Overlapping structures, pneumonia, hiatal hernia and lung cancer are among the causes of shadows appearing on the lungs on X-ray results, according to About.com. Some sources of the shadow effect are nothing of concern, while others may be more serious.
Overlapping structures are a common cause of shadows on X-rays and CT scans. This occurs when organs and blood vessels are at an angle when the X-ray is taken, creating a shadow effect. Pneumonia may cause shadows on the lungs, but it is difficult for radiologists to tell whether the shadow is caused by pneumonia or something else, explains About.com. Pleural effusions and a buildup of fluid in the lungs are other common causes.
Noncancerous tumors, broken ribs, hiatal hernias and tuberculosis can cause abnormal findings on X-rays, states About.com. In more serious cases an enlargement of the aorta, called an aortic aneurysm, may appear as shadows. Lung cancer and a collapsed lung also create a shadow effect on the lungs.
If shadows are found on the lungs, it does not necessarily mean a serious condition is to blame, notes About.com. To identify the source of the shadows accurately, a doctor may order additional tests, such as a chest CT scan, an MRI, a lung biopsy or a PET scan.