Advances in Early Detection of Lung Cancer
For some types of cancer, there are screening tests to detect small tumors while they're still potentially curable. Breast exams 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 15% of these patients will survive more than five years.
There are two tests that have been used in the past as early tests to look for lung cancer in people who have symptoms. 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 most doctors have not recommended lung cancer screening with these tests for the general population or even for people at higher risk, such as cigarette smokers.
The good news is that newer screening and diagnostic tests may 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.
Low-dose CT is more sensitive at detecting lung cancer than chest X-rays. 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.
While neither sputum cytology nor chest X-rays were found to reduce the risk of death from lung cancer, a recent large study found that spiral CT scans may indeed lower this risk. The National Lung Screening Trial (NLST) was a large clinical trial run by the National Cancer Institute. It compared spiral CT scans with chest X-rays in more than 50,000 people aged 55 to 74 who were current or former smokers with at least a 30 pack-year history of smoking (equal to smoking a pack a day for 30 years). People in the study got either yearly spiral CT scans or chest X-rays for three years. They were then followed for several years to see how many in each group died of lung cancer.
The study found that people who got spiral CT scans had a 20% lower chance of dying from lung cancer than those who got chest X-rays. But there are some questions that still need to be answered. For example, it's not clear if screening with spiral CT scans would have the same effect in people who smoked less (or not at all) or people younger than age 55. It's also not clear how often the scans should be done or how long they should be continued.
Spiral CT scans may also have some downsides that need to be considered. They also find a lot of abnormalities that turn out not to be cancer but that still need further testing to be sure. (About one out of four people in the NLST had such a finding.) This may lead to anxiety and possibly unnecessary tests such as more CT scans, or even more invasive tests such as biopsies or surgery in some people, when they don't have lung cancer. Spiral CT scans also expose people to a small amount of radiation during each test.
Some medical organizations, such as the American Society for Clinical Oncology, now recommend that doctors offer yearly spiral CT scans to people who would have met the entry criteria for the NLST. Other groups have not yet come out with screening guidelines since the publication of the study.
If you have risk factors for lung cancer, such as a history of smoking, talk with your doctor about whether you should consider using this test. The possible benefits, risks, and limitations of testing need to be taken into account when considering whether or not screening with spiral CT scans is right for you.
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 (for example, the bronchi) with a flexible fiber optic 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.
On the monitor, normal tissue appears green and cancerous tissue appears brown. With fluorescence bronchoscopy, a doctor can also take samples of abnormal tissue to confirm if it is cancerous.
Fluorescence bronchoscopy is able to find some early lung cancers that might be missed on routine bronchoscopy or other tests. It is now used at many cancer centers worldwide for the diagnosis of lung cancer. A limitation, however, is that the fiber optic tube does not fit into the smaller air passages located deep within the lungs. As a result, it would not be able to find cancers that are out of the fiber optic tube's reach.
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. Some might even be detectable in the blood. This could lead to the development of simple genetic screening tests.
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 shows the most promise as a screening test. 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.