Pathology dictionary -


Atypia is a word pathologists use to describe cells that look abnormal either in shape, colour, or size compared to normal, healthy cells in the same location. Pathologists may also describe these changes as cytologic atypia or atypical cells.

Why causes atypia?

There are many reasons why atypia may develop in a cell or group of cells. The most common causes include:


  • InflammationInflammation is the body’s natural defense against injury or disease.  The body also uses inflammation to repair tissue after an injury has taken place. The special cells that take part in inflammation are called inflammatory cells and they are part of the body's immune system. Normal, healthy cells can start to look atypical when they are close to inflammatory cells. In this situation, the atypia goes away when the inflammation stops.

  • Infection - Cells that become infected by a virus can look very atypical. Pathologists sometimes call this type of atypia viral cytopathic effects. This type of atypia will go away once the infection is removed from the body.

  • Radiation - Radiation is a common treatment for cancer. Normal, healthy cells that have been exposed to radiation commonly look very atypical when examined under the microscope. Your doctor should always let your pathologist know if you are receiving currently radiation or have received radiation in the past.   

  • Pre-cancerous diseases - The abnormal cells in most pre-cancerous diseases look atypical when examined under the microscope. Pre-cancerous diseases that show atypical cells include dysplasia and carcinoma in situ.

  • Cancer - Almost all cancers are made up of cells that look atypical compared to the normal, healthy cells around them. In this case, atypia is very important because it helps your pathologist make the diagnosis and determine the tumour grade.

Why is this important? It is important to keep in mind that atypia is a description of the way some cells look and not a complete diagnosis by itself. In many cases your pathologist will try to determine the cause of the atypical cells. If the cause is known, it will be described in your pathology report. However, your doctors may only be able to determine the cause of the atypia later with the help of additional information about you and your medical history. 

Reactive atypia

Pathologists use the term reactive atypia to describe cells that show atypia in response to inflammation, infection, or radiation. Reactive atypia is not cancer.

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