Positron emission tomography (PET) imaging can provide information beyond that available from magnetic resonance imaging (MRI) and computed tomography (CT). Dr. Kai Chen from the Keck School of Medicine, University of Southern California, and colleagues at the National Institutes of Health provided a review of PET scans in cancer.
PET uses radiotracer atoms that emit positrons of low energy. The most commonly used tracer is Fluorine-18, which has a half-life of 109.8 minutes. Positrons are a form of anti-matter. As soon as a positron meets an electron, they annihilate each other and produce two photons traveling in opposite directions.
For Fluorine-18 in the human body, the average positron travels about 1 mm before encountering an electron. The photons are detected in the PET scanner, and computer analysis is used to construct an image based on the timing and location of the photons.
The advantage of PET is that the tracer atom can be incorporated in a biological molecule of interest. The most commonly used tracer is [18F]fluorodeoxyglucose (FDG), which is taken up by cells in the same way as glucose. C
ancer cells often have a higher metabolism than normal cells, so they accumulate FDG faster. However, tissues with infection or inflammation may have increased metabolic rates as well, so PET alone is not diagnostic. It is used in combination with conventional imaging techniques.
Breast cancer is a paradigm for the potential of PET to improve cancer therapy, according to Dr. Ian N. Fleming and colleagues in UK. “PET currently has a limited role in this indication, but this could expand significantly as a result of ongoing tumour characterisation at the molecular level,” they reported.
Current research on PET applications includes staging, tumor subtyping, and assessing therapy response.
“One of the greatest clinical needs in breast cancer is to find an early and accurate way to determine which patients are responding to therapy,” Fleming noted.
PET can measure changes in glucose metabolism in tumors treated by chemotherapy. When the first chemotherapy agent does not work, PET can provide early warning that it's time to change to a different drug.
Newer tracers are in development to expand the role of PET in cancer treatment. PET is not expected to replace the mammogram in screening, but it may become a more common part of breast cancer care.
1. Chen K et al, “Positron emission tomography imaging of cancer biology: current status and future prospects”, Semin Oncol. 2011 Feb; 38(1): 70-86. http://www.ncbi.nlm.nih.gov/pubmed/21362517
2. Fleming I et al, “Opportunities for PET to deliver clinical benefit in cancer: breast cancer as a paradigm”, Cancer Imaging 2010; 10: 144-52.
Linda Fugate is a scientist and writer in Austin, Texas. She has a Ph.D. in Physics and an M.S. in Macromolecular Science and Engineering. Her background includes academic and industrial research in materials science. She currently writes song lyrics and health articles.
Reviewed September 14, 2011
by Michele Blacksberg
Edited by Jody Smith