Lung nodules are focal, round, or oval-shaped dense spots in the lungs. They’re usually 30 millimeters or smaller in diameter, with either clear or fuzzy borders. These nodules may appear alone or in groups. Most don’t come with other signs like lung collapse (atelectasis), enlarged lymph nodes in the hilum, or pleural effusion (fluid around the lungs).

Classifying lung nodules by cause

Doctors often classify lung nodules based on their underlying cause. Nodules fall into two main categories: benign (non-cancerous) or malignant (cancerous).

Benign nodules can come from many conditions. These include non-cancerous tumors, infections, inflammatory diseases like rheumatologic disorders, congenital abnormalities, or even bleeding in lung tissue. Malignant nodules, though, often link to lung cancers—such as bronchogenic carcinoma (including pre-invasive and invasive types). They can also come from lymphomas, sarcomas, or cancers that spread to the lungs from other body parts (metastases).

Classifying nodules by density: solid vs. subsolid

Radiologists also classify lung nodules by their density, using CT scans to evaluate this trait.

A solid nodule is round or oval. It’s dense enough to block the view of blood vessels or airways behind it. In contrast, a subsolid nodule (also called a ground-glass nodule or GGN/GGO) looks hazy and semi-transparent. This lets blood vessels and airways stay partly visible.

Subsolid nodules split into two subtypes. Pure ground-glass nodules (pGGNs) are entirely hazy. Mixed ground-glass nodules (mGGNs)—or part-solid nodules—combine hazy and solid areas. Note that subsolid nodules often tie to lung adenocarcinomas. These can range from pre-cancerous issues (like atypical adenomatous hyperplasia, AAH) to advanced stages: adenocarcinoma in situ (AIS), microinvasive adenocarcinoma (MIA), and invasive adenocarcinoma (IA).

Size and number: key classification traits

Size also matters for classifying lung nodules. Very small nodules are under 5 mm wide. Small nodules range from 5 to 10 mm. Standard nodules measure 11 to 30 mm.

Doctors also check if nodules are solitary (only one) or multiple (two or more). This detail influences how they diagnose and treat the nodules.

Why classification matters

Accurately classifying lung nodules is crucial. It directly affects how doctors monitor or treat patients. Knowing if a nodule is benign or malignant, solid or subsolid, and large or small helps physicians build tailored plans. This leads to better outcomes, less patient anxiety, and smarter follow-up care.