PET-CT is a combination of cross-sectional anatomic information provided by CT and the metabolic information provided by positron emission tomography (PET).

PET is most commonly performed with 2-[F-18]fluoro-2-deoxy-D-glucose (FDG). Fluorine-18 (F-18) is an unstable radioisotope and has a half-life of approximately 110 minutes. More recently, radiopharmaceuticals such as gallium-68 prostate-specific membrane antigen have been developed and approved for use in PET.

F-18 FDG is a glucose analogue, with broad clinical applications for the imaging assessment of certain types of cancer, as well as infection and inflammation. The indications for F-18 FDG PET-CT imaging in patients with cancer:

• staging of cancer which potentially can be treated radically (e.g. small cell lung cancer) ⁴

• establish baseline staging before commencing treatment (e.g. GIST)

• evaluation of an indeterminate lesion (solitary pulmonary nodule) ^4,5

• assessing response to therapy ³

• evaluation of suspected disease recurrence, relapse and/or residual disease (e.g. lymphoma, testicular seminoma) ⁴

• to guide a biopsy (e.g. pleural biopsy for mesothelioma)

• differentiate between radiation-induced necrosis and tumour recurrence

PET-CT can also be used as a problem-solving tool, for example:

• occult primary lesion (e.g. non-metastatic manifestation of neoplastic disease)

• evaluation of suspected recurrence in patients with equivocal conventional imaging ⁴

• evaluation of residual disease in patients with treated differentiated thyroid carcinoma and treated medullary thyroid carcinoma with negative/equivocal conventional imaging ⁴

• prior to radical nodal resection in patients with metastatic melanoma

• suspected malignant transformation in plexiform neurofibromata (neurofibromatosis type 1)

• differentiate between radiation-induced necrosis and tumour recurrence (e.g. primary CNS malignancy)

Timing of PET scans is of particular importance for insulin-dependent diabetic patients, as administration of exogenous insulin can decrease the image quality of certain PET scans (e.g. 18-F FDG PET). Blood sugar levels of up to 13mmol/L are accepted at some centres, where patients are then hydrated with oral or intravenous fluids. If short-acting insulin is administered, scans may have to be delayed by up to 4 hours.