Diffuse: Definition


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In pathology, diffuse describes a change or finding that is spread throughout the tissue examined, rather than being limited to one small area. A diffuse change can be seen when a pathologist looks at the tissue by eye (called the gross examination) or under the microscope. The opposite of diffuse is focal, which describes a change limited to one or a few specific spots. Diffuse describes a pattern or distribution; it is not a diagnosis on its own, and it does not by itself mean that a finding is harmless or serious. This article explains what diffuse means, how it differs from focal, and where you are likely to see it on a pathology report.

Diffuse versus focal

Diffuse and focal are opposite terms that pathologists use to describe how widespread a finding is:

  • Diffuse — The change involves the tissue widely and fairly evenly, so it is found throughout the sample rather than in just one spot.
  • Focal — The change is limited to one or a few small, well-defined areas, while the rest of the tissue looks unaffected.

Pathologists note whether a change is diffuse or focal because the pattern can help tell conditions apart and can affect how a finding is interpreted. For example, inflammation spread evenly throughout a sample (diffuse) may point to a different cause than inflammation found in a single small area (focal).

What does “diffuse” mean when it is part of a diagnosis?

For some diagnoses, the word diffuse is part of the name itself, describing how abnormal cells grow or spread. In these cases, diffuse usually means the cells spread throughout a tissue instead of forming a single, well-defined lump. Examples include:

  • Diffuse large B-cell lymphoma — A common, fast-growing lymphoma in which the cancer cells grow in sheets that spread throughout and replace the normal structure of a lymph node or other tissue.
  • Diffuse midline glioma — A brain tumor whose cells spread out and infiltrate the surrounding brain tissue rather than forming a single, clearly separated mass.
  • Diffuse-type stomach cancer — A type of stomach cancer in which the cells scatter and spread individually through the stomach wall instead of forming a distinct tumor. These cancers often contain signet ring cells.

In each of these examples, diffuse is describing the growth pattern of the disease and is only one part of the full diagnosis.

What does “diffuse” mean in immunohistochemistry?

In immunohistochemistry, a test that uses special stains to detect specific proteins in the tissue, a result may be described as diffusely positive. This means the protein was found throughout the cells being examined, rather than in only a few scattered cells (which would be described as focal). Noting whether staining is diffuse or focal helps the pathologist interpret what the result means for the diagnosis.

Questions to ask your doctor

  • Does “diffuse” on my report describe a normal finding, an inflammatory change, or a tumor?
  • Is “diffuse” part of the name of my diagnosis, or is it describing how widespread a finding is?
  • How does a diffuse pattern differ from a focal one in my case?
  • If a stain was diffusely positive, what does that mean for my diagnosis?
  • Does the diffuse pattern affect my treatment or follow-up?
  • Can you point to where on my report the diffuse finding is described?

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