How a PET Scan Can Diagnose Lung Cancer

When and Why are PET Scans Used for Lung Cancer?

images of a PET scan of the brain
A PET scan can help in the diagnosis and staging of lung cancer. Photo©wenht

Definition: PET Scan (Positron Emission Tomography)

A PET scan is a radiological test used to evaluate and stage lung cancer and is often used along with a CT scan.

How is a PET Scan Different From Other Tests?

Whereas CT scans and MRI look at the body's anatomy (bones, organs and other tissues,) PET scans look at how the body functions (how these organs and tissues are working.) Whereas CT and MRI are considered structural imaging techniques, PET is considered a molecular imaging technique.

Reasons for a PET Scan

There are several reasons why your doctor may recommend a PET scan.  Some of these include:

  • To diagnose cancer.
  • To see whether an undiagnosed lung nodule appears to be cancerous or instead appears more like scar tissue or a non-cancerous process. In some cases, a PET scan can determine the difference between a benign and malignant tumor without the need for a lung biopsy.
  • To stage cancer by determining how far a cancer has spread. A PET scan is more sensitive than CT in detecting metastases in non-small cell lung cancer and is a useful test in distinguishing between extensive and limited small cell lung cancer. PET scans are also very good at evaluating lymph nodes which are positive for cancer. For example, a PET scan can sometimes diagnose lymph nodes in the mediastinum without the need to do a mediastinoscopy (a surgical procedure used to evaluate this region.)
  • To plan the best treatment options for a particular lung cancer.
  • To see how treatment is working.
  • To monitor for a recurrence of lung cancer. For example, it may be hard on a CT to determine if an area in the lung is scar tissue as a result of radiation therapy or instead, a recurrence of a tumor.
  • To help determine prognosis. Tumors with less uptake of glucose may be less aggressive and have a better prognosis than those with greater uptake.

With lung cancer, a PET scan is often done in patients with early stage lung cancer in which surgery may be done with the intent to cure the disease. For people who have lung cancers which have spread beyond a certain degree, surgery is not the best option. In one study it was found that in 1 in 5 people who had cancers that appeared to be curable, a PET scan was able to diagnose spread of cancer that was otherwise unknown so that unnecessary surgery did not take place.

PET Scan Procedure

With a PET scan, a small amount of radioactive sugar (fluorodeoxyglucose or FDG) is injected into the bloodstream. Growing cells use sugar. Rapidly growing cells such as cancer cells take up the sugar and can be seen on 3-dimensional imaging.

Before a PET scan, people are often instructed to avoid eating or drinking (especially eating sugary foods) for a period of time and to reduce exercise for 24 hours. The radioactive sugar (fluorodeoxyglucose or FDG) is injected, and a patient waits for an hour for the body to absorb the sugar. Then a scan is performed which takes around 30 to 60 minutes.

It can be frightening to think about a radioactive substance being injected into your body, but in fact, only a small amount of radiation exposure occurs. This radiation decays rapidly after injection, though some oncologists recommend that people stay away from pregnant women and young children on the day the test is performed.

Limitations of PET Scans - False Positives and Negatives

As with most tests done for cancer, there can be both false negatives and false positives on a PET scan.  A false negative occurs when there is an abnormality that is present, but it is not detected on a PET scan.  A false positive occurs when a PET scan suggests that something is cancer, but in reality, it is nothing significant, such as scar tissue. Common conditions that cause false positives in PET scans for lung cancer include post-obstructive pneumonia (a pneumonia that occurs in the lungs peripheral to where a tumor is blocking an airway) and silicosis. False positives are often seen in areas in which healing is taking place, such as the sites where surgery has been performed.

In general, PET scanning for lung cancer has a high sensitivity and a low specificity. A high sensitivity means that the test is very good at picking up abnormal findings, and may distinguish benign and malignant areas as small as 1 cm in diameter.  A low specificity means that a finding does not necessarily mean cancer and that processes such as an infection or inflammation may raise unnecesary concern.  

Examples: Even though Jill's CT scan failed to identify lung cancer in tissues other than her lung, her PET scan showed that her lung cancer had spread and that surgery would not be the best treatment for her.

View Article Sources