Importance of Positron Emission Tomography (PET-CT) Scans in Cancer

A PET scan is a diagnostic test. It uses a radiotracer to measure important body functions such as blood flow, oxygen use, and sugar (glucose) metabolism

PET utilises short-lived radioactive isotopes; the PET camera detects the gamma rays given off at the site where a positron emitted from the radioactive substance collides with an electron in the tissue. Through a number of stages the PET camera enables the gamma rays to be converted to electrical signals that are processed by a computer to generate images; a series of many thin “slice” images of the body can then be assembled into a three dimensional representation of the patient’s body.

PET provides images of blood flow or other biochemical functions. PET is increasingly used in the detection and staging of cancers, particularly in metastatic spread, where glucose metabolism enables metastases to be readily identified

Through the use of PET scanning, the use of nuclear medicine for diagnosis of cancer and for identifying metastatic spread is increasing rapidly

Value of PET: major tumor types that PET can be useful

Depending on your clinical situation, PET scanning has been proven to be critical in the following tumor types:

How PET works

Cancer cells require a great deal of sugar, or glucose, to have enough energy to grow. PET scanning utilizes a radioactive molecule that is similar to glucose, called fluorodeoxyglucose (FDG). FDG accumulates within malignant cells because of their high rate of glucose metabolism. Once injected with this agent, the patient is imaged on the whole body PET scanner to reveal cancer growths which may have been overlooked or difficult to characterize by conventional CT, X-Ray, or MRI.

What are some common uses of the procedure?

PET scans are performed to:

  • detect cancer.
  • determine whether a cancer has spread in the body.
  • assess the effectiveness of a treatment plan, such as cancer therapy.
  • determine if a cancer has returned after treatment.
  • evaluate brain abnormalities, such as tumors, memory disorders, seizures and other central nervous system disorders.
  • map normal human brain and heart function.

How should I prepare for a PET and PET/CT scan?

Women should always inform their physician or technologist if there is any possibility that they are pregnant or if they are breastfeeding. 

You should inform your physician and the technologist performing your exam of any medications you are taking, including vitamins and herbal supplements. You should also inform them if you have any allergies and about recent illnesses or other medical conditions.

You will receive specific instructions based on the type of PET scan you are undergoing. Diabetic patients will receive special instructions to prepare for this exam.

If you are breastfeeding at the time of the exam, you should ask your radiologist or the doctor ordering the exam how to proceed. It may help to pump breast milk ahead of time and keep it on hand for use after the PET radiopharmaceutical and CT contrast material are no longer in your body.

Metal objects including jewelry, eyeglasses, dentures and hairpins may affect the CT images and should be left at home or removed prior to your exam. You may also be asked to remove hearing aids and removable dental work.

Generally, you will be asked not to eat anything for several hours before a whole body PET/CT scan since eating may alter the distribution of the PET tracer in your body and can lead to a suboptimal scan. This could require the scan to be repeated on another day, so following instructions regarding eating is very important. You should not drink any liquids containing sugars or calories for several hours before the scan. Instead, you are encouraged to drink water. If you are diabetic, you may be given special instructions. You should inform your physician of any medications you are taking and if you have any allergies, especially to contrast materials, iodine, or seafood.

You will be asked and checked for any conditions that you may have that may increase the risk of receiving intravenous contrast material

What are the benefits vs. risks?

Benefits

  • Nuclear medicine examinations provide unique information—including details on both function and anatomic structure of the body that is often unattainable using other imaging procedures.
  • For many diseases, nuclear medicine scans yield the most useful information needed to make a diagnosis or to determine appropriate treatment, if any.
  • Nuclear medicine is less expensive and may yield more precise information than exploratory surgery.
  • By identifying changes in the body at the cellular level, PET imaging may detect the early onset of disease before it is evident on other imaging tests such as CT or MRI.

Risks

  • Because the doses of radiotracer administered are small, diagnostic nuclear medicine procedures result in relatively low radiation exposure to the patient, acceptable for diagnostic exams. Thus, the radiation risk is very low compared with the potential benefits.
  • Nuclear medicine diagnostic procedures have been used for more than five decades, and there are no known long-term adverse effects from such low-dose exposure.
  • The risks of the treatment are always weighed against the potential benefits for nuclear medicine therapeutic procedures. You will be informed of all significant risks prior to the treatment and have an opportunity to ask questions.
  • Allergic reactions to radiopharmaceuticals may occur but are extremely rare and are usually mild. Nevertheless, you should inform the nuclear medicine personnel of any allergies you may have or other problems that may have occurred during a previous nuclear medicine exam.
  • Injection of the radiotracer may cause slight pain and redness which should rapidly resolve.
  • Women should always inform their physician or radiology technologist if there is any possibility that they are pregnant or if they are breastfeeding.

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