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Incidental Right Breast Carcinoma 1-cm |
Overview--What is PET? Positron Emission Tomography is a powerful medical imaging modality which can non-invasively trace molecular and physiologic processes within the body. These images can more accurately define a host of diseases processes than conventional, anatomic-based imaging alone (such as CT and MR). PET can play an important role in cardiac, neurologic, and whole body imaging. By far the most common use of PET today is for whole body oncologic imaging—diagnosis, staging, restaging, and measuring early therapy response. While indeed most of nuclear medicine imaging is actually "physiologic", and can also be tomographic (i.e. can obtain "slices" through the body), PET is unique in the type of decay it utilizes—positron—which, when combined with specialized machinery can yield higher resolution images than conventional nuclear medicine. In addition, because of this specialized equipment, abnormalities on PET images can be easily quantified for additional diagnostic information (i.e. standardized uptake values [SUV]). There are many positron decaying isotopes which can be used in conjunction with PET. O-15, N-13, and C-11 can be labeled to water, ammonia, and a host of carbon compounds respectively, and imaged. However, only several positron-emitting radiotracers have found their way out of the realm of research and into mainstream clinical practice. By far the most common of these is F-18 in the form of F-18 fluorodeoxyglucose (FDG). FDG is the agent used in the most common clinical PET application: whole body oncologic imaging. FDG-PET can also be used in many neurologic and some cardiac applications. As the name implies, FDG is an analogue of glucose. Increased FDG uptake is seen in most malignancies, and the higher the metabolic activity of the malignant cells, generally the greater the abnormal uptake. Increased FDG uptake is not specific for malignancy. Besides active normal organs (most notably the brain grey matter and often cardiac segments) any active infectious or inflammatory process can have elevated FDG uptake that is indistinguishable from tumor. Such uptake can therefore lower the specificity of PET findings. Nevertheless, there is a vast amount of published data that FDG-PET imaging, when added to conventional imaging algorithms, results in much more accurate staging and restaging of nearly all of the most common tumor types, with profound implications on patient management. PET can also be combined with CT in several ways with the newest improvement to PET imaging: PET/CT, such as is available at the University of Chicago. This dual modality combines the increased lesion detection seen with PET and the anatomic localization prowess of CT resulting in "fused" images of higher diagnostic quality and also a shorter patient scan time. See the PET/CT section for further details. A practical consideration in PET imaging relates to scheduling and preauthorization. PET operates in a heavily regulated environment and CMS (Medicare) and therefore most private insurers keep tight control over what they will authorize for PET reimbursement, much more so than for CT, for example. There are a number of reasons for this but keep in mind that a single dose of FDG costs the hospital several hundred dollars and decays away in several hours. The good news is that the list of approved cancer types and other indications for PET is growing every year and the hope is that in the future, the somewhat restrictive policy will be removed altogether. Until then, our schedulers at the University of Chicago PET Center will do most of the preauthorization inquiries for you. If coverage is denied, a patient may choose to pay for the exam themselves (although it is expensive—several thousand dollars). For more information on CMS approved indications, please see the Whole Body Oncologic PET entry and also see Scheduling an Exam. |
