New Tests for Early Detection... Health Article

For some types of cancer, there are screening tests to detect small tumors while they're still potentially curable. Breast self-examination and yearly mammograms, for example, often help detect breast cancer before it has spread to other areas of the body. For many cancers, however, reliable and effective tests for early detection still don't exist. The lack of effective methods for early detection of lung cancer is one of the reasons that most people diagnosed with lung cancer have advanced disease, and fewer than fifteen percent of these patients will survive more than five years.

There are two tests commonly used in the diagnosis of lung cancer. One is a chest X-ray. The other is a sputum cytology test, which involves collecting the mucus coughed up and looking at it under a microscope for cancer cells. Both of these tests have been studied extensively for use as screening tests for early detection, but neither approach has been proven to reduce the number of lung cancer deaths. As a result, the American Cancer Society and many oncologists do not recommend lung cancer screening for the general population or cigarette smokers.

The good news is that newer screening and diagnostic tests may prove to be more effective. If lung cancer can be detected earlier and more accurately, this may help reduce the number of people who die from the disease. Promising tests include low-dose spiral CT scanning, fluorescence bronchoscopy, and genetic screening.

Low-dose Spiral Computed Tomography

Low-dose spiral computed tomography (CT) scanning of the chest is one of the most promising new tests for detecting small, early lung cancers. CT scanners rotate a thin X-ray beam around the chest in a spiral pattern. A computer interprets the X-ray images and produces many thin cross-sectional views of the lungs. The low-dose screening CT differs from a standard CT scan in two major ways: it uses lower X-ray doses and does not require the use of special contrast agents injected into the blood to enhance the contrast of the images. As a result, the radiation dose, cost, and time required to perform a low-dose spiral CT scan are close to those for a standard chest X-ray.¹

Researchers have compared low-dose spiral CT to chest X-ray for lung cancer screening in both the general population and cigarette smokers. Within the general population, low-dose CT is about ten times more sensitive at detecting lung cancer than chest X-rays. Among high-risk cigarette smokers, it is about four times more sensitive. (The higher sensitivity in the general population may be due to the lower incidence of lung cancer in this population.) In addition, tumors detected by low-dose spiral CT are generally smaller and more likely to be surgically removed than those detected by chest X-ray. However, scientists still do not know if the increase in screening sensitivity of low-dose spiral CT will improve the survival of those diagnosed with lung cancer. Research is currently ongoing to determine if this new test has the potential to reduce deaths from lung cancer.

Information about the National Lung Screening Trial, which compares spiral CT scans to chest X-rays, can be obtained by calling 1-800-4-CANCER.

Fluorescence Bronchoscopy

Fluorescence bronchoscopy is a second test that shows promise in the early detection of lung cancers. What is fluorescence bronchoscopy and how does it differ from “standard” bronchoscopy? Standard bronchoscopy, also known as white light bronchoscopy, is examination of the air passages in the lungs (i.e., the bronchi) with a flexible fiberoptic tube that displays images on a video monitor. White light bronchoscopy is commonly used to evaluate the extent of lung cancer; it is not, however, a very sensitive test for finding lung cancer.

Fluorescence bronchoscopy is a relatively new diagnostic test that causes cancerous tissues to glow, or fluoresce, and thus look different from normal tissues. To picture how this might work, think about how a standard “black” light bulb causes light-colored fabrics or materials to glow in a darkened room.

The newest technique of fluorescence bronchoscopy is performed with a special piece of equipment known as a light imaging fluorescence endoscope, or LIFE for short. Developed during the 1990s, the LIFE system pairs a helium-cadmium laser with a standard white light bronchoscope. The laser light and white light alternate in a strobe-like manner. The laser light, which is invisible to the naked eye, causes cancerous tissues to light up, or fluoresce. This fluorescence is then enhanced by a computer and shown on a video monitor in real time. On the monitor, normal tissue appears green and cancerous tissue appears brown. The white light view of the LIFE system, which is displayed on the monitor next to the laser view for easy comparison, shows the actual appearance of the air passages and allows the doctor to easily move the fiberoptic tube throughout the lungs. With the LIFE system, a doctor can also take samples of abnormal tissue to confirm if it is cancerous.

Recently published reports show that the LIFE system is about 50 percent more sensitive than standard bronchoscopy at finding early lung cancers. LIFE has also found small tumors in the lung not seen by chest X-ray or CT scan. The LIFE system is now used routinely at about 35 cancer centers worldwide for the diagnosis of lung cancer. A limitation of LIFE, however, is that the fiberoptic tube does not fit into the smaller air passages located deep within the lungs. As a result, LIFE would not be able to find cancers that are out of the fiberoptic tube's reach.

Genetic Screening

Finding early lung cancers by low-dose spiral CT and fluorescence bronchoscopy may eventually save lives, but ongoing research in DNA sequencing and human genetics are setting the stage for a different diagnostic test—genetic screening. Scientists have learned that certain changes, or mutations, in the DNA of normal cells lead to the development of cancer. This knowledge may one day allow scientists to reliably look at tissue or mucus samples for genetic changes linked to lung cancer.

The challenge with genetic screening lies in knowing which DNA changes are linked to most types of lung cancer. Several such changes are known and can currently be detected by genetic tests, but they only appear in a fraction of lung cancers. Also, some mutations may not appear until late in the course of disease, rather than early when a chance of cure is still possible. Scientists are currently working to find the genetic changes that appear early in the development of lung cancer. Once these are known, genetic screening may become a valuable diagnostic tool.

Another problem with genetic screening is finding cancerous or precancerous cells and tissues to test. Although still useful for diagnosis, as a screening test, it would not be practical if doctors first had to locate small tumors by chest X-ray, CT, or bronchoscopy, and then take a tissue sample to test for DNA changes. However, some genetic changes may be found in mucus after coughing. This could lead to the development of simple genetic screening tests. “An ideal situation would be to have a blood test,” says Vikas Kundra, MD, PhD, a radiologist and imaging researcher at Brigham and Women's Hospital in Boston. “For example, if one could detect cancer cells or cancer cell components shed into the bloodstream, or genetic alterations that portend cancer, then genetic screening would be a true screen for early detection.”

Future Hope?

Will these new diagnostic tests ever receive recommendation for widespread use in the early detection and screening of lung cancer? To be recommended on a widespread basis, any cancer screening test must reduce the number of people dying from that cancer, and should be relatively inexpensive and easy to perform. As mentioned earlier, a good example of an effective test is the mammogram for breast cancer screening.

Of the lung cancer tests described above, low-dose spiral CT scanning has received the most attention in the medical literature. Whether it reduces lung cancer deaths, however, has yet to be proven. Fluorescence bronchoscopy may become a standard diagnostic test, but it may be too expensive, invasive, and time-consuming for widespread use as a screening test. In the future, genetic screening may be valuable, but for now, it's considered strictly experimental.