A positron emission tomography (PET) scan is a medical imaging procedure that provides images of organs and tissues. It uses a small amount of an injectable radioactive chemical, called a radiotracer, to visualize molecular activity within the body. PET scans are frequently used to help diagnose and assess cancer, along with certain heart and brain conditions.
How PET Scans Identify Cancer
PET scans detect metabolic activity within cells. A radiotracer, most often fluorodeoxyglucose (FDG), a sugar-like molecule, is injected into the bloodstream. Cancer cells typically have a higher metabolic rate than healthy cells, requiring more energy to grow and reproduce rapidly. This increased activity causes cancer cells to absorb more FDG, which the PET scanner identifies as “hot spots” on images. This ability to detect metabolic changes can often reveal disease in its early stages, sometimes before structural changes are visible on other imaging tests.
What a PET Scan Reveals About Cancer
A PET scan provides various insights into cancer, helping to detect cancerous lesions, confirm a diagnosis, and stage the disease by determining its spread to areas like lymph nodes or distant organs. The procedure also identifies if cancer has returned after treatment, a process known as recurrence. PET scans assess how effectively cancer treatment is working by monitoring changes in tumor metabolic activity. Observing these shifts allows doctors to understand treatment response early, even before changes in tumor size are apparent. PET scans can also aid in guiding biopsies by pinpointing metabolically active areas within a suspicious lesion for more accurate tissue sampling.
Preparing for and Undergoing a PET Scan
Preparation for a PET scan requires specific instructions. Patients typically fast for about six hours before the scan, consuming only plain water. Avoiding strenuous exercise for 24 hours prior is advised, as physical activity can affect radiotracer distribution. Patients with diabetes receive tailored instructions for managing blood sugar, as this influences tracer uptake.
Upon arrival, a radiotracer is injected intravenously. A waiting period of 30 to 90 minutes follows, allowing the tracer to circulate and be absorbed. During the scan, lasting about 30 minutes, the patient lies still on a padded table that moves through the scanner. Staying motionless is important for clear images. The entire appointment usually takes 1.5 to 2 hours.
Limitations and Important Considerations
PET scans have limitations. Non-cancerous conditions, such as inflammation or infection, can also exhibit increased metabolic activity and absorb the radiotracer, leading to false positives. Conversely, some slow-growing cancers, very small tumors, or certain types of cancer with low metabolic activity may not absorb enough tracer to be detected, leading to false negatives. Some prostate cancers or certain lung cancer subtypes, for example, may not show up clearly.
Patients are exposed to a small amount of radiation from the radiotracer during a PET scan. This radiation is generally low, and the radioactive material has a short half-life, breaking down quickly. Despite this, patients are advised to inform their healthcare provider if they are pregnant or breastfeeding. PET scans are often used with other imaging modalities, such as CT or MRI, for a more comprehensive diagnosis by combining metabolic and anatomical information.