Calculating the mitotic index in breast cancer biopsies
Doctors use tumour grade and age and general health, to determine a
patient’s prognosis (the likely course of a disease and the chance of
recovery). Generally, a lower grade indicates a better prognosis, slower
cancer growth and a greater chance of recovery. In breast cancer and
prostate cancer tumour grade is important for planning treatment and
they each that have their own grading systems.
A common technique to test tissue biopsies in cancer care uses visible features in the tissue. In breast
cancer grading, the Nottingham grading system (also called Elston-Ellis) these features include:
1. Tubule formation – how much of the tissue has normal cell structure (because cancer cells do
not differentiate properly causing the tissue to lose its normal structure).
2. Nuclear shape variations – cell nuclei look different from small round nuclei of normal cells
(because some cancer genes cause changes in nuclear membrane shape and size).
3. Mitotic index - the proportion of cells in one of the stages of mitosis (because cancer cells
divide more frequently than normal tissue).
In this activity you will analyse tissue samples as a pathologist would to calculate the mitotic index.
The mitotic index is calculated using the formula:
Key Terms
Mitotic index
The ratio of the number of cells in mitosis to the total number of cells.
Prognostic tool
A method of predicting the future growth of cancer cells. (e.g. mitotic index)
Five microscope views of biopsy tissue from a breast cancer patient are available for examination.
1. Identify any cells that are in a stage of mitosis. Use a red circle for each.
2. Count all the cells in the frame. Put a dot on each nucleus, circle groups of ten cells,
then count the number of groups, plus any remaining cells.
Note: Identifying individual cells is not easy. There is a variety of shapes and sizes,
even between nuclei. So count quite quickly, remember the cell count is an estimation
and a total count of 145 or 146 will make little different to the mitotic index.
3. Record the number of cells in mitosis and the total number of cells in the table below.
4. Then calculate the mitotic index as a decimal and a %.
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© David Faure, InThinking http://www.thinkib.net/biology
Sample 1 – From a patient called Jacintha who is 35 years old and in good general health.
Microscope view
A
B
C
D
E
Total
Number of cells
doing mitosis
2
0
2
2
3
9
Total number of
cells in the slide
30
30
36
29
21
146
Sample 2 – From a patient called Angela who is a slightly overweight 58 year old smoker.
Microscope view
F
G
H
I
J
Total
Number of cells
doing mitosis
2
2
1
1
1
7
Total number of
cells in the slide
30
37
34
20
24
145
Calculating mitotic index
Tissue Type
Patient 1 - Jacintha
Total Number of cells
Number of cells in
any phase of
mitosis
146
9
Mitotic index
0.062
Mitotic index
expressed as a %
6.2%
Patient 2 - Angela
145
7
0.048
4.8%
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Analysis of data
To convert a mitotic index expressed as a decimal, or a fraction, multiply this number by 100 to get
the percentage of cells undergoing mitosis in your sample.
Questions
1. What does your data show about mitotic index in each of the samples cancerous tissue?
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2. Who’s cancer appears to be the fastest growing? Explain your answer, using mitotic index.
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3. Compare the results to these examples, which show the three grades of breast tissue.
Suggest advice that you might give to each these patients if you were their doctor.
This could include, how aggressive the treatment would need to be, how serious the cancer is.
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