Pet of Oncological Disorders

1

What is ¹⁸ F-fluoro-2-deoxy-2-D-glucose (FDG)?

FDG is the most widely used radiotracer for positron emission tomography (PET) imaging and was first tested in humans in 1976. Its utility is based on the Warburg effect (described in the late 1920s by Otto Warburg), in which malignant cancer cells preferentially use glucose compared to other substrates. However, this radiotracer can be used to assess any organ or disease process that has accelerated glucose metabolism, including infectious and noninfectious inflammatory disease conditions.

2

What instructions do patients need to prepare for FDG PET imaging?

The most important issue regarding an FDG PET scan is that FDG competes with nonradioactive glucose. If a patient has recently eaten or has diabetes mellitus with a serum glucose level >150 mg/dl, the sensitivity of the scan to detect malignancy may be diminished. Typically, serum glucose levels greater than 200 mg/dl should exclude a patient from having an FDG PET scan. Nondiabetic patients should have nothing by mouth overnight for a scan scheduled in the morning or else for at least 4 to 6 hours for a scan scheduled later in the day. Diabetic patients should be managed carefully because some have difficulty forgoing eating. Also, insulin drives glucose (and FDG) into the muscles, so care should be taken regarding the administration of insulin too close to the scan time. It is preferred that the patient not inject insulin, eat, or exercise before the PET scan, but if this is impossible, the referring physician must balance glucose levels, eating, and insulin in the diabetic patient. The scan itself is typically acquired starting 1 hour after intravenous FDG administration from the skull base to the proximal thighs and takes about 30 minutes to perform. Whole body scanning from skull vertex to toes, which takes about 45 minutes to acquire, is performed in some patients with malignancies that more commonly involve the head, skin, or extremities such as melanoma and cutaneous lymphoma.

3

What is dual time point (DTP) PET, and what is its potential utility?

DTP PET is a scanning technique that involves acquisition of two sets of PET images (at early and delayed time points) after intravenous FDG administration. In general, malignant lesions tend to have increasing levels of glucose (and FDG) uptake over time, whereas nonmalignant lesions as well as normal tissues tend to have stable or decreasing levels of glucose (and FDG) uptake over time. Therefore, DTP PET generally helps to improve the sensitivity of detection of malignant lesions in the body compared to single time point (STP) PET, as malignant lesions become more FDG avid over time and background tissue FDG uptake decreases. Furthermore, DTP PET improves the specificity to characterize a lesion as malignant or benign, given the differential temporal patterns of FDG uptake.

4

What is the normal pattern of FDG uptake in the body?

High FDG uptake is observed in the gray matter of the brain. Also, high FDG activity is seen in the collecting systems, ureters, and bladder due to urinary excretion of FDG, which can be mitigated by patient hydration and voiding prior to image acquisition. Mild to moderate FDG uptake is usually seen in the tonsils, salivary glands, tongue, vocal cords, brown fat, bone marrow, liver, stomach, and large bowel. Mild FDG uptake is usually seen in the thyroid gland, esophagus, small bowel, spleen, and skeletal muscle. Minimal FDG uptake is seen in the lungs and pancreas. Variable FDG uptake is observed in the myocardium, as the heart uses glucose as well as fatty acids as sources of fuel. FDG uptake will generally be increased in the myocardium with a carbohydrate rich meal and decreased with a fatty rich meal prior to PET scanning ( Figure 76-1 ).

Minimizing talking and patient movement as well as keeping the environment quiet and dimly lit during the FDG uptake period are methods to decrease FDG uptake in the normal tissues of the head and neck and skeletal muscles. In general, FDG uptake in bilaterally located organs should be symmetric, and up to 70% of unexpected findings detected on FDG PET are malignant or premalignant in nature.

5

What is the significance of increased FDG uptake in the thyroid gland?

Focal FDG uptake in the thyroid gland is generally considered as suspicious for thyroid malignancy until proven otherwise and requires further evaluation with thyroid ultrasonography or tissue sampling. This is in contradistinction to diffuse FDG uptake in the thyroid gland, which may be a normal finding when mild, or due to thyroiditis or Graves' disease when moderate or marked.

6

What is the significance of increased FDG uptake in the bowel?

Focal FDG uptake in the bowel is generally considered as suspicious for malignancy until proven otherwise ( Figure 76-2 ) and requires further evaluation such as with computed tomography (CT), magnetic resonance imaging (MRI), fluoroscopy, or endoscopy with tissue sampling. Diffuse FDG uptake in the bowel is often physiologic, whereas segmental FDG uptake may be physiologic or inflammatory in nature.

7

What is the significance of increased FDG uptake in the endometrium or adnexa?

In premenopausal women, mild FDG uptake may normally be seen in the endometrium during the early menstrual cycle or in midcycle, and in the fallopian tubes or ovaries during midcycle. However, in postmenopausal women, increased endometrial or adnexal FDG uptake is generally considered as suspicious for malignancy until proven otherwise.