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

Abnormal X-rays are sometimes serious and sometimes not

Nurse examining chest x-rays in hospital
Paul Bradbury / Getty Images

One of the more unnerving yet all-too-common findings on a chest X-ray is something called a shadow on the lung. While we may assume it to mean something serious, the finding is actually not a diagnosis but rather an observation of an abnormality which is not clearly defined and needs further investigation.

While certain distinguishing patterns may suggest what it is, further tests would be needed before a definitive diagnosis could be made.

In the end, a shadow on the lung may be something serious or may mean nothing at all. Consider it the first step toward a diagnosis.

Reading an X-ray or Scan

It may be helpful to think of radiology studies (which include X-ray, CT scan, and MRI) as pictures in shades of black, white, and gray. Dense or solid structures such as bone, heart, and major blood vessels appear white. By contrast, air-filled structures such as the lungs will appear black. Overlapping structures or anything in between will appear in shades of gray.

Radiology scans are sometimes difficult to read because structures do overlap, and, even if you spot an abnormality, it can hard to discern what it is. While some abnormalities may be defined structures such as a mass, nodule, or tumor, at other times their appearance may not be so well defined. In such, we may refer to them as a lesion, spot, or shadow.

Possible Causes for a Shadow on the Lung

When a radiologist picks up a shadow on the lung, the doctor will begin to explore the possible causes based on whatever clues or symptoms there may be.

These may include the person's medical history, family history, lab tests, and factors like smoking or exposure to occupational toxins.

Among the possible causes:

  • Overlapping structures, such as organs and blood vessels, may be composited on the image in such way as to create a shadow.
  • Broken ribs can sometimes be mistaken for a mass on an X-ray.
  • Hiatal hernias (the herniation of the stomach into the chest cavity) can appear as a poorly defined abnormality on a chest X-ray.
  • Pneumonia is the infection of the air sacs of the lungs which often appears patchy or opaque on X-rays.
  • Pleural effusion is the appearance of fluid in the layer between the lungs and chest wall.
  • Pulmonary edema is a condition involving the accumulation of fluid in the lungs, often due to heart disease.
  • Aortic aneurysm (the enlargement of the aorta the heart) can cause a shadow on chest X-rays.
  • Lung cancer may appear as a shadow without a well-defined nodule or mass.
  • Benign tumors may similarly appear on an X-ray as a shadow or spot.
  • Tuberculosis is a bacterial infection of the lungs which often has no discernible features on X-rays in early disease.
  • Sarcoidosis is an inflammatory disease affecting multiple organs which cause the development of granulomas (granulated tissue).
  • Pneumothorax, better known as a collapsed lung, can cause irregularities on X-ray around the area of the collapse.

X-Rays Fall Short in Diagnosing Lung Short

When we think about cancer, we often picture a mass and expect to see that on an X-ray. In many cases, that just doesn't happen. In fact, between 12 percent and 30 percent of people with lung cancer will have a perfectly normal X-ray at the time of diagnosis.

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

The simple fact is that X-rays can miss lung cancer and, for this reason, are not used as a screening tool. 

Tests Performed If Cancer Is Suspected

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

  • Computed tomography (CT scan) uses X-ray to make detailed pictures of your lungs as it scans the entire chest area.
  • Magnetic resonance imaging (MRI) uses magnetic waves to create images. It distinguishes soft tissues better than a CT scan and is able to determine whether the malignancy has spread beyond the lungs and chest wall.
  • Positron emission tomography (PET scan) is an imaging test which shows the metabolic activity of a cell. Those that are hyperactive, like cancer cells, are more readily identified with this tool.
  • Bronchoscopy is a procedure in which a scope is inserted into lungs to make a visual evaluation.
  • 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.

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 might even the remnant of a past infection that has long since resolved.

Work with your doctor to pinpoint the cause, and, even if is something serious like cancer, catching it early—when it is most treatable—is always a plus.

Sources:

Long, B.; Rollins, J.; and Smith, B. (2016) Merrill's Atlas of Radiographic Positioning and Procedures,13th Edition. Maryland Heights, Missouri: Mosby/Elsevier.

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.

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