A Positron Emission Tomography (PET) scan is an imaging test that creates three-dimensional pictures of the inside of the body. This diagnostic tool observes how tissues and organs are functioning at a molecular level, rather than just their structure. PET scans are used to detect and manage various conditions, including cancer, by showing areas where cells are more active than usual.
How PET Scans Work
PET scans operate on the principle that cancer cells often have a higher metabolic rate than normal cells, particularly in their glucose consumption. To highlight these active areas, a small amount of a mildly radioactive liquid, known as a radioactive tracer, is injected into a vein. The most common tracer used is fluorodeoxyglucose (FDG), a modified sugar molecule.
After injection, the FDG tracer travels through the bloodstream and is absorbed by cells throughout the body. Cancer cells, due to their rapid growth and division, tend to absorb significantly more FDG than healthy cells. The patient then rests for approximately 60 to 90 minutes, allowing the tracer to distribute and accumulate in the tissues. During the scan, a large, ring-shaped machine detects the radiation emitted by the FDG, creating detailed images where areas of high sugar consumption appear as bright “hot spots”.
Applications for Breast Cancer
PET scans are valuable in the management of breast cancer, particularly for assessing the extent of the disease and monitoring treatment effectiveness. One primary application is cancer staging, which involves determining if the cancer has spread beyond the original site to other parts of the body, such as lymph nodes or distant organs. For patients with locally advanced breast cancer, FDG PET can identify previously unsuspected distant metastases.
PET scans are also used to detect recurrence, identifying if breast cancer has returned after initial treatment. These scans help assess how well cancer responds to therapy by showing changes in metabolic activity within tumors. A decrease in tracer uptake in follow-up scans can indicate that the treatment is working effectively. PET scans can also clarify ambiguous findings from other imaging tests.
Interpreting PET Scan Results
The images produced by a PET scan display areas of varying tracer uptake, with “hot spots” indicating regions of increased metabolic activity. These brighter areas suggest a higher concentration of the FDG tracer, which can be characteristic of cancerous activity due to increased glucose consumption by tumor cells. However, inflammation, infection, or even normal physiological processes can also lead to increased tracer uptake.
Radiologists use a quantitative measure called the Standardized Uptake Value (SUV) to assess the intensity of tracer accumulation. The SUV provides a numerical value for the level of radioactivity in a specific area compared to the overall distribution in the body. While higher SUV numbers correlate with increased metabolic activity and potential malignancy, these values are interpreted alongside a patient’s medical history and other diagnostic findings to differentiate conditions. For example, a significant decrease in SUV values in subsequent scans of a tumor can indicate a positive response to cancer treatment.
Complementary Role with Other Tests
PET scans are a powerful tool but serve a complementary role in breast cancer diagnosis and management. They are not used for initial breast cancer screening, unlike mammography, due to limitations like lower sensitivity for very small tumors and associated radiation exposure.
Instead, PET scans are used with other imaging modalities: mammograms, MRI, CT scans, and biopsies. Combining PET with CT (PET-CT) can provide more detailed anatomical information alongside metabolic activity, improving the detection of small tumors or multiple metastases. This integrated approach allows healthcare providers to gather a comprehensive picture of the disease, enabling more accurate diagnosis, precise staging, and informed treatment planning.