Shadow on the Lung

Possible Causes of a Chest X-Ray Shadow

physician looking at a shadow on a chest x-ray
What is the meaning of a shadow on the lungs?.

A frightening and all-too-common finding noted on chest x-rays and chest CT scans is a shadow on the lungs. What does this mean, what are some possible causes, and what tests may your doctor order to understand what this means?

Meaning of Shadows on Chest X-rays and CT Scans

It can help to think of radiology studies as pictures with shades of black and white and gray. Dense or solid structures such as bone, the heart, and major blood vessels appear white.

  In contrast, air-filled structures such as the lungs appear black. Structures in-between dense and air-filled, or overlapping structures often appear gray. 

Other Terms Used to Describe Abnormal Findings

Before talking more about shadows, it's helpful to define some other terms that your doctor may use when describing an abnormality on an X-ray, CT, or MRI.

  • Nodule - A pulmonary nodule is defined as an abnormality on a chest x-ray or CT scan of the chest that is smaller than 3 cm (1 ½ inches) in diameter.
  • Mass - A lung mass refers to an abnormality greater than 3 cm (1 ½ inches) in diameter.
  • Tumor - A tumor is an abnormality of any size, and can be either benign or malignant.
  • Lesion - A lesion simply refers to any form of abnormality seen in a radiology study of the chest.
  • Spot - A spot on the lung is a vague tumor, and may be a nodule, a mass, or a shadow.

These articles on causes of lung nodules and causes of a lung mass cover these abnormal findings in greater depth.

Why Can’t My Doctor Know for Certain What a Shadow Is?

It can be hard to understand at first why some radiology scans are hard to read, but scans show not only internal structures, but the overlap of organs, blood vessels, and other tissues. Another reason is because scans are essentially "black and white." Many medical students are surprised to learn how difficult it is to identify tissues - even during surgery - when those tissues are no longer in color such as in a textbook, but many shades of gray.

It's also important to note that a chest x-ray alone, with or without a shadow, is not enough to rule out lung cancer. In fact, it's thought that between 12 and 30 percent of people with lung cancer will have a normal chest x-ray at the time of diagnosis.

Possible Causes of a Shadow on the Lungs

There are many conditions as well as normal findings that may result in a radiologist reporting a "shadow" or "lesion" on an x-ray. A few of these include:

Overlapping Structures - In Other Words, Nothing of Concern - Sometimes an area on a chest x-ray can appear different simply because a few normal organs and blood vessels are at an angle that creates a shadow.

Pneumonia - It can sometimes be difficult to tell if a possible abnormality seen on an x-ray is pneumonia or not.

Lung Cancer - Lung cancer may appear as a shadow on an x-ray without a well-defined nodule or mass.

Benign lung - Noncancerous tumors such as hamartomas may appear as a shadow or spot on an x-ray.

Pleural Effusion - The presence of fluid in the pleural space (surrounding the lungs) may appear as a shadow.

Pulmonary Edema - Pulmonary edema is a condition involving the build-up of fluid in the lungs, often due to heart disease.

Broken Ribs - Broken ribs can sometimes be mistaken for a mass on x-rays, and though there is usually a history of trauma, sometimes rib fractures can occur during coughing or minimal trauma in people with osteoporosis.

Hiatal Hernia - Hiatal hernias - herniation of the stomach through the diaphragm and into the chest cavity -- can appear as an abnormality on a chest x-ray. Approximately 50 percent of people with a hiatal hernia do not have any symptoms.

Aortic Aneurysm - An aortic aneurysm (in the chest, not abdomen) - an enlargement of the aorta where it exits the heart - may appear as a shadow on x-ray studies.

Tuberculosis - Tuberculosis is an important cause of abnormal findings on chest x-rays, especially in countries where tuberculosis is common.

Sarcoidosis - Sarcoidosis - an inflammatory disease in which granulomas develop in body tissues - may appear at first as a shadow on an x-ray.

Pneumothorax - A pneumothorax - better known as a collapsed lung may appear as a shadow on a chest x-ray, both in the region where the lung is collapsed and due to structures that are pushed together due to the collapse.

Tests Your Doctor May Order

  • Chest CT scan – If the shadow is seen on a plain chest x-ray, your doctor may first order a chest CT. It's important to note that a chest x-ray isn't sufficient to rule out lung cancer. 
  • Bronchoscopy - A bronchoscopy is a procedure in which a scope is directed through the mouth into the large airways to evaluate abnormal x-ray findings.
  • Sputum Cytology - While sputum cytology has not been found to be a good screening test for lung cancer, sometimes a sputum sample may contain cancerous cells which lead to a diagnosis.
  • MRI - An MRI may be a good test for evaluating a shadow on the lung, as it distinguishes soft tissues better than a CT scan.
  • PET Scan - When a mass or nodule is found on a radiology study, it doesn't tell us whether that spot has been there a month or 20 years. A PET scan is a radiology study that identifies areas that actively growing, such as cancer.
  • Lung Biopsy - Your doctor may order a lung biopsy to clarify an abnormal finding or thickening seen on radiological studies. This may be done via an endobronchial ultrasound biopsy during a bronchoscopy, a needle biopsy, or an open lung biopsy.

Chances it is Cancer

Everyone's chest x-ray and CT scan are different, and shadows can mean many things. Of course, our greatest fear is an abnormality such as cancer. Yet there are a few things that may increase the likelihood that an abnormality is cancer.  Some of these include:


Guan, C., Ma, D., Cui, D. et al. Short Linear Shadows Connecting Pulmonary Segmental Arteries to Oblique Fissures in Volumetric Thin-Section CT Images: Comparing CT, Micro-CT and Histopathology. European Radiology 2016. 26(8):2740-8.

Yoshimoto, A. Reasons for the delays in the definitive diagnosis of lung cancer for more than one year from the recognition of abnormal chest shadows. Internal Medicine. 2002. 41(2):95-102.

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