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

A chest X-ray creates images of:

• Lungs
• Airways
• Heart
• Blood vessels
• Bones of the chest and spine

It is often the first imaging test a doctor will order if lung or heart disease is suspected. If lung cancer is present, chest X-rays can sometimes detect larger tumors. More often than not, though, they fail to diagnose the disease. Chest X-rays also fall short as a tool for lung cancer screening.

This article looks at chest X-ray and its use for the diagnosis of lung cancer. It also discusses some of the other diagnostic tools a doctor may use if lung cancer is suspected.

Limitations of Chest X-Rays

X-rays use radiation to create two-dimensional images of internal organs. Organs and tissues that absorb more radiation appear white or gray, while materials that absorb less radiation (such as gases in the lungs) appear black.

It is not uncommon to hear stories of lung cancer that was discovered on an X-ray. When this happens, though, the disease usually is in an advanced stage. The cancer was only seen because the tumor was large and hard to miss.

The problem with this is that advanced lung cancer found in stage 3b or stage 4 is more difficult to treat. With rare exception, these advanced lung cancers are incurable. This is why chest X-rays are poor tools for diagnosing lung cancer.

Low Resolution Images

X-ray images appear in shades of gray. They need to be interpreted by a radiologist who is trained to spot abnormalities. Even so, the images are not high-resolution, and it is easy to miss subtle details. Substances like blood, pus, and water, which have a similar density, can be hard to tell apart.

On a chest X-ray, any abnormal growth in the lung will appear as a relatively solid area of light gray. The growth might be cancer, but it could also be any number of benign or noncancerous conditions.

Based on the X-ray alone, all the radiologist can do is describe the growth in general terms, such as:

• Lung nodule: A spot 3 cm or approximately 1.5 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 that could describe anything from a tumor to the overlap of structures in the chest
• Lung neoplasm: A term that describes any form of "new growth" in the lungs, including anything benign
• Lung lesion: Any abnormality, whether benign or cancerous

Obscured Images

Normal body parts like bones can obscure tumors on an X-ray and make them hard to see. This is especially true if they are small. Disease processes can also make cancerous growths hard to see. Pneumonia, for example, commonly occurs with symptomatic lung cancer. If you have pneumonia, the pus and mucus that clog the airways can easily hide a tumor.

On an X-ray, tuberculosis (TB) also looks similar to certain lung cancers. These conditions often occur together. When this happens, TB may be diagnosed while the cancer is missed. Even after the TB infection clears up, doctors may assume remaining spots are scars and leave them uninvestigated.

Missed Diagnoses

In some cases, a patient may be told their chest X-ray is normal only to learn months or years later that they have cancer. In these cases, the cancer may come to light after advanced symptoms appear. These may include:

• Wheezing
• Unintended weight loss
• Coughing up blood

This may seem like negligence, but chest X-rays have fundamental limitations. This is especially true when it comes to detecting certain types and sizes of lung cancer. Your medical team can't diagnose what they can't see.

Cancer Type and Location

Cancers in certain parts of the lungs are harder to see and are more likely to be missed on a chest X-ray. A 2017 study found that 45% to 81% of missed lung cancers occurred in the upper parts of the lung where the collarbone and other structures obscure the view.

Lung adenocarcinoma is the most common form of lung cancer. It usually occurs on the outer edges of the lungs, where it can be hard to spot. This type of cancer is missed more often than cancers that occur near the large airways, such as small cell lung cancer and squamous cell carcinoma of the lungs.

Tumor Size

Tumors smaller than 1.5 cm (.6 inch) are more likely to be missed on a chest X-ray than larger ones.

Lung adenocarcinomas often have a diffuse "ground glass" appearance. These kinds of tumors may also be hard to see on an X-ray.

Risk Factors

Sometimes lung cancers are missed simply because no one was looking for them. Unless there are symptoms or your doctor knows you are at high risk of lung cancer, there may be no reason to suspect cancer.

Only 65% of physicians ask patients if and how much they smoke. Doctors also often fail to ask about a patient's past history of smoking if they say they are a "non-smoker."

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

In addition, many of the symptoms of lung cancer, such as shortness of breath or fatigue can be easily attributed to things like age or obesity.

As a result:

• Females tend to be diagnosed later than males.
• 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 that look at the incidence of missed lung cancer. The research that has been done, though, is sobering.

A 2019 review of 21 studies found that 20% to 23% of chest X-rays in people with lung cancer symptoms were falsely negative for lung cancer. This can be disastrous since it only takes an average of 136 days for a lung tumor to double in size.

Stage 1, stage 2, and stage 3a lung cancers are considered treatable with surgery. If the disease is allowed to progress, it can quickly move from treatable to inoperable. This not only makes it more difficult to manage, it also significantly reduces the patient's survival time.

How Lung Cancer Is Diagnosed

A chest X-ray might "accidentally" find lung cancer during the investigation of another condition. Still, it isn't one of the most common tools used to diagnose lung cancer because of the concerns outlined above.

Instead, healthcare providers generally rely on three tests:

• Computed tomography (CT). This scan takes multiple X-ray images and combines them into three dimensional "slices." This makes it easier to see abnormalities in the lungs.
• Bronchoscopy. During this procedure, a pulmonologist looks at the inside of the airways and lungs with a camera attached to a narrow tube.
• Lung biopsy. Suspicious tissue is extracted using a needle or other methods. This is usually ordered if a CT scan suggests cancer. The results can provide definitive evidence of the disease.

If cancer is diagnosed, other tests will help stage and grade the tumor. This process helps doctors understand how far the cancer has progressed so they can decide on the right treatment.

Positron emission tomography (PET) scans are not usually used for diagnosing lung cancer. They are considered more useful for staging the disease. PET scans don't take snapshots of the body. Instead, they visualize metabolic processes and can help tell the difference between a growing tumor and scar tissue or benign growths.

Lung Cancer Screening

Chest X-rays are also not sensitive or specific enough to screen for lung cancer.

A 2011 study followed 150,000 people at high risk of lung cancer. Researchers found that four years of annual chest X-rays did not change the death rate in the group. Some cancers were detected, but the tumors were generally advanced enough that finding them on X-ray did not change the patient's ultimate outcome.

Instead of chest X-rays, annual low-dose CT scans are recommended for people at high-risk for lung cancer. In this population of adults, screening can significantly reduce the risk of advanced cancer and premature death.

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

Most health officials feel the risks of annual CT screening in other groups outweigh the benefits. A 2019 study, though, suggests otherwise. The study's authors say 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's not clear if annual exposure to low-dose radiation might actually increase the risk of lung cancer over time.

What You Can Do

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 knows about and can access any old chest X-rays you've had. A comparison can often detect subtle changes that might otherwise be overlooked.
• Report symptoms: Make sure all your symptoms and risk factors are on the radiology order form. This can add context to the findings and encourage the radiologist to look for subtle signs.
• Be your own advocate: Don't give up if your respiratory symptoms don't go away but the cause hasn't been found. This especially true if you have only had a chest X-ray. Seek a second opinion and ask if a CT scan is a reasonable option.

Summary

Chest X-rays may occasionally find advanced lung cancers, but they are not good at diagnosing cancer in the early stages. This is because they are low-resolution, and it's easy for small cancers to be obscured behind other structures.

CT scans are generally better at diagnosing lung cancer. They may also be helpful for lung cancer screening in high-risk patients.

If you think you have symptoms of lung cancer, ask your doctor about a CT scan. Don't depend on a chest X-ray for a diagnosis.