# Nodule Types

A four-chapter lesson on what a pulmonary nodule is and on the three features radiologists
read off one: size, density, and margin. Together these determine the follow-up interval
and the urgency of any workup.

## Chapter 1 — What is a nodule?

A pulmonary nodule is a small, rounded opacity in the lung, typically 30 mm or less in
diameter. Most are found incidentally on CT scans ordered for unrelated reasons.

- **30 mm or less in diameter.** Larger opacities are classified as *masses* and managed
  under a different protocol.
- **About 96% are benign.** Most are caused by old infections, scar tissue, calcified
  granulomas, or inflammation.
- **Three key features.** Radiologists assess size, density, and margin to determine
  follow-up interval and urgency.

*Source: MacMahon et al., Fleischner Society Guidelines, Radiology 2017; ACR Lung-RADS 2022.*

## Chapter 2 — Size: the primary predictor

Size — measured as the longest or the average diameter — is the primary predictor used to
guide follow-up. Both the Fleischner Society guidelines and Lung-RADS are built around size
thresholds.

| Size | Risk | Typical management |
|---|---|---|
| Under 6 mm | Very low | No routine follow-up needed for average-risk patients (Fleischner 2017). |
| 6–8 mm | Low to intermediate | Follow-up CT in 6–12 months recommended. |
| 8–30 mm | Intermediate to high | Consider CT, PET-CT, or tissue sampling. |
| Over 30 mm | Urgent | Classified as a mass — prompt workup regardless of other features. |

*Source: MacMahon et al., Fleischner Society Guidelines, Radiology 2017.*

## Chapter 3 — Density: solid, hazy, or mixed?

Density (attenuation) describes how opaque the nodule appears on CT. It reflects the
nodule's composition and correlates strongly with growth behaviour and malignancy risk.

- **Solid.** Completely opaque — obscures the underlying vessels. The most common type, and
  typically faster-growing when malignant.
- **Ground glass opacity (GGO).** Hazy and cloud-like; blood vessels are still visible
  through it. Often represents adenocarcinoma in situ or minimally invasive adenocarcinoma.
- **Part-solid.** Contains both a solid component and ground-glass haze. Carries the highest
  malignancy risk per unit size of all three types.

*Source: Naidich et al., Radiology 2013; ACR Lung-RADS 2022.*

## Chapter 4 — Margin: reading the edge

The margin — the border of the nodule — reflects how it interfaces with the surrounding
lung tissue. Irregular margins suggest infiltrative growth and independently predict higher
malignancy risk.

- **Smooth.** A well-defined, regular border. Generally associated with benign aetiology or
  lower malignancy risk.
- **Lobulated.** A scalloped or bumpy edge, suggesting uneven, asymmetric growth.
  Intermediate malignancy risk.
- **Spiculated.** The "corona radiata" sign — radiating projections into the adjacent lung.
  A strong independent predictor of malignancy, and one of the six variables in the Mayo
  Clinic risk model.

*Source: Swensen et al., Arch Intern Med 1997; MacMahon et al., Radiology 2017.*

## Related

- [3D and AR viewer](https://nodule.app/viewer) — each margin and density type rendered at
  four true-to-life diameters.
- [Risk calculator](https://nodule.app/calculator) — how size and spiculation feed a
  validated malignancy risk model.
- [Terminology library](https://nodule.app/glossary) — definitions for every term above.

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*Educational use only · Not a medical device.* This document mirrors the page at
<https://nodule.app/tutorial>.