Types of SPN
One of the most important goals in diagnosing SPNs is establishing whether the nodule is benign (noncancerous) or malignant (cancerous). About 60 percent are benign, and most of these are a type of lesion known as a granuloma (an inflammatory, immune system reaction). There are many granulomatous disorders, including tuberculosis and other infectious diseases.
The other 40 percent of SPNs are malignant, three-quarters of which are primary lung cancers, and one-quarter of which are metastases from other parts of the body.
About 40 percent of all SPNs are granulomas—small, granular inflammatory lesions. The word "granuloma" comes from the Latin word "granum," meaning "grain" or "seed."
Granulomas are characterized by a nodular appearance and a unique cellular pattern that can be seen through a microscope. They can form on nearly any part of the body, internal or external. There are many different types of granulomatous diseases, including tuberculosis.
Hamartomas are the third most common cause of SPNs, after cancer and granulomas. Hamartomas are abnormal masses of normal tissue—the cells are normal, but they are poorly organized. They are often classified as a neoplasm (a neoplasm is an abnormal tissue growth that resembles a cancerous tumor but does not spread and invade surrounding tissue). Some researchers suspect, however, that hamartomas may be a developmental abnormality and not actually a neoplasm.
Hamartomas can develop anywhere in the body and occur in about 0.25 percent of the general population. Hamartomas that develop in the lungs are usually asymptomatic. They tend to occur in people between 50 and 60 years old, and they are more common in men than women (3:1).
Pulmonary hamartomas tend to grow slowly, about 1.5 mm/year, doubling in size every 14 years. They usually occur singly. Multiple hamartomas often signify an underlying disease such as Carney's Syndrome or Cowden's Disease.
Hamartomas are usually well-defined on a chest x-ray or CT. They tend to have smooth edges and sharp margins. Twenty-five percent are calcified; 30 to 75 percent have a collection of fat; 90 percent are solitary lesions; and about 80 to 90 percent of them are located peripherally.
Other Benign Tumors
Other types of benign tumors that manifest as SPNs include fibromas (a tumor made of fibrous connective tissue), lipomas (a benign tumor composed of fat), leiomyomas (a benign tumor made of smooth muscle), hemangiomas (a tumor made of dilated blood vessels), and papillomas (a tumor made of epithelial cells).
Histoplasmosis is an infection caused by a common fungus known as histoplasma that occurs worldwide. In some areas of the United States (e.g., Kentucky, Tennessee), histoplasma is so common that nearly 90 percent of adults show evidence of exposure.
Bats are carriers of histoplasma, and soil that contains bat excrement is usually infected with the fungus. Localized outbreaks of histoplasmosis often occur when bat-inhabited areas are disturbed by construction or other activity.
Coccidioidomycosis and blastomycosis are other fungal infections that can affect the lungs and lead to solitary pulmonary nodules. Coccidioides immitis, the cause of coccidioidomycosis, inhabits soil of the southwestern Sonoran desert and can spread on high winds.
Blastomycosis is caused by the fungus Blastomyces dermatiitides and is endemic to the central United States. It has a more northern distribution and is commonly diagnosed in Illinois and Wisconsin. Like the other fungi that cause SPNs, Blastomyces lives in the soil. Blastomycosis most commonly occurs in people who work outdoors or spend a lot of time in the woods (e.g., hunters).
Most malignant SPNs represent Stage I lung cancers and can often be surgically removed. About 25 percent of all bronchogenic carcinomas (cancers originating in the bronchi, 90 percent of lung cancers are bronchogenic) initially manifest as solitary pulmonary nodules.
The prognosis for lung cancer patients diagnosed on the basis of an SPN is better than for other lung cancer patients on average because of the early stage of disease. The 5-year survival rate for these patients is 50 percent, compared to 10 to 15 percent for lung cancer patients diagnosed after symptoms appear.
About one-quarter of malignant SPNs are metastases from other other parts of the body. Upon closer inspection (i.e., on a CT scan or during a bronchoscopy), additional "satellite" SNPs are commonly found.