Use of Chest X-Ray in the Diagnosis of Lung Cancer

A chest X-ray can produce images of your lungs, airways, heart, blood vessels, and bones of the chest and spine. It is often the first imaging test your doctor will order if lung or heart disease is suspected. If lung cancer is involved, chest X-rays can sometimes detect larger tumors—but more often than not fail to diagnose the disease. Chest X-rays also fall short as a tool for lung cancer screening.

Limitations of Chest X-Rays

X-rays use penetrating radiation to produce two-dimensional images of internal organs. Organs and tissues that absorb less X-ray radiation will show up more prominently on the image. This includes the lungs and gasses in them that are more clearly visualized than surrounding tissues.

It is not uncommon to hear that a chest X-ray detected cancer in someone who may or may not have been suspected of having the disease. However, when this occurs, the disease is usually in an advanced stage (i.e., the tumor is large and hard to miss). The problem with this, of course, is that advanced lung cancer (stage 3b and stage 4) is more difficult to treat and, with rare exception, incurable.

When it comes to diagnosing lung cancer, chest X-rays have serious shortcomings that limit their use.

Differentiation of Structures

Chest X-rays are seen in shades of gray and require interpretation by a radiologist trained to spot abnormalities. Even so, the images are not high-resolution, and it is easy to miss subtle details. Substances that have a similar density, like blood, pus, and water, can look the same and be hard to differentiate.

Any abnormal growth in the lung will appear on a chest X-ray as a relatively consolidated area of light gray. While the growth may be cancer, it could also be any number of benign (non-cancerous) conditions.

All doctors and radiologists can do at this point is describe the growth in the most general terms, such as:

• Lung nodule: A spot that is 3 centimeter, cm (1½ inches) or less in diameter
• Lung mass: A spot greater than 3 cm or more in diameter
• Shadow on the lung: A non-precise term meaning anything from a tumor to the overlap of structures in the chest
• Lung neoplasm: A term translated as "new growth" that describes any form of growth in the lungs, including anything benign
• Lung lesion: A term used to describe any abnormality, whether benign or malignant

Obscured Images

Overlapping structures can obscure tumors on an X-ray and make them hard to visualize, especially if they are small.

Disease processes can also obscure cancerous growths. Pneumonia, which commonly occurs with symptomatic lung cancer, can easily conceal a tumor as pus and mucus start to clog the airways.

Tuberculosis (TB) also looks similar to certain lung cancers on X-ray. If these occur together, as they sometimes do, TB may end up being diagnosed and cancer missed. Even after the TB infection resolves, any remaining spots on the lungs may be presumed to be scarring and left uninvestigated.

Missed Diagnoses

Arguably more concerning than misdiagnoses is the number of times a tumor is entirely missed on a chest X-ray.

It is not uncommon for someone to be told that a chest X-ray is normal only to find out, months or years later, that cancer is present. In such cases, this typically only comes to light when advanced symptoms (such as wheezing, unintended weight loss, or the coughing up of blood) develop.

While this may suggest negligence is the sole cause of missed diagnoses, chest X-rays fundamentally have limitations, particularly when it comes to detecting certain types and sizes of lung cancer. Your medical team cannot diagnose what they cannot see.

Cancer Type and Location

Anatomically, cancers in certain parts of the lungs are more difficult to visualize and are more likely to be missed on a chest X-ray.

Research published in Diagnostic and Interventional Radiation reported that 45% to 81% of missed lung cancers occurred in the upper lobes where clavicles and other structures obscure the view.

Cancer found in the periphery of the lungs⁠—such as lung adenocarcinoma, the most common form of lung cancer⁠—are more frequently missed than those that occur centrally near the large airways (such as small cell lung cancer and squamous cell carcinoma of the lungs).

Size

Generally speaking, tumors smaller than 1.5 cm (3/5 inch) are more likely to be missed on a chest X-ray than larger ones.

Tumors that have a diffuse "ground glass appearance"—something often found with lung adenocarcinomas—also increase the risk that they will not be seen.

Risk Factors

Another reason that lung cancers are missed is simply that no one was looking. Unless there are symptoms or your doctor knows that you are at high risk of lung cancer, they may not even think about ordering a chest X-ray.

Only 65% of physicians query a patient about their smoking status and how much they smoke, according to a 2015 study in Substance Abuse Treatment, Prevention, and Policy. Moreover, doctors often fail to question a patient's past smoking history if they describe themself as a "non-smoker."

Never-smokers often fall between the cracks because lung cancer is a less likely cause of respiratory illness in this group (although it should be noted that diagnoses are increasing in this group). Similarly, former smokers are often assumed to be of lesser risk even if their past use was high.

In addition, many of the symptoms of lung cancer, such as shortness of breath or fatigue, are non-specific and easily attributed to things like age or obesity.

As a result:

• Women tend to be diagnosed later than men.
• Non-smokers are diagnosed later than smokers.
• Young people are diagnosed later than older adults.

How Often X-Rays Miss Lung Cancer

There are surprisingly few recent studies looking at the actual incidence of missed diagnoses of lung cancer, but the research that has been done is sobering.

A review of 21 studies published in the British Journal of General Practice found that 20% to 23% of chest X-rays in people who had symptoms of lung cancer were falsely negative for lung cancer.

This can be disastrous given that it only takes 136 days, on average, for a lung malignancy to double in size.

If the disease is allowed to progress silently with few if any symptoms, it can quickly move from being treatable (stage 1, stage 2, and stage 3a) to being inoperable. This not only makes the disease more difficult to manage but significantly reduces a person's survival time.

How Lung Cancer Is Diagnosed

Even though a chest X-ray might "accidentally" detect lung cancer during a routine exam (or the investigation of another disease), it is not one of the tools commonly used in the diagnosis of lung cancer because of the concerns outlined here.

Instead, doctors will generally rely on three tests for preliminary investigation:

• Computed tomography (CT) scans take multiple X-ray images and combine them in dimensional "slices" so that abnormalities in the lungs can be seen more clearly.
• Sputum cytology involves coughing up phlegm so that the sample can be checked for cancer cells in the lab. A negative sputum test does not exclude cancer as a cause.
• Lung biopsy, involving the extraction of suspicious tissues by needle or other methods, is ordered if a CT scan suggests cancer. This can provide definitive evidence of the disease.

If cancer is diagnosed, other tests will be performed to stage and grade the tumor so that the appropriate treatment can be delivered.

Positron emission tomography (PET) scans are less commonly used for the initial diagnosis of lung cancer and are considered more useful for the staging of the disease. Rather than taking snapshots of the body, PET scans visualize metabolic processes in the body and can help differentiate a growing tumor from inert scar tissue or benign growths.

Lung Cancer Screening

In the same way that chest X-rays aren’t sensitive or specific enough to diagnose lung cancer accurately, they are not an effective means of screening for lung cancer either.

A study published in JAMA involving 150,000 people at high risk of lung cancer reported that four years of annual chest X-rays did nothing to alter the death rate in the group. Although some cancers were detected, the tumors were generally advanced enough that they did not change the ultimate outcome.

In place of chest X-rays, annual low-dose CT scans are recommended in high-risk individuals. It is in this population of adults that screening can significantly reduce the risk of advanced malignancy and premature death.

Used according to these guidelines, CT screening could reduce the lung cancer death rate by 20% in the United States.

Although the general consensus among health officials is that the risks of annual CT screening in other groups outweigh the benefits, a 2019 study in the Journal of Thoracic Oncology suggests otherwise.

According to the researchers, low-dose CT screening in non-smokers detected a significant number of cancers in the early stages that would have otherwise been missed. On the downside, it is unclear if annual exposure to low-dose radiation might actually increase the risk of lung cancer over time.

What You Can Do

As stunning as some of these statistics can be, there are things you can do to reduce your risk of a missed lung cancer diagnosis:

• Compare results: If you have a chest X-ray, make sure your doctor is aware of—and has available for comparison—any old chest X-rays you have had. The comparison can often detect subtle changes that might otherwise be overlooked.
• Report symptoms: Make sure that all of your symptoms and risk factors are listed on the radiology order form. This can add context to the findings and encourage the radiologist to take a second look at subtle signs.
• Be your own advocate: Don't give up if respiratory symptoms persist but the cause is not found in the preliminary investigation. This especially true if you have only had a chest X-ray. If needed, seek a second opinion and ask if a CT scan is a reasonable option.

A Word From Verywell

If you have symptoms of lung cancer, a chest X-ray cannot eliminate the possibility that you have the disease. As reassuring as a "normal" result may seem, don't allow it to give you a false sense of security if the cause of persistent symptoms remains unknown or if the diagnosis you were given can't explain them. This is even true for never-smokers in whom lung cancer is the sixth leading cause of cancer-related deaths in the United States.

If you are at high risk for lung cancer, ask your doctor about annual low-dose CT screening. If you meet the USPSTF criteria, the cost of the screening may be fully or partially covered.