Positron Emission Tomography–Computed Tomography (PET-CT)

Positron emission tomography–computed tomography (PET-CT) combines functional PET imaging with the anatomical detail provided by CT. In cancer care, the most commonly used PET-CT scan uses fluorodeoxyglucose (18F-FDG), a glucose-like radioactive tracer, to show areas of increased metabolic activity.

Because PET-CT can evaluate multiple areas of the body in a single examination, it is used for staging, treatment response assessment, and evaluation of suspected recurrence or metastasis in selected blood cancers and solid tumors. It is particularly important for initial staging and response assessment in certain types of lymphoma.

PET-CT is not routinely required for every patient with cancer. Its use depends on the cancer type and the specific clinical question.

For FDG-avid lymphomas, including Hodgkin lymphoma and diffuse large B-cell lymphoma, PET-CT is commonly used for initial staging, response assessment during or after treatment, and evaluation of suspected relapse.

For solid tumors such as lung cancer, melanoma, and selected head and neck or esophageal cancers, PET-CT may help assess lymph node involvement and distant metastases, support staging, or identify sites of suspected recurrence.

PET-CT may also provide additional information when other imaging tests identify an indeterminate lesion, although pathology may still be needed for a definitive diagnosis.

For a typical 18F-FDG PET-CT scan, patients are usually asked to fast for several hours, and blood glucose is checked before the examination. After the tracer is injected, the patient rests quietly while it distributes throughout the body before imaging begins.

During the scan, the patient lies still while images are acquired. Because the examination includes both a tracer uptake period and the scan itself, the overall process generally takes longer than a standard CT examination.

Patients with diabetes, those who are pregnant or breastfeeding, and those who have recently received certain cancer treatments should inform the healthcare team so the examination can be planned appropriately.

Increased metabolic activity on PET-CT does not necessarily mean that cancer is present. Infection, inflammation, and some benign conditions can also show increased FDG uptake, so PET-CT findings need to be interpreted together with the patient's clinical history, other imaging studies, and pathology when appropriate.

Not all cancers take up FDG to the same degree. Small tumors, tumors with relatively low metabolic activity, or cancers with certain biological characteristics may be less visible on FDG PET-CT. PET-CT therefore does not replace tissue pathology or other imaging methods such as CT and MRI.

PET-CT also involves exposure to ionizing radiation and should be used when clinically appropriate. Inflammation after cancer treatment can affect scan results, so the timing of PET-CT for response assessment needs to be selected carefully.