From the Guidelines
Pulmonary nodules are caused by a variety of conditions, ranging from benign to malignant processes, with the most common causes including infections, benign tumors, malignant tumors, inflammatory conditions, and vascular abnormalities, as well as environmental exposures and scars from previous infections or inflammation. The causes of pulmonary nodules can be broadly classified into several categories, including:
- Infections, such as tuberculosis, fungal infections like histoplasmosis, or bacterial abscesses
- Benign tumors, like hamartomas
- Malignant tumors, including primary lung cancer or metastases from other cancers
- Inflammatory conditions, such as rheumatoid arthritis, sarcoidosis
- Vascular abnormalities
- Environmental exposures, including occupational dust exposure, silicosis, and coal worker's pneumoconiosis
- Scars from previous infections or inflammation
According to the most recent study 1, incidental pulmonary nodules are common, with reported frequencies ranging from 5.6% to 51% on CT and 0.1% to 7% on chest radiographs. While it is estimated that 70% to 97% of incidental pulmonary nodules are benign, most are indeterminate for malignancy when first encountered, making their management challenging. The risk of malignancy increases with nodule size, especially those larger than 8mm, irregular borders, upper lobe location, and in patients with risk factors like smoking history, older age, or previous cancer, as noted in 1 and 1.
The management of pulmonary nodules should be based on the estimated lung cancer risk, with follow-up tests recommended in patients with an estimated lung cancer risk of 1% or higher, and flexibility to accommodate a patient’s risk factors and preferences in management. Most small nodules (less than 6mm) in low-risk individuals are benign and may simply require monitoring with follow-up imaging, as suggested in 1. However, larger nodules or those with suspicious features often require further evaluation with PET scans, biopsy, or surgical removal to determine their exact nature, as recommended in 1 and 1.
From the Research
Causes of Pulmonary Nodules
Pulmonary nodules can be caused by a variety of factors, including:
- Infectious diseases, such as histoplasmosis, nontuberculous mycobacterial infection, and aspergillosis, as identified in 19% of patients in a study 2
- Nonmalignant diagnoses, such as nonspecific granuloma, which was the most prevalent diagnosis in a study, accounting for 26% of cases 2
- Malignant diagnoses, which accounted for 17% of cases in a study 2
- Rheumatoid arthritis, which can cause intraparenchymal lung nodules, as described in a case report 3
- Sarcoidosis, which can manifest as nodular lung disease, resembling metastatic neoplasm disease, as described in a case report 4
- Pulmonary tuberculosis, which can show clusters of small nodules on CT scans, similar to sarcoidosis 5
- Necrobiotic nodules, which can be a presenting feature of rheumatoid arthritis, as described in a case report 6
Infectious Causes
Infectious diseases are a common cause of pulmonary nodules, with histoplasmosis being the most common diagnosis in one study, followed by nontuberculous mycobacterial infection and aspergillosis 2.
Non-Infectious Causes
Non-infectious causes of pulmonary nodules include rheumatoid arthritis, which can cause intraparenchymal lung nodules, as described in a case report 3. Sarcoidosis is another non-infectious cause, which can manifest as nodular lung disease, resembling metastatic neoplasm disease, as described in a case report 4.