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PET, CT scans work together

Despite the important role played by computerized tomography in the management of lung cancer over the years, by current standards, it is considered suboptimal as a sole imaging method.

While providing anatomic detail, CT is not designed to provide functional information regarding the images it generates. Therefore, additional invasive surgical procedures such as mediastinoscopy and thoracotomy are frequently performed to establish the true nature of the abnormalities seen on CT.

Thus, CT tends to increase the overall morbidity that the patient must incur and the length of the pre-treatment work-up. This ultimately delays the selection and application of a definitive treatment modality.

Positron emission tomography provides functional information because it includes intravenous administration of a radio pharmaceutical, FDG (fluoro-deoxy-glucose) prior to imaging. The uptake of the FDG into a tissue means the tissue it more likely to be malignant. Thus, the PET provides functional information while the CT gives anatomic detail only.

A meta-analysis assessing the accuracy of PET in the evaluation of lung nodule showed sensitivity was 97 percent and specificity was 78 percent. Another meta-analysis assessing the value of PET for the examination of the mediastinum (involvement of this area usually precludes surgical resection), generated a sensitivity of 84 percent and specificity of 89 percent, with a positive predictive value of 79 percent and a negative predictive value of 93 percent.

The same study found a corresponding sensitivity and specificity of 57 percent and 82 percent, respectively, with a positive predictive value of 53 percent and a negative predictive value of 83 percent for CT. The value of a CT scan in this study was inferior to that of the PET scan.

PET is considered more accurate in detecting bone metastasis as compared to the conventional bone scan and more likely to reveal distant spread of the tumor as compared to other imaging methods.

PET, however, has the following limitations:

- With a positive predictive value of only 80 percent, some patients will have a false positive scan.

- It is not very accurate in detecting small lesions of a variety of lung cancer known as bronchoalveolar carcinoma. The pattern of spread of this cancer is "lacy" rather than lumpy, which makes it difficult to detect.

- PET does not provide anatomical detail.

- PET is more expensive than single-site CT. However, to get the whole body image comparable to what a PET provides, multiple CT scans will be required.

The advantages of PET are:

- It is more accurate than CT. A negative predictive value of 93 percent ensures that a patient with a negative scan has an extremely high chance of being truly negative.

- It delivers less radiation when performed by itself. When combined with CT, as the latest generation of PET/CT machines do, the radiation is somewhat greater, but still less than conventional multiple CTs.

- It compresses preoperative evaluation time and facilitates the application of definitive treatment earlier.

- The intensity of the PET image correlates with prognosis, and decrease in intensity on follow up scans during chemotherapy indicates response to treatment. This provides a positive reinforcement to both physician and patient that they are on the right tract.

Conversely, an increased intensity denotes non-response and provides the oncologist an early opportunity to change the treatment.

Combined evaluation of the results of PET and CT yields greater information. Side-by-side examination or fusion of both images acquired by two different machines can be accomplished by specially designed software.

The latest generation of scanners house both PET and CT equipment together and yield the best information. Such machines are available in Citrus County.

V. Upender Rao, MD, FACP, practices at the Cancer and Blood Disease Center in Lecanto.

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