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Clinical Investigations |
1 Department of Diagnostic Imaging, University of Rome "Tor Vergata," Rome, Italy
2 Department of Nuclear Medicine, University of Sassari, Sassari, Italy
3 Department of Radiologic Sciences, University of Rome "La Sapienza," Rome, Italy
4 Department of Surgery, University of Rome "La Sapienza," Rome, Italy
5 Department of Respiratory Diseases, University of Sassari, Sassari, Italy
| ABSTRACT |
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1 cm), with an accuracy (91.7%) significantly higher (P < 0.05) than that of CT (66.7%). Conclusion: 99mTc-Tetrofosmin SPECT is a useful presurgical noninvasive method to assess mediastinal lymph node involvement in NSCLC. In particular, it could play a clinical role in reducing the number of invasive staging surgical procedures in selected patients, especially in those with enlarged lymph nodes at CT. Fusing SPECT with CT images could further improve the interpretation of the scintigraphic data.
Key Words: nonsmall cell lung cancer 99mTc-tetrofosmin mediastinal lymph nodes SPECT CT
| INTRODUCTION |
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Chest CT is the most commonly used noninvasive imaging method for mediastinal staging in NSCLC, but its limits are widely acknowledged: The criterion used for diagnosis is based only on lymph node size, and thus it is less accurate than that used in invasive surgical staging (6,7). Therefore, there is a need for noninvasive imaging methods able to accurately assess metastatic mediastinal involvement in patients with NSCLC and thus reduce the number of staging mediastinoscopies and mediastinotomies and avoid unnecessary thoracotomies (8).
Nuclear medicine procedures have also been used in NSCLC staging; some authors (9) have demonstrated the usefulness of 18F-FDG PET in the preoperative assessment of patients affected by this type of cancer, also suggesting that the addition of this procedure to conventional imaging methods can improve the selection of surgically curable patients (9). The usefulness of some myocardial perfusion imaging tracers such as 201Tl (10,11) and 99mTc-sestamibi (12,13) has also been reported in mediastinal lymph node detection using SPECT acquisition; in particular, recent data have demonstrated a higher specificity and accuracy for 99mTc-sestamibi SPECT than for CT, but a lower sensitivity (13).
More recently, another cationic lipophilic complex, 99mTc-tetrofosmin, largely used for myocardial perfusion scintigraphy, has also been used as a tumor-seeking agent; previous clinical studies have demonstrated that 99mTc-tetrofosmin clears rapidly from the lungs and the liver, thereby permitting early high-quality images of the chest (14). Published data indicate that 99mTc-tetrofosmin clearance from the lungs and the liver appears to be faster than the 99mTc-sestamibi clearance (14,15). 99mTc-Tetrofosmin accumulates in different kinds of neoplasms (16) and proved useful in imaging lung cancer (17,18).
The aim of this prospective study was to evaluate the usefulness of 99mTc-tetrofosmin imaging in the presurgical assessment of mediastinal lymph nodes in patients with NSCLC and to compare scintigraphic data with CT results.
| MATERIALS AND METHODS |
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All the patients had been selected on the basis of the presence of clinical signs and radiographic or CT signs suggestive of lung cancer and with a definitive diagnosis of NSCLC ascertained from specimens obtained by percutaneous guided CT or bronchoscopy biopsy. In all cases, distant metastases had also been ruled out by CT and bone scintigraphy. The patients demographic and clinicopathologic characteristics are summarized in Table 1.
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A commercial tetrofosmin preparation (Myoview; Amersham International) was used. The labeling and quality control procedures were performed according to the manufacturers instructions. The scintigraphic images were obtained using a rectangular, large-field-of-view, double-head gamma camera (Millennium MG; General Electric Medical Systems, or Helix; Elscint) equipped with low-energy, high-resolution, parallel-hole collimators. After an intravenous injection of 740 MBq of 99mTc-tetrofosmin, SPECT images of the chest were acquired 20 min later over 360° (180° per head) using the following parameters: a 64 x 64 word matrix, a 3° angular step, and 30 s/frame. After prefiltration with a count-optimized Metz filter, images were reconstructed with a ramp filter to produce 2-pixel-thick transaxial slices. Coronal and sagittal views were then obtained.
SPECT images were always preceded by simultaneous acquisition of anteroposterior (600 s) planar lung images starting 10 min after 99mTc-tetrofosmin injection. However, for the present study we have reported only the SPECT results.
Two experienced nuclear medicine physicians evaluated the SPECT scintigrams; they were unaware of the mediastinal CT findings and, obviously, the final pathologic diagnosis, which was always made after scintigraphy, but they were informed of the clinical reason for the scintigraphy and of the site of the suspect primary lesion. At qualitative analysis, SPECT images were classified as positive for lymph node mediastinal involvement when there were 1 or more foci of well-defined increased 99mTc-tetrofosmin uptake in the mediastinum compared with background activity; disagreements were resolved by consensus, with a third observer as referee.
Chest CT was performed after intravenous injection of nonionic iodinated contrast medium with a window for the mediastinum and a window for the lung parenchyma and evaluated by experienced radiologists. For the interpretation of the CT images, the mediastinal lymph nodes were divided into 2 groups: Those with a diameter of less than 1 cm on the shortest axis were considered normal in size and those with a diameter of 1 cm or more were considered enlarged.
All patients underwent surgical evaluation by means of mediastinoscopy with mediastinal lymph node sampling (14 patients) or thoracotomy (69 patients); surgical node specimens were examined by histopathology with hematoxylineosin staining. The results of SPECT were compared with those of CT and both with the histopathologic findings; per-patient sensitivity, specificity, accuracy, and positive and negative predictive values were then determined for each procedure; accuracy values of CT and SPECT were mutually compared by the McNemar test for correlated proportions.
| RESULTS |
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99mTc-Tetrofosmin SPECT yielded a per-patient sensitivity of 85.7%, a specificity of 89.6%, and an accuracy of 88.0% in detecting mediastinal lymph node metastases, whereas CT showed a sensitivity of 68.6%, a specificity of 75.0%, and an accuracy of 72.3% (Table 2). SPECT accuracy was thus significantly higher than that of CT (P < 0.05). We had no difficulty in localizing the foci of increased 99mTc-tetrofosmin uptake in the different regions of mediastinum; nevertheless, unlike CT, which has a better spatial resolution than SPECT, 99mTc-tetrofosmin imaging could not always determine the exact anatomic location of metastatic lymph nodes.
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| DISCUSSION |
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The first step in mediastinal staging in patients with NSCLC involves radiologic imaging with CT; the malignancy criterion is based on lymph node size by measuring the short-axis diameter in the axial images, and a dimension of
1 cm is considered suspect for cancer. However, benign nodes may also be enlarged for other reasons, such as reactive hyperplasia, anthracosis, inflammation, or infection; on the other hand, malignant lymph nodes may also be normal in size (7).
Per-patient CT accuracy was 72.3% in our series, and these data are in line with previous results from other studies and with the many prospective data from the Radiologic Diagnostic Oncology Group (21). Moreover, it has been shown that MRI is not superior to CT in mediastinal node staging in NSCLC because of its limited spatial resolution and inability to obtain images during held respiration. These limitations can cause the separate nodes to become blurred on images and to appear as 1 large node (19).
The shortcomings of anatomic images obtained by conventional methods have led clinicians to explore alternative imaging techniques and, in particular, to consider whether functional images may have an additional role in the assessment of mediastinal lymph node involvement in patients with NSCLC. A wide range of radiopharmaceuticals has been used for this purpose, such as 67Ga-citrate, 201Tl-chloride, 99mTc-sestamibi, 111In-pentetreotide, radiolabeled monoclonal antibodies, and, more recently, 18F-FDG (9,22). However, the recommended nuclear medicine imaging procedures with these tracers for mediastinal lymph node detection should be those that are most reliable and easily available; those that are more expensive and unnecessary should be avoided (23).
99mTc-Tetrofosmin is largely used for myocardial perfusion scintigraphy; it accumulates in viable myocardium, and the uptake is proportional to regional blood flow (14). The biodistribution and cellular uptake characteristics of 99mTc-tetrofosmin allow different clinical applications, and its use has also been reported in patients with different tumor types (16). In tumor cell lines, uptake of 99mTc-tetrofosmin depends on both cell membrane and mitochondrial potentials, but only a small fraction of the radiopharmaceutical accumulates inside the mitochondria; the low uptake observed in dead cells indicates that 99mTc-tetrofosmin accumulates in viable tumor tissue (24).
In 2 recent studies performed on a small number of patients with lung cancer, 99mTc-tetrofosmin was used to detect mediastinal lymph node metastases. Shiun et al. (25) reported data on 34 patients with large primary tumors (T2 or greater), in whom 99mTc-tetrofosmin SPECT showed a sensitivity of 85.0%, a specificity of 85.7%, and an accuracy of 85.3%; Buccheri et al. (26), in studying 16 patients with pathologically staged mediastina, found sensitivity, specificity, and accuracy values of 73%, 100%, and 81%, respectively.
Our series was a larger number of patients and demonstrated that 99mTc-tetrofosmin SPECT was superior to CT, which is considered the radiologic procedure of choice for diagnosing mediastinal node metastases from NSCLC. In this comparative study, 99mTc-tetrofosmin imaging resulted in a nonsignificant increase in accuracy in patients with normal-sized lymph nodes at CT, although it is important to emphasize that in 8 of the 11 patients who had mediastinal lymph node metastases but false-negative CT findings, SPECT correctly diagnosed the metastases. However, when SPECT was used to evaluate the mediastinum in the patients for whom CT showed enlarged nodes, a significantly higher accuracy was obtained. This outcome is interesting, since the most important role of a new imaging technique for assessing mediastinal lymph nodes would be to evaluate enlarged nodes and to distinguish tumoral nodes from those with hyperplasia due to nonmalignant conditions, thus reducing the need for invasive staging procedures (8).
Our clinical series proved the usefulness of the noninvasive functional imaging method with the radiopharmaceutical 99mTc-tetrofosmin, uptake of which is related to tumoral cell viability. However, 99mTc-tetrofosmin SPECT presents some diagnostic limitations. In particular, its high accuracy refers only to a per-patient evaluation. Even though the analysis is made with 3-direction sections, 99mTc-tetrofosmin SPECT has a limited spatial resolution, which makes it difficult to define the exact number of involved nodes and thus to calculate the data on a per-node basis. On the other hand, knowledge of the number of mediastinal lymph node metastases is not crucial for NSCLC staging.
We found no difficulty in localizing the foci of increased tetrofosmin uptake in the different regions of the mediastinum; however, the exact anatomic location with respect to the surrounding vascular structures within the mediastinum could not always be estimated. Thus, in this sense, although SPECT imaging is simple to perform, SPECT findings may not always be easily interpreted. Unlike CT, 99mTc-tetrofosmin SPECT lacks anatomic landmarks, potentially leading to an incorrect interpretation of the images because of interference from the great vascular structures. However, in only 3 of the 5 false-positive SPECT results of our series were no specific histologic findings found. It is possible that we erroneously considered some areas of 99mTc-tetrofosmin physiologic uptake as a pathologic accumulation in the mediastinum. However, this interpretation is speculative only when referring to interference from vascular structures, since it is well known that 99mTc-tetrofosmin can also accumulate in lymph nodes with nonspecific changes.
Correlation of images obtained by SPECT and CT and, even more, the combination of image registration and fusion (i.e., precise matching of the data from the 2 imaging modalities) can more accurately identify the tumoral site (27). SPECT/CT fusion has also been used in the application of radiolabeled antibodies in lung cancer, proving useful in evaluating mediastinal adenopathy and especially in distinguishing a normal blood pool from pathologic areas of accumulation (28). We think that the application of SPECT and CT image fusion to 99mTc-tetrofosmin studies, too, could lead to better anatomic localization of areas of abnormal activity at SPECT, thus helping to differentiate tumoral lesion from physiologic radiopharmaceutical uptake; moreover, image fusion is easy to perform using dedicated devices (29,30).
| CONCLUSION |
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| FOOTNOTES |
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For correspondence or reprints contact: Orazio Schillaci, MD, Viale G. Mazzini 121, 00195 Rome, Italy.
E-mail: oschil{at}tiscalinet.it or orazio.schillaci@uniroma2.it.
| REFERENCES |
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