By: Dan Sperling, MD

This is the first in a series of five articles about PET scans and their application in prostate cancer.

The ever-evolving world of medical imaging has changed the world of detection and diagnosis. It has vastly reduced the use of exploratory surgery, in which a surgeon cuts into an area of the body to look for a disease condition such as a cancerous tumor, and possibly take samples (biopsy). Today’s imaging can reveal injury or disease within the body without the need for invasive surgery. In fact, certain imaging techniques can give important clues as to the nature and aggression of cancer even before a biopsy is performed, and help guide a minimum number of biopsy needles precisely into the center of the tumor.

There are several types of imaging that are available almost everywhere in the U.S. The most commonly used include X-rays, ultrasound, computed tomography (CT) scans, magnetic resonance imaging (MRI), and nuclear medicine imaging. Nuclear medicine imaging is sometimes called an “inside out X-ray” because it records radiation coming from a radioactive agent within the body rather than radiation beamed into or through the body, such as X-rays.

The radiation agent is one of a class of drugs called radiopharmaceuticals. These are chemical compounds composed of a radiotracer (a man-made isotope with a low dose of radiation that is short-lived due to rapid decay) bonded with a biologically active molecule. Such molecules are either derived from a substance the body normally uses, such as glucose (sugar), water, etc., or molecules that bind to specific sites in the body. Different biologically active molecules are attracted to different kinds of tissue, so the choice of radiotracer will depend on which organ or system is to be scanned.  The radiotracer is administered either by intravenous injection (into a vein), taking it by mouth, or inhaling an aerosol preparation. As the radiotracer reaches the bloodstream, it travels with the blood flow until it reaches the target tissue where it is taken up and collected, usually within a period of several minutes. (Some types of scans may be performed a day or two after the drug is administered.)

Once the uptake period is complete, the patient is placed on a table under special equipment that pinpoints and records the levels of radioactive emissions and transforms them into images. It is completely noninvasive and painless. Unlike X-rays or ultrasound, which show anatomy, nuclear medicine imaging provides information on how tissue is functioning, not just what it looks like. After the scan, the radiopharmaceutical substance will be naturally eliminated from the body. In some cases, the doctor will give instructions to facilitate this process. The results of the imaging are read and interpreted by a trained radiologist who creates a report for the patient’s doctor.

PET scans

A particular type of nuclear medicine imaging developed in the 1950s – 60s is called a Positron Emission Tomography (PET) scan. Positrons are subatomic particles with a positive charge. Positrons from a man-made radioactive isotope that is administered as a radiotracer indirectly emit gamma rays, which are registered by the “camera.” Sophisticated computer software constructs 3-dimensional (3D) images of the region of interest, revealing functional details about the tissue in which the tracer has concentrated. PET scans are an established source of diagnostic information before cancer treatment. They are especially useful in monitoring how well tissue has responded to cancer treatment, to search for any cancer spread, and to monitor for recurrence (cancer returning).

PET scans can identify the biological pathway of any clinical radiotracer. In the majority of body sites, the most commonly used biological molecule is fluorodeoxyglucose (FDG), a form of sugar that is radioactively labeled with the isotope fluorine-18 (F-18 FDG PET). However, when used to locate prostate cancer it can be confusing, as it may bind with normal tissue or benign gland enlargement in addition to inadequate concentrations in a prostate cancer tumor. Therefore, subsequent articles in this series will explore radiotracers that are specifically used for prostate cancer detection and diagnosis, including choline C-11 and carbon-11-acetate, and discuss studies that have demonstrated their effectiveness.

In addition, there will be content on “fusion” or co-registration imaging such as PET-MRI and PET-CT.