What Does It Mean to Have a Shadow on the Lung?

Possible causes and evaluation

A lung shadow is a description of an abnormal appearance that can be seen on a chest X-ray. This finding is not actually 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 not have any significance in terms of your health. Consider it the first step toward a diagnosis. While certain distinguishing patterns may help narrow down the cause of your lung shadow, further investigation would be needed before a definitive diagnosis could be made.

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 black, white, and gray.

Patterns include:

  • Dense or solid structures, such as bone, heart, and major blood vessels, appear white
  • Air-filled structures, such as the lungs, will appear black
  • Overlapping structures or material that is moderately solid, will 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, we may refer to them as a lesion, spot, or shadow.

Possible Causes

If your radiologist reports that you have a shadow on your lung, your doctor 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:

  • Overlapping structures, such as organs and blood vessels, can be composited on the image in a way that creates the appearance of a shadow.
  • When fractures, like broken ribs, heal, callous can develop (extra bone on the surface) at the site of the fracture, and the appearance can sometimes resemble a shadow on an X-ray.
  • Hiatal hernia (herniation of the stomach 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, a defect that causes an enlargement of the aorta as it leaves the heart or as it begins to descend through the chest, 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 appear to look like shadows.
  • Pneumothorax (collapsed lung) can cause irregularities on X-ray around the area of the collapse.

X-Rays Can Miss Lung Cancer

Lung cancer is often associated with a lung mass. But a mass might not be present in all cases of lung cancer. In fact, between 12 and 30% of people with lung cancer will have a normal X-ray at the time of diagnosis.

A 2006 study further demonstrated that almost 25% of chest X-rays performed on patients with lung cancer were still negative within the 12 months following diagnosis.

It's also important to note that never smokers can and do develop lung cancer. At the current time, the majority of people who develop lung cancer are non-smokers (either never smokers or former smokers).

For reasons unknown, the incidence of lung cancer in never smokers is increasing in the United States and Europe, particularly in young women.

Tests When Cancer Is Suspected

In the event there is a shadow on your X-ray, your doctor may order a battery of tests to better evaluate the cause.

Among the options:

  • Computerized tomography (CT) scan uses X-ray to make detailed 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 also 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. 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 lung biopsy.

All of these tests have advantages and limitations. Tests such as X-ray, CT, and MRI are structural tests. They can tell us that 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 doctors important information about the molecular characteristics of the mass and whether it is cancerous.

A Word From Verywell

While a shadow on a chest X-ray can be distressing, you shouldn't jump the gun and assume the worst. There can be many reasons for the abnormality and, in some cases, it is only the remnant of a past infection that has long since resolved or an overlap of normal structures found in the chest.

You may need other tests to pinpoint the cause. Even if it is something serious like cancer, catching it early—when it is most treatable—is always a plus.

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Article Sources
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Additional Reading
  • Fishman's Pulmonary Diseases and Disorders, 5th Ed. McGraw-Hill Education. 2015.

  • Long, B.; Rollins, J.; and Smith, B. (2016) Merrill's Atlas of Radiographic Positioning and Procedures,13th Edition. Maryland Heights, Missouri: Mosby/Elsevier.
  • Pass HI. Principles and Practice of Lung Cancer: The Official Reference Text of the IASLC. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2010. Print.
  • Stapley, S.; Sharp, D;. and Hamilton, W. "Negative chest X-rays in primary care patients with lung cancer." Brit J Gen Practice. 2006; 58(529);570-579.