Unlike techniques that provide anatomical images, such as X-ray, CT and MRI, PET scans show chemical and physiological changes related to metabolism. This is important because these functional changes often occur before structural changes in tissues. PET images may therefore show abnormalities long before they would be revealed by X-ray, CT, or MRI.
Before a PET scan, a patient will receive an injection of a radiopharmaceutical, which is a drug labeled with a basic element of biological substances, called an isotope. These isotopes distribute in the organs and tissues of the body and mimic natural substances such as sugars, water, proteins, and oxygen. This radioactive substance is then taken up by the cancer cells, thereby allowing the radiologist to visualize areas of increased activity.
After the patient has received the injection, a small amount of radiation is passed through the body, which detects the isotopes and reveals details of cellular-level metabolism. Although the radiation is different from that used in radiography, it’s roughly equivalent to what is administered in two chest x-rays. After the scan is complete the radiation does not stay in the body for very long.
PET is useful for diagnosing lung and breast cancer, and for monitoring response to therapy. Effective therapy leads to rapid reductions in the amount of glucose that is taken up by tumors. PET imaging can easily reveal this drop in metabolic activity and show—sometimes within minutes or hours—whether a patient is responding positively to a particular course of treatment. PET has been shown effective for predicting outcomes, detecting spread of cancer, and/or monitoring therapeutic response in a wide range of cancers, including breast, colon, lung, ovarian, head, neck, and thyroid cancers, as well as melanoma and lymphoma.
