What Does It Mean to Have a Shadow on the Lung?
Possible causes and evaluation
Medically reviewed by Doru Paul, MD
A lung "shadow" is a description of something abnormal that can be seen on a chest X-ray. This finding is not considered a diagnosis, but rather an observation that could be caused by several different lung conditions.
A shadow on the lung can be a sign of something serious, or it may have no effect on your health. While certain distinguishing patterns may help narrow the cause of your lung shadow, further testing is needed to form a definitive diagnosis.
This article explores the many possible reasons why a shadow might appear on a lung. It also describes the imaging tests that might be used to make a diagnosis.
Verywell / Ellen Lindner
Reading an X-Ray or CT Scan
Diagnostic imaging studies, which include X-ray, computerized tomography (CT) scan, magnetic resonance imaging (MRI), and ultrasound, are pictures in shades of white, black, and gray.
Patterns on the images include:
Dense or solid structures, such as bone, heart, and major blood vessels, appear white.
Air-filled structures, such as the lungs, appear black.
Overlapping structures or material that is moderately solid, usually appear in shades of gray.
Typically, the structures seen in these radiology studies overlap. While some abnormalities may be defined structures—such as a mass, nodule, or tumor—their appearance may not always be so well defined. As such, they may be referred to as lesions, spots, or shadows.
Possible Causes
If your radiologist reports that you have a shadow on your lung, your healthcare provider will begin to explore the possible causes based on your symptoms and other health issues.
Helpful clues include your medical history, family history, lab tests, and factors like smoking or exposure to occupational toxins.
Among the possible causes of your lung shadow are:
Overlapping structures, such as organs and blood vessels, can appear on the image in a way that creates the appearance of a shadow.
When fractures like broken ribs heal, a callous (extra bone on the surface) can develop at the site of the fracture. Your physician may spell it "callus," which is also correct. Either way, the appearance can sometimes resemble a shadow on an X-ray.
Hiatal hernia (when the stomach pushes up into the chest cavity) can appear as a poorly defined abnormality on a chest X-ray.
Pneumonia is an infection of the air sacs of the lungs that often produces a patchy or opaque appearance on X-rays.
Pleural effusion is fluid in the layer between the lungs and chest wall, and it can cause a cloudy appearance on X-rays.
Pulmonary edema is an accumulation of fluid in the lungs, often due to heart disease.
An aortic aneurysm is a defect that causes an enlargement of the aorta as it leaves the heart or as it begins to descend through the chest. It can cause a shadow on chest X-rays.
Lung cancer may appear as a shadow, with or without a well-defined nodule or mass.
Benign tumors may similarly appear as a shadow or a spot.
Tuberculosis is a bacterial infection of the lungs that often has no discernible features on X-rays in early disease but can cause the appearance of a shadow.
Sarcoidosis is a multi-organ inflammatory disease characterized by granulomas (granulated tissue) in the lungs that can resemble shadows.
Pneumothorax (collapsed lung) can cause irregularities on an X-ray around the area of the collapse.
COVID-19 may cause abnormal lung CT scans. A study conducted in the early days of the COVID-19 pandemic found that up to 95% of hospitalized patients had "patchy shadows" on their CT scans at discharge.
X-Rays Can Miss Lung Cancer
Lung cancer is often associated with a lung mass. Still, a mass might not be visible on X-ray in all cases of lung cancer, especially in the early stages.
A systematic review study found that X-rays may miss more than 20% of lung cancer cases in people who have symptoms and are later diagnosed with lung cancer. Missed cancers are often located in the upper parts of the lung or behind ribs where they are difficult to see. It is also important to remember that some of these people may not have had cancer yet when they were initially scanned.
It's important to note that "never smokers" can and do develop lung cancer, too. Never-smokers are those people who have never smoked or have smoked fewer than 100 cigarettes in their lifetime.
Today, as many as 20% of the people who die from lung cancer are never smokers, according to the American Cancer Society. Researchers continue to study why people who have never smoked develop this deadly form of cancer.
Related: Why Is Lung Cancer Increasing in Never Smokers?
Tests When Cancer Is Suspected
If there is a shadow on your X-ray and your healthcare provider suspects cancer, they may order a battery of tests to home in on the cause. Among the options:
Computerized tomography (CT) scan uses multiple X-rays to make detailed 3-D pictures of your lungs as it scans the entire chest area. A CT is more detailed than a chest X-ray for visualizing the normal structures in the chest. And it can detect abnormalities that are too small to be seen on a chest X-ray.
Magnetic resonance imaging (MRI) uses magnetic waves to create images. It often distinguishes soft tissues better than a CT scan.
Positron emission tomography (PET scan) is an imaging test that shows the metabolic activity of a cell (essentially, how much energy it is using). Those that are hyperactive, like cancer cells, are more readily identified with this tool. PET scans can also help determine if lung cancer has spread beyond the lungs to other regions of the body.
Bronchoscopy is a procedure in which a camera-equipped scope is inserted into the lungs for a visual evaluation. It is most helpful in diagnosing lung tumors that lie in or near the major airways (bronchi) of the lungs.
Lung biopsy is the removal of a tissue sample for evaluation. It may be done as part of the bronchoscopic procedure, by inserting a needle into the chest cavity, or with an open incision.
All of these tests have advantages and limitations. Tests such as X-ray, CT, and MRI are structural tests. They can tell an abnormality is present, but don't provide information about the lesion's activity.
A PET scan can assess levels of metabolic activity. Most cancers have increased metabolic activity, which is why PET scans are useful in oncology.
However, a PET scan detects the increased metabolic activity associated with active cell growth, which can also be caused by inflammation or infection. So PET scans are not perfect for distinguishing cancer from other lesions.
Even with both structural and functional imaging tests, a biopsy might be needed to confirm or rule out a diagnosis.
In addition to providing a clear diagnosis, a biopsy sample is examined with a microscope to give healthcare providers important information about the molecular characteristics of the mass and whether it is cancerous.
Does lung cancer show up in blood tests?
Blood tests can't be used to diagnose lung cancer. In people who have already been diagnosed, blood tests can help identify certain genetic mutations that may guide treatment.
Related: How Lung Cancer is Diagnosed
Summary
A shadow on the lung can be anything from cancer to an overlap of completely normal structures. This is quite a range, and your healthcare provider will work with you to pinpoint the cause.
Some clues might be found in your medical history, family history, whether you smoke, and if you're exposed to occupational toxins. If your provider does suspect cancer—or at least cannot rule it out—they may order a number of tests, including a CT scan, MRI, or PET scan. A biopsy may be needed to make a sure diagnosis.
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