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Causes of Lung Nodules

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Updated July 14, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

What causes lung nodules? With 150,000 people being diagnosed with a lung nodule on a chest x-ray every year in the U.S., this is a common question. What are some of the benign (non-cancerous) and malignant (cancerous) causes?

First, it is important to define what a lung nodule is. A lung nodule is a “spot” on the lung that is less than 3 cm (or 1½ inches) in diameter. If a spot is larger than 3 cm, it is considered a lung mass, rather than a lung nodule. The overall chance that a lung nodule is cancer is 40%, but that risk varies a lot depending on factors such as your age, if you have smoked, and what the nodule looks like on x-ray.

For an indepth discussion about lung nodules, check this article:

Benign (Non-Cancerous) Lung Nodules

Benign lung nodules are responsible for 60% of “spots” seen on a lung x-ray. A nodule is more likely to be benign if:
  • You are young (in people less than 35 years of age, 99% of lung nodules are benign)
  • You are a non-smoker
  • You haven’t worked in occupations that raise the risk of lung cancer
  • The nodule is slow-growing (or grows very rapidly)
  • The nodule has calcifications
  • The nodule is smooth and regular in appearance
  • You don’t have a history of cancer in the past
  • You have traveled outside of the United States

Causes of benign lung nodules include:

Inflammatory lung nodules (granulomas):

Bacterial infections:

Fungal infections:

Parasitic infections:

Benign lung tumors:

Other benign lung nodules:

Malignant (Cancerous) Lung Nodules

A lung nodule is more likely to be malignant if:
  • You are older – half of lung nodules in people over age 50 are malignant
  • You have a history of cancer in the past
  • You smoke or have smoked in the past
  • You work in an occupation where you are exposed to substances that cause lung cancer
  • The nodule is irregular or “spiculated”
  • The nodule is not calcified
  • The nodule is growing on repeat x-rays

Causes of malignant lung nodules include:

As noted above, there are many possible causes of lung nodules. While the likelihood of cancer is less than 50%, it is very important to discuss a lung nodule carefully with your doctor. When lung cancer is found at an early stage, the chances of a cure is much higher than at later stages of the disease.

Further Reading:

Sources:

Cronin, P. et al. Solitary Pulmonary Nodules: Meta-analytic Comparison of Cross-sectional Imaging Modalities for Diagnosis of Malignancy. Radiology. 2008. 246:772-782.

Gould, M. et al. Evaluation of Patients With Pulmonary Nodules: When Is It Lung Cancer? ACCP Evidence-Based Clinical Practice Guidelines (2nd Edition). Chest. 2007. 132(no 3 suppl):108S-130S.

MacMahon, H. et al. Guidelines for Management of Small Pulmonary Nodules Detected on CT Scans: A Statement from the Fleischner Society. Radiology. 2005. 237:395-400.

Wahidi, M. et al. Evidence for the Treatment of Patients With Pulmonary Nodules: When Is It Lung Cancer? ACCP Evidence-Based Clinical Practice Guidelines (2nd Edition). Chest. 2007. 132(no 3 suppl):94S-107S.

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