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PET’s role in cancer diagnosis is generally limited to the following important clinical situations:
- patients with radiographically indeterminate solitary pulmonary nodules
- women with mammographically difficult breasts such as dense or surgically altered breasts
- the case of indeterminate pancreatic enlargement or mass present on CT scan.
- patients with metastatic cervical node cancer, in the search for on occult head and neck primary.
- suspected pancreatic lesions, but only if active pancreatitis is excluded.
With a few notable and important and notable exceptions, PET does not now, and is not anticipated to play a role in the diagnosis of malignancy. The exceptions consist of situations where other imaging methods are of low sensitivity or specificity for the diagnosis of malignancy and where invasive methodologies have a high potential for morbidity or, are of themselves, costly.
PET plays a strong role in the staging of several malignancies and is under active evaluation in a number of additional tumor types. Here, the advantage of PET relates to it’s ability to more specifically identify the underlying cause of an anatomic abnormality such as lymph node enlargement on CT or MRI examinations. Obviously, nodes may be enlarged by benign causes, even in patients with malignancy, and conversely, nodes not reaching the size criteria for enlargement on CT or MRI may contain tumor. Another major advantage of PET in this and other situations is the ability to examine the whole body in a single study. This has been shown to add to the diagnostic yield in staging and also means that PET can be highly cost-effective compared with the multiple anatomic imaging examinations needed to image the entire body. It is likely that as PET becomes more widely used for staging, there will be significant “staging creep” and new data on therapy results for a given patient stage will have to be obtained. Evaluation of this possibility is now under way for lung cancer in a SWOG trial.
This is an area where PET plays an extremely useful role since other imaging methodologies suffer from poor specificity. This frequently results in inability to differentiate recurrent tumor from therapy-induced alterations in anatomy and from residual changes such as scarring in a node which was previously enlarged and remains enlarged after therapy.
Many studies in several common malignancies show that PET results in alterations in subsequent therapy on the order of 40% of the time when it is performed after and in addition to the usual imaging studies. In this regard, PET is extremely useful in situations where patients are being considered for major surgical procedures such as in major hepatic resections for metastatic colorectal cancer, and where the presence of extrahepatic disease would preclude such surgery. The ability to study the whole body in a single study is a major advantage of PET in comparison with other imaging methods such as CT or MRI where the cost of whole body monitoring would be prohibitive.
Much current research is directed at determining the potential of PET to assess early response of malignancy to therapy. Current methods suffer from inability to show change until even months after therapy initiation, whereas PET imaging can show change in tumor metabolism almost on a real time basis. For therapy monitoring, PET can be performed even as soon as a day or two after the therapy whereas for restaging after therapy, PET should be performed at as great a temporal distance from the therapy as is practical. If used to help decide if additional chemotherapy is to be given, perform the PET scan just before that therapy would be given. Note that PET has not been approved as yet for real-time therapy monitoring. (Dave, more specific guidelines would be helpful here.)
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