What Is the Most Common Cause of Multiple Nodules in the Lungs?

Multiple nodules in the lungs or multiple pulmonary nodules (MPN) refer to two or more lesions in the lungs.

Multiple nodules in the lungs or multiple pulmonary nodules (MPN) refer to two or more lesions in the lungs. The most common cause of MPN is metastatic cancer or cancer spread from other parts to the lungs. Causes of MPN can be malignant (cancerous) or benign (noncancerous). The most common ones include

Metastatic cancer

It is the leading cause of MPN, especially if a person has a family history of lung cancer. The most common metastases to the lungs include

• Breast cancer
• Colon cancer
• Prostate cancer
• Bladder cancer
• Medullary thyroid cancer
• Melanoma
• Squamous cell cancer of the head and neck

Lung cancer (primary cancer)

Lung cancer such as bronchoalveolar carcinoma and lymphoma are the most common causes of MPN.

Benign causes include

• Benign tumor: Abnormal growth or mass in the lungs may be benign or malignant. Types of benign tumor include
  • Hamartoma (abnormal grouping of normal tissues)
  • Neurofibroma (a lump made up of nerve tissues)
  • Blastoma (growth made up of immature cells)
  • Fibroma (a lump of fibrous connective tissue)
  • Adenoma (a tumor arising from mucous gland)
  • Lymphoma (a tumor made up of lymphoid tissue)
• Lung infections: Infections associated with MPN include
  • Mycobacterium tuberculosis
  • Mycobacterium avium intracellulare
  • Aspergillosis
  • Histoplasmosis
  • Coccidiomycosis
  • Cryptococcosis

These infections result in inflammation, which further forms a granuloma. A granuloma is a small cluster of cells that form when the lung tissue is inflamed. It develops when the immune system isolates any foreign substance in the lungs or other parts of the body. It can become calcified over time because calcium tends to accumulate in the healing tissue.

• Inflammatory disorders (noninfectious): Other inflammatory disorders that may cause granuloma formation in the lungs include
  • Rheumatoid arthritis (a generalized disease of the connective tissues)
  • Granulomatosis with polyangiitis (inflammation of the small blood vessels characterized by lesions that kill the cells in different organs of the body)
  • Sarcoidosis (a disease characterized by granular lesions of unknown cause that involve various organs of the body)
  • Lipoid (resembling fat) pneumonia
• Black lung disease: It mainly occurs in coal miners and often causes nodules that are 1-2mm in diameter. It involves the formation of fibrous tissue due to a reactive process. 
• Scar from past infections: These may show up as nodules in chest X-rays. These may be common in people who never realized that they had an infection.

Congenital issues

• Arteriovenous malformation (failure of proper or normal development of the arteries and veins)
• Lung cyst (an abnormal sac that contains gas, fluid or a semisolid material)
• Sequestration (a piece of the lung tissue that has become separated from the surrounding healthy tissue)