When people who have no symptoms are tested to detect a cancer, that process is called “screening.”
Screening is not a single test, but a process. The initial test results must be carefully evaluated. If cancer is suspected, additional testing will be required to confirm the results. These additional tests may involve invasive procedures that carry certain risk. Anxiety during this process is common.
The chance of developing some type of cancer generally increases over the course of an individual’s lifetime. In the United States, lifetime risk for men is 44% and for women 38%. The goal of cancer screening is to detect cancer before it has spread (metastasized) to other sites in the body, because early localized cancer has a greater chance of being cured.
Screening tests are not available for all cancers.
Why is cancer screening important?
Cancer is the leading cause of death in the United States of people under 85, and the second leading cause of death, after heart disease, for people over 85. Cancer is one of the most expensive diseases to treat, especially when detected at late stage.
Screening increases the chance of being diagnosed at early when cancer can be more successfully treated and even cured. Early stage diagnosis is associated with longer survival rates, as can be seen below with three cancers that have screening tests: Mammography for breast cancer; PSA testing for prostate cancer; and colonoscopy for colon cancer.
Breast Cancer
Stage Distribution and 5-year Relative Survival Rate by Stage at Diagnosis for 2001-2007, All Races, Females
| Stage at Diagnosis | Stage Distribution (%) | 5-year Relative Survival (%) |
| Localized (confirmed to primary site) | 60 | 98.0 |
| Regional (spread to regional lymphnodes) | 33 | 83.6 |
| Distant (cancer has metastacized) | 5 | 23.4 |
| Unknown (unstaged) | 2 | 57.9 |
Prostate cancer
Stage Distribution and 5-year Relative Survival Rate by Stage at Diagnosis for 2001-2007, All Races, Males
| Stage at Diagnosis | Stage Distribution (%) | 5-year Relative Survival (%) |
| Localized (confirmed to primary site) | 82 | 100.0 |
| Regional (spread to regional lymphnodes) | 11 | 100.0 |
| Distant (cancer has metastacized) | 4 | 30.2 |
| Unknown (unstaged) | 3 | 75.0 |
Colon Cancer
Stage Distribution and 5-year Relative Survival Rate by Stage at Diagnosis for 2001-2007, All Races, Both Sexes
| Stage at Diagnosis | Stage Distribution (%) | 5-year Relative Survival (%) |
| Localized (confirmed to primary site) | 39 | 90.4 |
| Regional (spread to regional lymphnodes) | 37 | 69.5 |
| Distant (cancer has metastacized) | 19 | 11.6 |
| Unknown (unstaged) | 5 | 38.3 |
Until now, however, there has been no proven screening test for lung cancer. Symptoms of early stage lung cancer are often not evident or vague. Usually by time a more definitive symptom becomes obvious, such as blood in the sputum, the disease has already progressed to late stage. As can be seen below, the lung cancer has already spread beyond the original site in three out of four new cases.
Lung Cancer
Stage Distribution and 5-year Relative Survival Rate by Stage at Diagnosis for 2001-2007, All Races, Both Sexes
| Stage at Diagnosis | Stage Distribution (%) | 5-year Relative Survival (%) |
| Localized (confirmed to primary site) | 15 | 52.9 |
| Regional (spread to regional lymphnodes) | 22 | 24.0 |
| Distant (cancer has metastacized) | 56 | 3.5 |
| Unknown (unstaged) | 8 | 8.7 |
Overall, the 5-year survival rate for breast cancer is 89%, for prostate cancer 99% and for colon cancer 65%. All three have widely accepted screening tests available to the entire population. The overall 5-year survival rate for lung cancer is 15%, and until now, no screening test for those at high risk has been available.
Examples of cancer screening
Health agencies recommend that all adults over the age of 50 should have a colonoscopy at least once every ten years. All women are encouraged to have regular PAP tests which can screen for cervical cancer, and to have mammograms to screen for breast cancer at least every two years. The question of when to start having mammograms - at age 40 or 50 - continues to be a subject of intense debate.
Whether PSA testing reduces prostate cancer mortality is also still controversial, and there have been conflicting results on trials carried out in the United States (negative) and Europe (positive). But most men have the blood test annually starting at age 50.
No screening test is perfect. Screening will frequently yield abnormalities (a positive screen) that require additional testing and may even go to surgery before cancer is definitely ruled out. These are called “false positives.” In mammography screening, half of all women will have a false positive over the course of ten years of screening.
Alternatively, a screening test could miss a genuine cancer. One in five cancers may be missed by mammography. These are called “false negatives.”
Screening can often lead to biopsies and other invasive testing that carries some risk. Participating in a screening process frequently increases the anxiety level in people undergoing the tests.
The cost and risk involved in screening the large numbers of the adult population at risk for cancers are high but must be weighed against the high cost of late stage treatment and the value of lives saved by early diagnosis, as well as the cost of lives lost.
Research is ongoing to identify genetic factors and biomarkers in the blood, urine, breath or sputum that may indicate the presence of cancer at even earlier stages. These new tools may allow for a more efficient screening process that may reduce costs or side effects.
How does a CT scan work?
Unlike a chest x-ray which produces flat, 2-dimensional images of the lungs, a CT scanner can explore the entire volume of the lungs by taking a continuing series of x-rays in a helical spiral around the chest. The images taken by a CT scanner can be examined on a computer in “slices” and reconstructed in 3-dimensions giving far more detailed information about the volume and shape of lung nodules.
The capability of CT scanners to detect the size and shape of lung nodules, and to compare the nodules and measure any change in size and volume in subsequent scans, is key to the process of screening.
How do you screen for lung cancer?
The only method proven to be effective in finding lung cancer early and reducing the number of lung cancer deaths in people at high risk is computed tomography (CT) screening. In November, 2010, the National Cancer Institute’s (NCI) eight year long, randomized controlled trial of 53,000 person at high risk for lung cancer was halted as soon as 20% fewer deaths occurred among those screened with CT scans than those screened with chest x-rays.
To put this profound benefit in perspective, the overall reduction in breast cancer deaths by mammography screening is estimated to be 15%.
Compared to no screening at all and with the advances in imaging, surgical treatments and diagnostic methods that have occurred since the NCI trial was started in 2002, the actual mortality benefit (number of deaths that can be avoided) of CT screening is probably closer to 50%.
However, as with all screening procedures, there are risks involved. This is discussed in “What are the risks involved.”
Research is ongoing to find biomarkers in the blood, urine, breath or sputum that would give an early indication that lung cancer may be present, or to find a genetic clue that a person is predisposed to lung cancer. Accomplishing this is proving to be more elusive and complicated than initially thought and may take decades to realize, as the Director of the National Cancer Institute noted in his recent book. A recent paper indicated that a single lung cancer in a single patient was found to have 50,000 genetic mutations. Yet there are promising research avenues that may enhance routine detection of early, curable lung cancer.
How did CT screening come about?
In 1991, radiology researchers led by Claudia Henschke, MD, began investigating the potential of computed tomography (CT) scans to identify lung cancer at early stage. Early Lung Cancer Action Program (ELCAP), launched two years later began developing the step by step process (protocol) for screening people at high risk for lung cancer. The dramatic advances in imaging technology that occurred over the next two decades were continuously incorporated into the protocol, as were refinements in the process of identifying cancerous nodules and reducing radiation dose. The program has grown to a multi-national study, International ELCAP or I-ELCAP, with over 55,000 participants at 50 sites in the United States and around the world. Results of this large observational research study, published in the New England Journal of Medicine in 2006, showed that the I-ELCAP protocol for CT screening could lead to ten year survival rates of nearly 80% among those diagnosed with lung cancer.
The I-ELCAP observational trial - a single arm study in which all participants were screened - showed a dramatically improved survival rate. But many were not convinced. Lung cancer screening, they said, could be finding tumors that never would have caused death (over diagnosis) had they been undetected, or could just be finding cancers earlier but not preventing eventual death (lead time bias), or only finding the slow-growing tumors and not the aggressive ones (length time bias). CT screening for lung cancer had to pass the “gold standard” test: a randomized controlled trial, which would compare two groups of patients -one getting a CT scan and the other a chest x-ray - and see if those in the CT arm had fewer lung cancer deaths.
For more information on I-ELCAP, click here.
In 2002, the National Cancer Institute launched the National Lung Screening Trial (NLST) and recruited over 53,000 people, current and former smokers between the ages of 55 and 74. Half were screened with chest x-rays and the other half with CT scans. The endpoint or target was to see if CT screening could reduce the number of lung cancer deaths by at least 20% Each participant received three annual screens starting in 2005 and ending in 2007. Although the NLST was funded to run through 2012, in November, 2010, the trial was stopped as soon as it become evident that there were 20.3% fewer lung cancer deaths (technically referred to as a mortality benefit) among those who were screened by a CT scan. Those in the chest x-ray arm were immediately advised of the significant benefit of CT scans and urged to contact their doctors. The NCI announced the results and held a press conference in November, 2010.
For the more information on the NLST, click here: www.cancer.gov.
To put the 20.3% figure in context, the overall mortality benefit of mammography screening for breast cancer is 15%. PSA testing for prostate cancer failed to show any mortality benefit in U.S, randomized controlled trials but did show a 20% benefit in a Swedish trial.
The actual mortality benefit of CT screening could be much greater. Recent studies from I-ELCAP suggest that it could be as high as 37-65%.
Since the majority of new lung cancer cases fall into the same category of age and risk as those in the trial, a 20.3% reduction in lung cancer deaths could translate to 25,000 lives saved a year - three times that number if the upper end figure is correct. Given that lung cancer causes nearly one in every three cancer deaths, such a large drop in lung cancer deaths would also bring about the biggest reduction in overall cancer deaths ever achieved.
The NLST also demonstrated that, in addition to 20.3% fewer deaths from lung cancer in just three years, those screened for lung cancer also had 7% fewer deaths from all causes. While the details have not been released yet, I-ELCAP and other studies have shown that CT screening of the chest can help diagnose early heart disease and COPD as well.
Since tobacco-related lung cancer, heart disease and other lung diseases are the cause of half of all the premature deaths in our society, integrating effective cessation programs with CT screening can profoundly change outcomes and the public health care system.
