CT Neoplasm Imaging

11

18F-FDG

PET/CT Neoplasm Imaging

INDICATIONS PATIENT PREPARATION FOR 18F-FDG IMAGING NORMAL 18F-FDG DISTRIBUTION AND VARIANTS

Positron emission tomography (PET) in clinical practice has been replaced by positron emission tomography/computed tomography (PET/ CT). The most commonly used radiopharmaceutical is 18F fluorodeoxyglucose (FDG). There are other PET agents for tumors, but they are not currently in mainstream clinical use. PET physics and basic properties of FDG have been discussed in Chapter 1, and PET/CT instrumentation and quality control (QC) have been presented in Chapter 2. Use of FDG for brain and thyroid neoplastic applications are also included in Chapters 3 and 4, respectively. FDG imaging in patients with infections and inflammatory conditions is covered in Chapter 12. PET/CT skeletal scintigraphy using 18F sodiumfluoride is presented in Chapter 8. This chapter is devoted to PET/CT imaging with 18F-FDG and includes image interpretation and quantitation in the setting of neoplasm imaging. While FDG PET/CT is a powerful tool for neoplasm imaging, the sensitivity and specificity vary widely among tumor types. In addition, while many tumors are FDG-avid, in some cases, PET/CT may not be helpful in determining local staging, whereas, in other cases, PET/ CT is helpful for staging but not for follow-up. INDICATIONS PET/CT in the setting of neoplasm is useful for a number of indications, especially when it replaces other conventional imaging procedures, guides, or obviates the need for invasive diagnostic or therapeutic procedures, or results in a change in patient management and/or treatment outcome. Depending on the type of neoplasm, PET/CT may be used for diagnosis (such as with solitary

QUALITATIVE IMAGE INTERPRETATION PET IMAGE QUANTITATION WHOLE-BODY 18F-FDG PET/CT NEOPLASM IMAGING

pulmonary nodules and patients with metastases from unknown primary tumors), staging and restaging disease, monitoring therapeutic regimen, and in patient follow-up in a number of settings. Table 11-1 compares the relative values of common imaging procedures (including PET/CT) when diagnosing or assessing a neoplastic disease. PATIENT PREPARATION FOR 18F-FDG IMAGING The biodistribution of 18F-FDG is affected by blood glucose levels. Although there can be competitive displacement of the 18F-FDG by high levels of blood glucose, the primary adverse effect of elevated serum glucose is the resultant elevation of insulin levels. Patients should fast for 4 to 6 hours before a scan (preferably overnight) so that basal insulin levels will allow for optimal images. Elevated insulin levels degrade scans by increasing muscle uptake of FDG. In general, serum glucose levels should be below 150 mg/dL, but glucose levels up to 200 mg/dL are usually acceptable for satisfactory image quality. Imaging diabetic patients can be especially challenging. In type 1 diabetes, insulin is not recommended, and imaging is usually performed in the morning after an overnight fast. In type 2 diabetes, short-acting insulin can be used, but this increases muscle uptake. If possible, regular insulin should not be used 2 to 4 hours before the examination. In fact, in general, the use of insulin is best avoided altogether. Because there is significant excretion of 18F-FDG via the kidneys (and 18F-FDG is not reabsorbed as glucose is), good hydration is 361