What Are Solitary Pulmonary Nodules?

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A solitary pulmonary nodule (SPN) is an isolated growth on the lung that's surrounded by normal tissue, with no other evidence of cancer. SPNs have a diameter less than or equal to 3 centimeters (cm), or 1½ inches. A larger growth would be defined as a lung mass.

A nodule on the lung can develop for many different reasons, including lung cancer. Some solitary pulmonary nodules turn out to be malignant (cancerous), but most end up being benign (noncancerous).

Types of Solitary Pulmonary Nodules

An SPN can be classified as one of three types. Classification is based on its appearance on an X-ray or other imaging studies.

Types include:

  • Solid nodules, the most common type, will appear on an X-ray as a homogeneous mass of tissue.
  • Ground-glass nodules are non-uniform and have a hazy, ground-glass like appearance on X-ray.
  • Part-solid nodules have both solid and ground-glass features.

These characteristics, along with the size of the nodule, can predict the likely cause of the growth and whether cancer may be involved.

SPN Symptoms

With an SPN, there will often be no signs or symptoms. If present, the symptoms would be related to the underlying cause.

Signs of cancer may include enlarged lymph nodes. If a nodule is caused by squamous cell carcinoma, a type of lung cancer mainly affecting the airways, it can be associated with a persistent cough or bloody phlegm. If it caused an infection, you may experience fever, chills, and shortness of breath.

This differs from multiple pulmonary nodules, which are mainly caused by systemic (whole-body) diseases and tend to manifest with more noticeable symptoms.

Causes

Pulmonary nodules are not all that uncommon, with around 150,000 reported in the United States each year, according to a 2019 review in the Annals of Thoracic Medicine.

There are many different causes of SPN, some of which are harmless or readily treatable, and others of which are serious and even life-threatening.

The most common causes of solitary pulmonary nodules are:

The odds that a solitary pulmonary nodule is cancerous are between 30% and 40%, but this can vary based on several factors.

Among the factors that can influence the risk of lung cancer are:

  • Age: An SPN is more likely to be benign in younger people and more likely to be cancerous in people over 50.
  • Smoking history: A history of smoking greatly increases the risk of a malignant SPN. Studies suggest that a solitary nodule in current or former smokers is five times more likely to be cancerous than those found in never-smokers.
  • Prior history of cancer: Having had cancer in the past increases your risk of a malignant SPN three-fold, although a family history of cancer does not.
  • Nodule type: Generally speaking, solid nodules are more likely to be cancer than ground-glass or part-solid nodules.
  • Nodule size: Large nodules are more likely to be cancerous than very small ones. Similarly, nodules that are stable and do not grow are less likely to be cancer. 

SPNs that have not changed in size for two years are, more often than not, benign.

Diagnosis

If an SPN is found on a chest X-ray or other imaging study, the diagnosis will mainly be directed by the size and characteristics of the nodule. In some cases, immediate action is required; in others, a "watch-and-wait" approach is more appropriate.

The diagnostic approach may involve:

  • Observation: If an SPN is smaller and uncharacteristic of cancer, the doctor may recommend routine computed tomography (CT) scans to monitor for any changes at least every 12 months. Depending on the type and size of the nodule, the testing interval can range from three to 12 months.
  • Positron emission tomography (PET) scan: At a certain threshold (usually when a nodule reaches a certain size or grows quickly), the doctor may order a PET scan along with CT to better determine if cancer is involved. A PET scan measures metabolic activity in tissues and can detect areas of increased activity (such as occurs with cancer).
  • Lung biopsy: If the characteristics of a nodule are suggestive of cancer, the doctor may recommend a lung biopsy in which affected tissue is sampled for microscopic evaluation. A biopsy is the only way to definitively diagnose lung cancer and can be done either with bronchoscopy, fine-needle aspiration biopsy (FNA), laparoscopic surgery, or open surgery.

The American College of Chest Physicians (CHEST) offers guidance on the most appropriate actions to take with a single nodule, based on its size and relative risk of cancer.

Type Size Risk CHEST Recommendations
Smaller nodules (including SPNs) Under 5 millimeters (mm)  Less likely to be cancer • Further evaluation may not be needed • Monitoring is advised in high-risk individuals (such as heavy smokers with a greater than 20 pack-year history of smoking)
Intermediate solid nodules with no risk factors for lung cancer Under 8 mm Between a 0.5% and 2% risk of cancer The frequency of surveillance is chosen according to the size of the nodule. .Nodules measuring less than or equal to 4 mm in diameter don't need to be followed, but the patient should be informed about the potential benefits and harms of this approach .Nodules measuring between 4 mm and 6 mm should be reevaluated at 12 months without the need for additional evaluation if unchanged .Nodules measuring between 6 mm up to 8 mm should be followed at six to 12 months, and then again between 18 to 24 months if unchanged
Larger solid nodules Over 8 mm Greater than 2% risk of cancer Either repeat CT scan in three months or immediately perform a PET/CT or non-surgical or surgical biopsy
Small part-solid nodules Under 8 mm Less likely to be cancer Monitor with CT scans at three,12, and 24 months, followed by annual surveillance for an additional one to three years
Larger part-solid nodules Over 8 mm Slightly more likely to be cancer Repeat chest CT at three months, followed by further evaluation with PET, non-surgical biopsy, and/or surgical resection for nodules that persist
Small ground-glass nodule Under 10 mm Less likely to be cancer Monitor with CT scans every two to five years, depending on the nodule size
Larger ground-glass nodules Over 10 mm More likely to be cancer Perform PET/CT or non-surgical or surgical biopsy

Treatment

The treatment of an SPN depends on the underlying cause. For example, infections may be treated with the appropriate antibiotic, antifungal, or antiparasitic drug. Autoimmune diseases are treated with medications that reduce inflammation.

Congenital disorders like CCAM and arteriovenous malformation may benefit from surgery to remove abnormal tissues or blood vessels.

Benign lung cysts or tumors often don't require treatment but can be surgically removed if they are causing an obstruction in the airways.

If cancer is involved, remember that there are many different types and stages of lung cancer, some of which are less aggressive and more readily treatable. The treatment approach will depend on these factors, as well as your general health.

Options include:

A Word From Verywell

It is important to remember that a diagnosis of SPN is not the same thing as a diagnosis of lung cancer. Statistically speaking, a solitary nodule on the lung is more likely to be benign than malignant. Even if it turns out to be cancer, new and more effective treatments are being introduced every year.

Take things one step at a time. If you don't get the answers you need or are faced with an inconclusive finding, do not hesitate to seek a second opinion from a lung specialist (pulmonologist) or a radiologist specializing in pulmonary diseases.

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