After a doctor removes tissue or cancer from your bowel, they send a sample of cells to the laboratory. A doctor called a pathologist does various tests on the cells.
These tests can diagnose cancer and look for gene changes. Some of these tests can show how well particular treatments might work, such as targeted cancer drugs and immunotherapy.
Finding the type of cancer
A pathologist looks at the cancer cells under a microscope to see which type and grade of bowel cancer you have.
Testing for gene changes
Your doctors might look to see if your bowel cancer cells have changes (mutations) in certain genes.
What are genes?
Genes are coded messages that tell cells how to behave. They control how our bodies grow and develop. All cancers develop because something has gone wrong with one or more of the genes in a cell. A change in a gene is called a ‘fault’ or ‘mutation’.
Doctors can do various tests. The results of each test can be useful in different ways. The tests can:
- show which targeted drug treatments might work for your bowel cancer
- help doctors find new ways of treating bowel cancer
- help you and the doctors understand more about the cause of your cancer
The doctor might test your bowel cancer to look for changes (mutations) in a group of genes called RAS genes. Mutated RAS genes can cause cancer cells to grow and spread.
You might have:
- a normal RAS gene, which is also known as RAS wild type
- an abnormal RAS gene, which is known as a mutated RAS gene
Targeted cancer drugs such as cetuximab and panitumumab can work well for people with advanced bowel cancer who have a normal RAS gene (RAS wild type).
The doctor might look to see if your bowel cancer cells have changes (mutations) in the BRAF gene. This gene makes a protein called BRAF which is involved in cell growth. A change to this gene can cause it to make an overactive BRAF protein. This makes cells grow and divide too fast.
You might have:
- BRAF positive bowel cancer, which means there are changes in the BRAF gene
- BRAF negative bowel cancer, which means you don’t have these changes
Doctors know that people with BRAF positive bowel cancer don't respond as well to treatment. Researchers have been looking at different ways of treating this type of bowel cancer.
Microsatellite instability/ Mismatch Repair status
A small number of bowel cancers have DNA changes called
Microsatellites are short, repeating arrangements (sequences) of DNA inside cells. Every time a cell divides, it makes new copies of these DNA sequences. Cells can correct any mistakes that happen in this process. Mismatch repair proteins identify and repair any mistakes made when cells make copies of DNA.
But if these mismatch repair processes are faulty, mistakes can happen when the cell divides causing changes in the new copies of DNA. Changes in the length of the new DNA sequence are called microsatellite instability. These changes can cause cells to grow abnormally.
Dependent on which test your doctor arranges they might describe your test results as:
- microsatellite stable (MSS) – this means they didn’t see any instability
- microsatellite low (MSI low) – this means they saw a low level of instability
- microsatellite instability high (MSI high) – this means they saw a high level of instability
- mismatch repair deficient (MMRd) – this means that one, or more of the mismatch repair proteins aren’t identified on testing
- mismatch repair proficient (MMRp) – this means that the MMR proteins are expressed normally
If a bowel cancer shows MMRd it is normally also MSI-high.
Your doctor can use the test results to help you decide whether to have further genetic tests. Further genetic tests can tell you whether there is an inherited cause for your bowel cancer.
You might not need any extra tests. The doctor can sometimes do genetic tests on your bowel cancer cells that they removed during surgery or when you were first diagnosed.
But sometimes the doctor needs to take another sample of your bowel cancer cells. They can take a sample from your bowel or from a secondary site if your cancer has spread. They will tell you more about this and about what the test will involve.