Whole Body FDG – PET / CT: Imaging Cancer

By Paul T. Finger, MD

Abdominal Section: Note that the fusion of the black and white forms seen on CT with the color (metabolic activity seen on PET) reveals liver metastasis.
Abdominal Section: Note that the fusion of the black and white forms seen on CT with the color (metabolic activity seen on PET) reveals liver metastasis.

Whole body PET/CT technology combines positron emission tomography (PET) with computed radiographic imaging (CT) to put FUNCTION and FORM on the same diagnostic page (PET/CT).

Spiral computed tomography CT is used to generate anatomic images of the entire body. When suspicious areas or tumors are found, CT allows your doctor to see their size, shape and internal radiographic density. Though computed radiographic tomography (and magnetic resonance imaging – MRI) are excellent methods to determine if an abnormality exists, its shape and location, it cannot reveal if the abnormality (e.g. tumor, lesion) is metabolically active.

In contrast, Positron Emission Tomography (PET) is used to determine if tissues or tumors are metabolically active. This is important, because metabolically active tumors are more likely to be malignant. In the case of imaging of malignant melanoma, PET imaging can differentiate between benign cysts and metastatic tumors in the liver (and other sites). PET requires a small injection of radioactive material (e.g. FDG – radioactive glucose) that is preferentially absorbed by malignant tumors. It is important to note that any glucose absorbing process (inflammations, infections, working muscles and excretory systems) will also concentrate glucose and appear as a “hot-spot” on PET. However, PET does not give you shape or location. That is why and additional CT is needed. It is the addition of the anatomic information provided by CT that allows the physician to differentiate between benign and malignant tumors. The PET/CT computers unify the PET information over the CT information, placing form and function on the same diagnostic page.

Dr. Finger has shown the though the liver is the most common initial site of metastatic choroidal melanoma. It can occur in the bones (in 50% of cases). Clearly, a whole-body PET/CT scan from the top of the head to the bottom of the feet will include all the bones in the body. It is important to realized that not all metastases will “light-up” with PET and that sometimes a dedicated CT or MRI will offer better anatomic imaging. So don’t be surprised if there is suspicious area seen on PET/CT, your doctor may request an additional radiographic examination.

We expect more melanoma specific radioactive materials (other than FDG – radioactive glucose) to be used for PET/CT. This will improve the specificity and sensitivity of this test. To see the research work being conducted thus far, visit the links below.

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