FDG-PET and Beyond: Molecular Breast Cancer Imaging

Abstract

Positron emission tomography (PET) scanning has gained widespread acceptance for the diagnosis, staging, and management of a variety of malignancies, including breast cancer. This has heralded an exciting new era of molecular imaging research of which using FDG as the primary PET tracer is only the beginning. The fundamental strength of PET over conventional imaging is the ability to convey functional information that even the most exquisitely detailed anatomic image cannot provide. As the standard PET radiotracer in current clinical use, FDG is a glucose analog that is taken up by cells in proportion to their rate of glucose metabolism. The increased glycolytic rate and glucose avidity of malignant cells in comparison to normal tissue is the basis of the ability of FDG-PET imaging to accurately differentiate cancer from benign tissue irregardless of morphology. The level or intensity of FDG uptake on PET is semiquantified and reported as the standardized uptake value (SUV). A multitude of new PET tracers are under development, many of which are aimed at targeting cellular processes that are more specific than glucose metabolism. In relation to breast cancer, these tracers include thymidine analogs such as [F-18]fluoro-L-thymidine (FLT) that target DNA replication as a measure of cell proliferation, annexin V derivatives that evaluate apoptosis, estrogen receptor (ER) tracers such as 16 -[F-18]fluoroestradiol-17 (FES), and engineered antibody fragments that directly target HER-2/neu receptors. In addition to new tracers, scanner technology is also rapidly evolving. Chief among these is the advent of the dual modality PET/CT scanner, which at the very least increases patient convenience by permitting PET and computed tomography (CT) imaging in a single appointment. But perhaps more importantly, initial studies indicate that the sum of the two modalities is better than either used separately and also may be an extremely useful tool in preradiation therapy planning. Other new scanning devices are also being developed, including small gantry PET scanners designed specifically for breast imaging, and handheld PET probes for direct intraoperative localization of tracer-avid tumor foci.