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

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

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.
  • 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 in 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 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.

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:

  • 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 the 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.

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 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

  1. Miller A. Practical approach to lung ultrasoundBJA Education. 2016;16(2):39-45. doi:10.1093/bjaceaccp/mkv012

  2. Stapley S, Sharp D, Hamilton W. Negative chest X-rays in primary care patients with lung cancer. Br J Gen Pract. 2006;56(529):570-3.

  3. Flechsig P, Mehndiratta A, Haberkorn U, Kratochwil C, Giesel FL. PET/MRI and PET/CT in Lung Lesions and Thoracic Malignancies. Semin Nucl Med. 2015;45(4):268-81. doi:10.1053/j.semnuclmed.2015.03.004

  4. Collins LG, Haines C, Perkel R, Enck RE. Lung cancer: diagnosis and management. Am Fam Physician. 2007;75(1):56-63.

  5. Winokur RS, Pua BB, Sullivan BW, Madoff DC. Percutaneous lung biopsy: technique, efficacy, and complications. Semin Intervent Radiol. 2013;30(2):121-7. doi:10.1055/s-0033-1342952

Additional Reading

  • 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.