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J Thorac Cardiovasc Surg 2000;119:1135-1140
© 2000 The American Association for Thoracic Surgery

GENERAL THORACIC SURGERY

THE ROLE OF COMPUTED TOMOGRAPHIC SCANNING IN DIAGNOSING MEDIASTINAL NODE INVOLVEMENT IN NON–SMALL CELL LUNG CANCER

From the Division of Thoracic Oncology,^a National Cancer Center Hospital East, Chiba, and Division of Thoracic Surgery,^b National Cancer Center Hospital, Tokyo, Japan.

The study was supported in part by a Grant-in-Aid for Cancer Research from the Ministry of Health and Welfare, Japan.

Address for reprints: Kazuya Takamochi, MD, Department of Thoracic Oncology, National Cancer Center Hospital East, 6-5-1, Kashiwanoha, Kashiwa, Chiba 277-8577 Japan (E-mail: ktakamoc{at}east.ncc.go.jp ).

	Abstract

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Objectives: The reliability of computed tomographic scanning in evaluating mediastinal node involvement is controversial because of the high false result rate. We attempted to identify significant factors responsible for false-positive and false-negative scans.
Methods: From August 1992 through April 1997, 401 patients with lung cancer who underwent major lung resection and systematic lymph node dissection were enrolled in this study. We retrospectively examined mediastinal node size, tumor location, maximum tumor dimension, the presence or absence of obstructive pneumonia, atelectasis, pulmonary fibrosis, and lymph node calcification on contrast-enhanced computed tomographic scans. We identified clinical and radiologic factors responsible for the false results by using univariate and multivariable analysis.
Results: Central tumor location proved to be a significant factor of false-positive scans. Elevated carcinoembryonic antigen level and larger tumor dimension were significant factors of false-negative scans. In patients with a peripheral tumor smaller than 40 mm and normal levels of serum carcinoembryonic antigen, sensitivity, specificity, positive predictive value, and negative predictive value were 6%, 93%, 8%, and 90%, respectively. The reliability of computed tomographic scanning in this low-risk subgroup was high in detecting N0-1 disease but low in diagnosing N2 disease.
Conclusion: It is not possible to accurately diagnose N2 disease by using lymph node size on computed tomographic scanning alone, especially in patients with a central tumor, an elevated serum carcinoembryonic antigen level, or a tumor of 40 mm or larger. A preoperative invasive staging procedure is indicated in these populations and may not be indicated in the population with normal computed tomographic scan results without any of these risk factors.

	Introduction

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Computed tomographic (CT) scanning is a widely used noninvasive imaging procedure for diagnosing mediastinal node involvement (N2) in patients with non–small cell lung cancer (NSCLC). However, the reliability of CT scanning in predicting N2 disease is controversial. Various results have been reported concerning the accuracy of CT scans in diagnosing N2 disease. ^1-9

Dales and associates ¹⁰ performed a meta-analysis of CT accuracy in diagnosing N2 disease in patients with NSCLC by using data from 42 studies published between 1980 and 1988. They concluded that the overall accuracy of mediastinal CT scanning was only 80%, with approximately 20% false-positive and 20% false-negative results. A high false-positive rate reduces specificity and the positive predictive value, whereas a high false-negative rate impairs sensitivity and the negative predictive value of an examination. The high frequency of false-positive and false-negative results substantiated the poor reliability of CT scanning in mediastinal lymph node evaluation.

Therefore, we attempted to identify clinical and radiologic factors responsible for false-positive and false-negative results of CT scanning. We also re-evaluated the efficiency of CT scans in predicting N2 disease on the basis of lymph node size, taking these factors into account.

	Methods

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From August 1992 through April 1997, 634 patients with lung cancer underwent surgical intervention at our institute. Among them, 401 patients who underwent major lung resection and systematic lymph node dissection were enrolled in this study. Seven of them underwent preoperative mediastinoscopy, which revealed no mediastinal node involvement. The following patients were excluded: (1) patients who received induction therapy, (2) patients with multiple primary lung cancers, and (3) patients with a primary tumor directly invading the mediastinum. The time interval between CT scanning and surgical dissection was less than 1 month in all the patients. CT scans were done on an X-Vision/SP system (Toshiba, Tokyo, Japan), and contiguous 10-mm thick sections were obtained from the pulmonary apices to the bases in a supine position at full inspiration. Dynamic incremental scanning was always performed, and scans were nearly always obtained after bolus injection of 100 mL of iopamidol by using an automatic injector.

All CT scans were reviewed by 3 authors (K.T., Y.O., and K.S.), who were not informed of the surgical outcome, to obtain the following information: the maximum dimension and location of the primary lesion, the sizes of mediastinal lymph nodes, and the presence of obstructive pneumonia, atelectasis, pulmonary fibrosis, or lymph node calcification. If multiple nodes within a single station were enlarged, the size of the largest node was recorded. The location of a tumor was considered to be central when it was located in the inner one third of the lung field on CT scanning and peripheral when in the outer two thirds. Histologic typing was determined according to the World Health Organization classification. ¹¹ The stage of the disease was based on the TNM classification of the International Union Against Cancer (UICC). ¹² The mediastinal lymph node evaluation on CT scanning and node mapping on thoracotomy were based on Naruke’s lymph node mapping. ¹³ All resected lymph nodes were fixed in formalin and examined microscopically by means of standard hematoxylin and eosin stain. Clinical N2 disease was defined when any mediastinal lymph node was 1.0 cm or larger in the shortest dimension. Pathologic N2 disease was defined when any mediastinal node was histologically involved. A true-positive result was documented when a patient had both clinical N2 and pathologic N2 findings.

The clinical record of each patient was reviewed for age, sex, histologic typing, smoking status, and serum carcinoembryonic antigen (CEA) level. Serum CEA was measured by means of the two-site immunoenzymometric assay (Tosoh Inc, Yamaguchi, Japan), and the upper normal limit for this assay was 5 ng/mL.

Univariate and multivariable analyses were performed by the logistic regression procedure on StatView 5.0 software (SAS Institute, Inc, Cary, NC) with a Macintosh Performa 6420 computer (Apple Computer, Cupertino, Calif) to determine the relationship between false-positive and false-negative scans and the following clinical or radiologic findings: age, sex, smoking status (pack-years >40 vs 40), histologic typing (squamous cell carcinoma vs others), serum CEA level, tumor location, maximum tumor dimension, the presence or absence of obstructive pneumonia, atelectasis, pulmonary fibrosis, and lymph node calcification on CT scan. The variables, such as age, node, and tumor dimension, were not skewed. The serum CEA level and smoking (pack-years) were severely skewed. Thus, we used the continuous variables for age, node, tumor dimension, and the log-transformed serum CEA level. For smoking status, we used the categories of pack-years of greater than 40 versus pack-years of 40 or less because the pack-years showed a two-peak distribution. The potential factors of false results in univariate analyses were also evaluated by using multivariable analyses. Differences were considered statistically significant when the P value was less than .05.

	Results

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The clinical characteristics of the patients are presented in Table I, and the radiologic characteristics of the tumors are presented in Table II. There were 57 (14%) cases of false-positive scans and 56 (14%) cases of false-negative scans.

	Discussion

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The CT scan is a routine examination for the diagnosis and locoregional staging of lung cancer at most institutes. CT scans have revealed evident reliability in examining the primary tumor in terms of size, location, and extrapulmonary extension. ⁴ However, they are far from satisfactory and less accurate than mediastinoscopy in node involvement evaluation. ^19,20 Detection of lymph node metastasis on CT scan is based on the demonstration of lymph node enlargement. However, several investigators reported that benign nodes might be enlarged as a result of reactive hyperplasia, obstructive pneumonia, or atelectasis attributable to primary tumor or prior granulomatous disease, whereas metastatic nodes may appear normal in size if the metastasis is microscopic. ^5,21,22 These factors can lead to false-positive or false-negative scans, which impair the reliability of CT scans in detecting node involvement. Therefore, we attempted to clarify which clinical and radiologic factors were significantly related to false results on CT scans.

Multivariable analysis revealed only one significant factor of false-positive scans: central tumor location. Mediastinal lymph nodes may be swollen reactively by obstructive pneumonia or atelectasis associated with the tumor. ^5,21 In our study, patients with obstructive pneumonia or atelectasis were more likely to have false-positive scans. However, once tumor location was accounted for, obstructive pneumonia or atelectasis per se did not provide additional information concerning false-positive results. Several reports compared the reliability of CT mediastinal evaluation between central and peripheral tumors. Daly and associates ³ reported that CT detection of positive mediastinal lymph nodes had an accuracy of 79% for central tumors and 90% for peripheral ones. Moreover, the false-positive rate was higher in central tumors (14%) than in peripheral ones (5%). Our results were generally consistent with theirs. However, our series revealed a higher false-positive rate (32%) in the central tumor population than that found in their series. Because these false-positive populations can benefit from primary surgery alone, an invasive staging procedure, such as mediastinoscopy, may be indicated.

Elevated serum CEA level and larger tumor dimension were significant factors of false-negative scans in both univariate and multivariable analyses. The larger a tumor has grown, the more normal-sized lymph nodes may have microscopic metastases. Some investigators reported a relationship between elevated serum CEA levels and advanced disease or poor prognosis. The patients with marked elevation of preoperative serum CEA levels are highly likely to have occult metastases. ^23-25 In patients with an elevated CEA level, metastatic nodes may appear normal in size. The prognosis of patients with false-negative scans for mediastinal node involvement has been reported to be poor, even though it was better than that of patients with true-positive scans. ^26,27 The false-negative subgroup may benefit from induction therapy, which might indicate an invasive staging procedure.

In patients without risk factors of false results (ie, patients with a peripheral tumor smaller than 40 mm and an elevated serum CEA level), CT scanning was extremely reliable in detecting the population without mediastinal node involvement. One hundred thirty-seven (90%) of 153 patients with clinical N0-1 disease on CT scanning had pathologic N0-1 disease. Thus, patients with normal CT results in this low-risk population can proceed directly to thoracotomy. A preoperative invasive staging procedure may not be indicated. Phillips and colleagues ²⁸ reported that as prevalence of disease decreases, the reliability of a positive result will drop, and the reliability of a negative result will increase. The prevalence of N2 disease in our proposed low-risk subgroup of false results was extremely low, which results in low reliability of CT scanning in diagnosing mediastinal node involvement.

In conclusion, it is not possible to accurately diagnose N2 disease by using lymph node size on CT scan alone, especially in patients with a central tumor, an abnormal serum CEA level, or a tumor 40 mm or larger. A preoperative invasive staging procedure, such as mediastinoscopy, is indicated in these populations to decide the appropriate treatment strategy and may not be indicated in the population of normal CT results without any of these risk factors.

	Acknowledgments

 
We thank Dr Satoshi Sasaki, Epidemiology and Biostatistics Division, National Cancer Center Research Institute East, for his technical supports in statistical analyses. We also thank Professor J. Patrick Barron, International Medical Communications Center, Tokyo Medical University, for reviewing the English manuscript.

	References

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