Vital Signs - December 2001

PET Scans:

The Latest Weapon Against Cancer

by Kathleen Foley

Mrs. X, a fifty-year-old mother of two, had been treated for colon cancer many years ago. Recently, a routine chest X-ray showed a suspicious shadow. A computerized tomography (CT) scan was performed, but the results could not verify whether or not her cancer had recurred. She considered surgery to biopsy the shadow, but opted instead for a PET scan, a new non-invasive diagnostic test. The results of her PET scan were clear - no cancer, and no need for more biopsies or surgery.

What is a PET scan? PET, or Positron Emission Tomography, is a unique method of studying human function, using a special scanner to track microscopic amounts of radioactive material placed into the body. Conventional tests, such as CT scans, ultrasound and MRI, rely on anatomy. For example, a CT scan of a lump on a patient’s neck might be able to tell what the growth looks like, how big it is and what other structures are sitting next to it. Sometimes it might even suggest whether or not the lump looks like cancer. However, separating a cancer from a benign growth is not always possible with a CT scan or an MRI. Biopsies or even surgeries may be necessary to determine whether a growth is cancerous. PET scans, on the other hand, rely on physiology - not what the lump looks like, but how it functions. Using the same example, a PET scan of the lump provides the unique ability to determine, with a very high accuracy, whether or not the lesion is cancerous.

On the day of a PET scan, the patient receives an intravenous injection of the PET agent (a radiolabeled compound very similar to glucose). Most patients also receive a small dose of valium, both for muscular relaxation and for mild sedation. The patient lies on a table for the scan, which takes about 30 minutes. Although glucose is used by all cells in the body, cancerous cells have a much higher rate of metabolism. Because they absorb more of the glucose tracer than neighboring cells, cancer cells are easily seen on the PET scan. The results of the test are generally available on the same day. The quantity of radiation is minimal, similar to the more familiar CT scan, but the potential impact of PET on the medical community is huge.

In addition to detecting new cancers, PET scans have been shown to be extremely useful in monitoring the effectiveness of therapy in patients with a known malignancy - for example, showing whether chemotherapy is working. Since the PET scan works at the molecular level, it can demonstrate cancer improvement before CT or MRI, because these tests rely on larger, anatomic changes.

"PET will make a clear difference in cancer patient care," says Dr. Alan Weissman, director of PET at Desert Radiologists. "PET will not replace CT and MRI, but rather complement these standard anatomic tests. Because of the different mechanisms of the tests, information from both will be extremely useful. I do think that PET will succeed in reducing some unnecessary surgeries and biopsies."

Several years ago, insurance companies began paying for PET scans. Particularly with oncology, the growing body of literature describing the usefulness of PET as a complement to routine CT and MRI testing was compelling. In addition, the PET camera itself underwent revision and refinement, which made outpatient PET scanning a practical and feasible plan. The state-of-the-art LSO crystal PET scan, which was released in 2000, allows for faster and more accurate scanning than ever before. Currently, the only LSO crystal PET scanner in Nevada is operated by Desert Radiologists

Kathleen Foley
Kathleen Foley is a freelance writer based in Southern Nevada.