Solitary Pulmonary Nodule

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Overview

A solitary pulmonary nodule (SPN) is a small, round or egg-shaped lesion (abnormal tissue) in the lungs. SPNs are typically asymptomatic, and they are usually noticed by chance on a chest x-ray that has been done for another reason. They are usually less than 3-4 cm in diameter (no larger than 6 cm) and are always surrounded by normal, functioning lung tissue.

SPNs are fairly common abnormalities on chest x-ray images: nearly one of every 500 chest x-rays shows a newly diagnosed SPN. In the United States, physicians are challenged each year by more than 150,000 new cases of SPNs.

Sixty percent of all SPNs are benign. In certain geographical areas where there are infectious agents (especially fungi) that cause SPNs, the percentage of benign SPNs increases remarkably (in some areas as high as 90% to 95%). Malignant SPNs may be primary Stage I lung cancer tumors or metastases from other parts of the body.

Determining the malignancy of an SPN is an integral and challenging part of diagnosis. One of the goals of diagnosis is to avoid unnecessary invasive procedures, such as surgically removing part of the lung because of a benign SPN. Benign SPNs can be treated in simpler, noninvasive ways.

Risk Factors

There is always the chance that a nodule on a chest x-ray is malignant (i.e., cancerous). SPNs should be considered potentially cancerous until proven otherwise. Risk factors that increase the chance that a nodule is malignant include:

• a history of cigarette smoking (some researchers include marijuana smoking as well, but the evidence is controversial);
• age older than 45 years;
• male;
• presence of respiratory symptoms; and
• a history of cancer elsewhere in the body.

Factors that increase the chance that the nodule is benign include:

• the patient has never smoked;
• the patient is younger than 35 years old; and
• the patient lives in an area with a high incidence of histoplasmosis or coccidioidomycosis.

Risk for chronic infections that can lead to SPNs
If the patient has lived in a geographical area with a high incidence of infection that can lead to SPNs, that person is at a higher risk for developing an SPN. For example, in the United States, histoplasmosis is more prevalent in the Ohio River Valley, especially Tennessee and Kentucky, than in other parts of the country. Blastomycosis is more common in the upper Midwest.

Many people who harbor fungi that can cause SPNs never actually develop an SPN or other respiratory symptoms. Patients with weakened immune systems due to disease, surgery, etc. are at greater risk for developing infectious SPNs. Patients with chronic lung disease are at an increased risk for fungal infections that can cause SPNs.

Occupational risks
Certain occupations, such as bridge building and farming, are associated with exposure to fungi that can lead to SPNs.

A history of extensive asbestos exposure has been linked to the development of malignant solitary pulmonary nodules.

Very rarely, pet owners or people who work with animals (particularly dogs) can become infected with Dirofilaria immitis, a type of worm that can lead to the development of solitary pulmonary nodules.

Patients with a history of cancer or benign neoplasm
In patients with a history of extrathoracic malignancy (cancer outside of the chest region) or benign neoplasms, there is a 50% to 70% chance that an SPN is metastatic.

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