Pulmonary nodules are round spots less than 3 cm in diameter, or a little over 1 inch. Seen in the lungs on chest X-rays or CT scans, pulmonary nodules appear on around one in 500 X-rays, the University of Rochester reports. Nodules on the lungs occur even more frequently, appearing on as many as 51 percent of CT scans of smokers over age 50, lead author Heber MacMahon, M.B. of the University of Chicago states in the November 2005 issue of "Radiology." A number of diseases can cause pulmonary nodules.
Around 40 percent of lung nodules turn out to be cancerous, the University of Rochester reports. Lung cancer can be primary, meaning the lung was the first place the cancer appeared, or secondary, meaning the cancer originated elsewhere and traveled, or metastasized, to the lung. Cancerous nodules grow quickly, often doubling in size within as little as four months. Cancerous nodules also have more irregular shapes, rougher surfaces and color variations, the University of Rochester adds. Larger nodules more often are cancerous than larger nodules. Cancerous lung nodules often form in the upper lobes of the lung, MacMahon reports.
At least 99 percent of all nodules less than 4 mm, or 0.1 inches in size are benign, according to MacMahon. Benign tumors often appear smoother, more rounded and even in color than cancerous tumors. Fat in the nodule most often indicates hamartoma, lipoid granuloma or lipoma, all benign tumors. Lung cysts and intrapulmonary lymph nodes can also appear as nodules on lung scans.
Several autoimmune diseases can cause benign or non-cancerous lesions on the lungs. Wegener's granulomatosis and rheumatoid lung disease can cause pulmonary symptoms such as pleural effusion, accumulation of fluid around the lining of the lungs and scarring. Cough, shortness of breath and joint pain may also occur.
Infections can cause pulmonary nodules, even after the disease is gone. Tuberculosis and fungal infections such as aspergilloma or histoplasmosis can cause scarring with nodules. Bacterial abscesses or a history of pneumonia or other lung disease can also lead to nodule formation. A cluster of nodules in one area of the lung often indicates an infectious disease, MacMahon states.
Diseases or malformations of the blood vessels in the lungs such as hemangioma, cavernous angioma and pulmonary telangiectasis can lead to pulmonary nodules, the Merck Manual states.