Positron Emission Tomography (PET) Scanning in Breast Cancer
Positron emission tomography (PET) is a noninvasive nuclear medicine study that has been in existence for almost 30 years, but has been gaining acceptance in oncologic imaging rapidly during the past 5 years. It involves injection of a short-lived positron-emitting radiopharmaceutical, 2-deoxy-[F-18]fluoro-D- glucose (FDG), a glucose analog with an approximate 2-hour half-life. When F-18 decays, it emits a positively charged electron, or positron, which travels a few millimeters in soft tissue before combining with an electron. Two high-energy "coincident" gamma photons are emitted at 180 degrees apart from each other as a result of this positron-electron annihilation reaction and are detected by a circular array of detectors in the PET scanner as they exit the patient's body.
The rationale for use of this radiopharmaceutical is that most malignant lesions have accentuated glucose metabolism, which is mirrored by increased uptake of FDG. Since FDG cannot be metabolized within the cell like glucose, it is effectively "trapped" within cancer cells. Malignant lesions appear visually as areas of increased activity ("hot spots") on a PET scan. PET images are analyzed routinely by qualitative visual methods, but also are often analyzed semiquantitatively, using the "standardized uptake value" (SUV). This method assigns a numerical value to the intensity of FDG uptake within a region of interest incorporating the neoplasm. The SUV relates the activity concentration in a certain volume of tissue to the amount of the injected dose and the patient's body weight. In general, malignant lesions have an SUV in the range of 2.5 to 15.
Other radiologic studies, such as mammography, sonography, CT, and MRI, provide detailed anatomic information about the size and location of masses, but not the unique metabolic information available with PET. This metabolic information generally affords PET several advantages over the anatomic modalities, including: earlier detection of malignancy; differentiation of scar or benign lesion from active malignancy; detection of metastatic disease in normal-size lymph nodes; and assessment of early tumor treatment response.
Breast cancer is the most common non-dermatologic malignancy in women in the United States, where approximately 192,000 women were diagnosed with breast cancer in 2001, with 40,000 mortalities. The incidence is increasing and a woman now has a 1 in 8 chance of developing breast cancer in her lifetime. 1 Accurate staging of breast cancer has important therapeutic and prognostic implications for optimal patient care.
Recently, the Centers for Medicare and Medicaid Services (CMS), previously known as HCFA, considered the utility of PET in breast cancer, after granting approval for the use of PET in non-small-cell lung cancer, colorectal carcinoma, lymphoma, esophageal cancer, melanoma, and head-and-neck-cancer since 1998. They commissioned the Blue Cross and Blue Shield Association (BC/BS) Technology Assessment Center, an evidence-based practice center, to evaluate the pertinent literature on PET in breast cancer and also polled PET experts and clinical oncologists with PET experience. Four clinical applications of PET in the evaluation of breast cancer were considered by CMS: 1) initial diagnosis of breast cancer; 2) initial staging of axillary lymph nodes; 3) detecting locoregional recurrence or distant metastases; and 4) evaluating response to treatment.
