Pathology dictionary -


Cancers start from cells that were previously normal and healthy. These normal cells can usually be found surrounding a tumour. Pathologists use the word differentiated to describe the difference between the cancer cells and the surrounding normal, healthy cells.

Why do pathologists use the word differentiated?

Pathologists use the word differentiated in their report because not all cancers look the same. Some cancers look very similar to normal, healthy cells while others look very different. 

Pathologists commonly use five levels to describe these changes:

  • Well differentiated - These cancers look very similar to the surrounding normal cells. In some cases it can be difficult for the a pathologist to tell the difference between the cancer and the normal cells.

  • Moderately differentiated - The cells in these cancers are clearly abnormal looking but they still share some features with the surrounding normal cells.

  • Poorly differentiated - These cancers look very abnormal. When the cells from a poorly differentiated tumour travel to a lymph node or other part of the body, additional tests such as immunohistochemistry may be required to determine the type of tumour and where it started. The movement of cancer cells to another part of the body is called metastasis.

  • Undifferentiated - These cancer look completely different from normal cells anywhere in the body. Even with additional tests, it is often very difficult for a pathologist to determine where this type of tumour tumour started.

  • Dedifferentiated - Dedifferentiated is used to describe a tumour that is made up of two different types of cancer cells. The first type of cancer cell looks similar to the previously normal cells. The second type of cancer cell looks very little or nothing like the normal cells. Dedifferentiated means that the less abnormal cells in the tumour have changed to become more abnormal and less like the original normal cells.

Why is this important? In most parts of the body, well and moderately differentiated cancers tend to grow more slowly and are less likely to spread to other parts of the body. In contrast, poorly differentiated and undifferentiated cancers are aggressive tumours that grow quickly and spread earlier to other parts of the body.

Differentiation and grade

Grade is another word pathologists use to describe the difference between cancer cells and the normal, healthy cells they have replaced. Grade is usually described using numbers to represent different levels. For example a tumour may be given a grade from 1 through 4. In this system, tumours that look similar to normal cells would be given a grade of 1 while those that look nothing like normal cells would be given a grade of 4. Grade can also be described using two levels, low and high.


For some types of cancer, the differentiation of the tumour is used to determine the grade. For example, each level of differentiation may equal a grade as shown below:


  • Well differentiated = Grade 1.

  • Moderately differentiated = Grade 2.

  • Poorly differentiated = Grade 3.

  • Undifferentiated = Grade 4.


Or some levels of differentiation may be grouped together to make a grade:

  • Well differentiated and moderately differentiated = Low grade.

  • Poorly differentiated and undifferentiated = High grade.


Why is this important? Like differentiation, grade is used to help predict how the tumour will behave over time. Lower grade tumours tend to grow more slowly and are less likely to spread to other parts of the body. In contrast, higher grade tumours tend to grow faster and are more likely to spread to other parts of the body.


Talk to your doctor about how the tumour differentiation or grade described in your pathology report will be used to select the treatment options that are best for you.

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