Positron emission tomography (PET) is one of the most exciting advances in the diagnosis of many cancers, including that of the lung. Briefly, the technique uses the radiolabeled glucose analog F 18 fluorodeoxyglucose. The compound is injected intravenously and is taken up preferentially by metabolically active cells, such as those found in tumors.The patient is placed in a detector similar in appearance to a computed tomography (CT) scanner and the data is recorded. Images are reconstructed and can be viewed in the axial plane (the same as a standard CT scan) or in coronal and sagittal planes. An area of high metabolic activity can be quantified and the number obtained is referred to as the standard uptake value (SUV). In general, areas with SUVs of 2.5 or greater are considered suspicious for tumor.
The clinical utility of PET scanning in patients with known or suspected lung cancer includes the evaluation of the primary lesion, determination of mediastinal lymph node involvement, and detection of distant metastases.
For patients with a new pulmonary nodule or mass, a positive PET scan is highly suspicious for malignancy and mandates tissue confirmation. A negative scan, although not necessarily ruling out malignancy, might encourage a period of observation.The sensitivity and specificity of PET scan for primary lung lesions are 97% and 89%, respectively.1 False positive readings can occur in non-malignant lesions of high metabolic activity, such as tuberculosis or sarcoidosis. Conversely, small tumors (<1.5cm) and tumors of low metabolic activity, such as bronchioalveolar carcinoma and renal cell carcinoma, have low SUVs, and can therefore give false negative results.
The advent of PET scanning has improved the detection of mediastinal lymph node metastases compared with chest CT. In conventional CT, suggestion of mediastinal lymph node involvement is based on size criteria, i.e., a lymph node greater than 1cm in diameter is considered suspicious for metastatic disease. Biopsy is required for confirmation.
In a recent study, PET scanning of the mediastinum had a sensitivity of 81% and a specificity of 96%, similar to that achieved by nodal biopsy via mediastinoscopy.2 In this study, PET scanning also identified positive nodes in areas not accessible to mediastinoscopy and redirected the diagnostic approach. Because of these advances, PET scan has been touted as an alternative to mediastinoscopy. However, biopsy is still advised because of the false negative and false positive rates and the potential consequences of both under-staging and over-staging the patient.
PET scanning has the ability to detect metastatic tumor at distant sites. Unsuspected metastases from lung cancer have been demonstrated in about 15% of patients, thus avoiding a surgical resection.2 Currently, many surgeons are including a PET scan in the preoperative evaluation of lung cancer patients.