan. These false-positive results may have been caused by reactive hyperplasia or active inflammation due to granulomatous diseases such as tuberculosis [20]. Risk factors for false positives in the present study were old age, well differentiation, low SUVmax of primary tumor and non-adenocarcinoma. Therefore, we still recommend biopsies of all suspicious lymph nodes with a high SUVmax which should not be equated with malignancy until tissue confirmation is obtained.

Our study has several limitations. Firstly, it suffers from selection bias, because we included patients that received surgical nodal dissection. Therefore, many more cases of early stage of NSCLCs were included, which may have contributed to observed sensitivity reduction. Secondly, it was inevitable that false negative findings are calculated on relatively small numbers. In addition, the analysis was based on a per-patient basis rather than on a per-node station basis.

In conclusion, the SUVmax of HMLN is a predictor of malignancy. The PET/CT had the best diagnostic efficiency at the cut-off SUVmax of 2.34 in our cohort. False negatives and false positives were inevitable but predictable according to some risk factors. Lymph node staging of PET/CT is far from equal to pathological staging.