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

Possible causes and evaluation

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 CT Scan

It may be helpful to think of radiology studies (which include X-ray, CT scan, MRI, and ultrasound) 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

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 a way as to create a shadow.
  • Broken ribs can sometimes be mistaken for a mass on an X-ray. When fractures heal, extra bone is deposited on the surface at the site of the fracture (referred to as callous), and this can sometimes resemble a nodule.
  • 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 (an enlargement of the aorta as it leaves the heart or 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 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) in the lungs.
  • Pneumothorax, better known as a collapsed lung, can cause irregularities on X-ray around the area of the collapse.

X-Rays Can Miss Lung Cancer

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% and 30% 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% 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, and in fact, at the current time, the majority of people who develop lung cancer are non-smokers (they are 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 and cancer is suspected, 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 much more accurate than a chest X-ray both in discriminating the normal structures in the chest, and can also find 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 a lung cancer has spread beyond the lungs to other regions of the body.
  • Bronchoscopy is a procedure in which a scope is inserted into the lungs to make 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.

With these tests, there are two important principles to point out. Tests such as X-ray, CT, and MRI are "structural" tests. They can tell us that an abnormality is present, but tell us little about what that abnormality may be.

PET scans, in contrast, are "functional" tests. When combined with CT they not only tell us if a lesion is present but if that lesion is actively growing.

This becomes much more important if a person has scars in their lungs from previous radiation therapy, a previous case of pneumonia, or even a previous fungal infection of which they were unaware (such as coccidiomycosis and others).

Even with both structural and functional imaging tests, a biopsy is often needed to confirm or rule out a diagnosis. In addition to providing a clear diagnosis, a biopsy can give doctors important information about both what is seen under the microscope and molecular characteristics of the mass if it is a tumor.

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.

Work with your doctor to pinpoint the cause. Even if it is something serious like cancer, catching it early—when it is most treatable—is always a plus. Certainly, most people who hear the word "shadow on the lung" fear lung cancer.

What the general public is less aware of, however, is that treatments for lung cancer are improving and survival rates are increasing. Even with advanced-stage lung cancers, some of these tumors can be kept in check for a lengthy period of time due to treatments such as targeted therapies and immunotherapy drugs.

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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.