Despite the many recent technological advances made in CT scans and MRI of the chest, these modalities continue to show some limitations in the following specific settings: in the SPN; after extensive postsurgical and postradiation changes; and in subcentimeter-sized lymph nodes in patients with prior malignancy. PET is a functional imaging technology that has shown great promise in this arena.
The majority of PET scans are performed utilizing (F)fluorodeoxyglucose (FDG), which is the radiotracer of choice for tumor imaging. The use of FDG is based on the fact that increased glucose metabolism has been noted in many neoplasms. Recently, we have noted that uptake of FDG by tumors increases over time, while the metabolic activity of inflammatory sites either remains stable or declines between 60 and 90 min following injection of the radiotracer. We are currently testing the validity of this dynamic FDG uptake curve in lung tumors. natural asthma inhaler
Many lung malignancies present as SPNs with a potential for cure after surgical resection. Unfortunately, up to one third of patients with SPNs who undergo surgery are found to have benign lesions at thoracotomy. This problem is even greater in geographic regions where there is a high prevalence of granulomatous disease, such as the Northeastern United States. PET scanning with FDG appears extremely useful in characterizing SPNs, as studies have reported sensitivities and specificities of > 80%. However, most studies exclude patients with diabetes, in whom sensitivities are somewhat lower. The criterion that characterizes an SPN as malignant on a PET scan is the degree of FDG uptake 1 h after injection, which should be greater than that seen in the mediastinum (Fig 9). Semiquantitative analyses have revealed that an SPN with a standard uptake value (SUV) > 2.5 is likely to represent a malignant lesion; however, visual inspection by an experienced observer is as accurate in making such distinctions as SUV analysis. Most false-positive results are due to increased FDG uptake in inflammatory conditions such as granulomas. In geographic regions where the prevalence of inflammatory conditions is high, an SUV threshold of 3.8 has been suggested to reduce the likelihood of false-positive results. Two time-point imaging, which we are currently studying, may obviate the need for such SUV modification.
Figure 9. PET in lung cancer. This 79-year-old female underwent a left upper lung cancer resection 14 months prior to the CT scan of the lower chest shown above. Top: this image reveals a new 2-cm right lower lobe nodule, while the rest of the study including the mediastinum and the left lung, appeared without evidence of recurrent disease. A curative surgery was planned if the PET-FDG scan revealed no other abnormalities elsewhere. Bottom: a PET-FDG scan of the chest and abdomen (selected coronal planes are shown above) revealed significant metabolic activity in the lesion seen on the CT scan (solid arrow), which was interpreted to represent a malignant tissue at this site. However, three more abnormalities were identified (broken arrows) in the upper mediastinum, left hilum, and the posterior chest wall to the left of the midline that also were suggestive of malignant lesions at these sites. These findings resulted in a significant alteration of the initial plans for the management of this patient. LUL = left upper lobe; RLL = right lower lobe.