Mediastinal Staging of Non-Small-Cell Lung Cancer

Christian Lloyd, MD, and Gerard A. Silvestri, MD, FCCP, Division of Pulmonary and Critical Care Medicine at the Medical University of South Carolina, Charleston.

Cancer Control. 2001;8(4) 

In This Article

Positron Emission Tomography

Given the shortfalls of CT staging, a more accurate method of staging patients nonsurgically is needed.Positron emission tomography (PET) was introduced recently as an alternative or complementary means of staging disease. Cancer has enhanced metabolic activity and avidly utilizes glucose. Patients are injected with fluorine-18 fluorodeoxyglucose (FDG) and imaged using PET. To identify malignancy, PET depends on demonstrating metabolic differences between tumor and normal tissue. This is determined by tumor size and metabolic activity. Potential advantages of PET imaging include the identification of tumor foci in normal-sized lymph nodes, which could reduce the number of surgical procedures performed for unresectable disease.

To measure the ability of PET to detect metastases in mediastinal lymph nodes, Pieterman and colleagues[13] prospectively compared PET imaging with a standard approach to staging. The sensitivity and specificity of PET were found to be 91% and 86%, respectively, while CT was 75% sensitive and 66% specific. The use of PET resulted in different clinical staging in 62 of 102 patients,with stage being lowered in 20 patients and raised in 42 patients. However, 17% of patients had false-positive "hot spots" in the mediastinum or at distal sites. The false-positive results were mainly due to inflammation, often from obstructing endobronchial tumors. Other studies have also noted the risk of false-positive results due to inflammatory processes.[14,15] Valk and colleagues[16] examined PET for whole-body staging as well as for mediastinal staging. Mediastinal PET and CT findings were compared with results of surgical staging in 76 patients. The sensitivity and specificity of PET for diagnosing mediastinal disease were 83% and 94%, respectively, while CT had a sensitivity of 63% and specificity of 73%. Overall, they found a 20% improvement in accuracy of PET over CT imaging for mediastinal staging of NSCLC. With respect to the size of mediastinal lymph nodes, a recent study by Gupta et al[17] compared small nodes (less than 1 cm) and large nodes (greater than 1 cm) using CT, PET, and histologic sampling. PET was found to be reliable and accurate for detecting lymph nodes of less than 1 cm, with sensitivity and specificity of 97% and 82%, respectively.

In addition to staging the mediastinum, PET has shown promise for identifying distant metastasis. In the study by Valk et al,[16] PET revealed extrathoracic metastasis not suspected by conventional means in 11% of patients. Surprisingly, there were no reported false-positive results. In 19 of 99 patients, CT imaging suggested metastasis at distant sites and PET imaging did not. Follow-up in 14 of these patients remained negative with one false-negative result.

The brain and the genitourinary system are two organ systems where PET is less accurate for identifying malignancy. The brain takes up glucose, and the genitourinary system concentrates and excretes the radio tracer.These areas avidly enhance, making it difficult to differentiate metastatic disease from normal activity.

In summary, PET appears to be more accurate than conventional CT scanning for staging the mediastinum ( Table ). Unlike CT, PET relies on increased metabolic activity rather than solely on the size of the lymph nodes. It can identify not only disease in nodes less than 1 cm, but also unsuspected extrathoracic metastasis.Disadvantages include difficulty with accurate anatomic placement of lesions as well as with extent of local tumor involvement,both necessary for staging. CT scanning is still needed in the majority of patients. Also, in two extrathoracic areas, the brain and the genitourinary system, PET is not as accurate for imaging. As the brain is one of the most common sites of distant metastasis,symptoms of spread to the brain need to be investigated with MRI or CT.

Although PET scanning is still in its infancy, it has been shown to be a cost-effective tool in staging lung cancer and is now covered by Medicare for the staging of lung cancer.[28] Currently, this technology is expensive and is generally performed only in tertiary care medical centers. It requires a special scanner, production of the radioactive isotope, and staff trained in nuclear medicine.

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