What is myelofibrosis (MF)?

Myelofibrosis (MF) is a type of blood cancer that causes scarring (fibrosis) in your bone marrow and stops blood cells being produced normally. Bone marrow is the spongy material inside our larger bones where most blood cells are made.

In the early stages of MF, you may have little or no bone marrow scarring. Doctors will make a diagnosis based on the number of blood cells you have (your blood cell count), what the cells look like under a microscope and how they are positioned in your bone marrow.

Some people don’t have any symptoms of MF at this stage. Others have symptoms caused by inflammation in their bone marrow, or because their bone marrow starts to make too many blood cells such as platelets.

As the MF progresses, bone marrow scarring starts to develop further. This makes it harder for your body to keep your blood cell counts at a normal level. For example, you may have too few red blood cells, and be anaemic.

Sometimes other parts of the body try to take over blood cell production, including the spleen, a fist-sized organ which sits under your ribs on the left side. This can lead to your spleen becoming swollen.

Treatment for myelofibrosis aims to control your blood cell production, help with your symptoms and shrink your spleen if it is very swollen.

Cancer is the name for a disease where something goes wrong with the production of cells in one or more parts of the body. Myelofibrosis (MF) is classed as a type of blood cancer because it affects blood cell production.

Because MF is considered a blood cancer, anyone diagnosed with it, even if they don’t need treatment, has rights in law and may be able to support including benefits. See our information about blood cancer, money and work.

Myelofibrosis (MF) can develop in people who haven’t had blood or bone marrow problems before. This is known as primary MF.

In some cases, MF develops from another type of blood cancer: essential thrombocythaemia (ET) or polycythaemia vera (PV). The general term for this is secondary MF, or you may be told you have post-ET myelofibrosis or post-PV myelofibrosis. Only a small number of people who have ET or PV go on to get MF.

Primary and secondary MF are treated in the same way, so this information about MF is relevant whether or not you have previously had ET or PV.

ET, PV and MF are all types of blood cancer called myeloproliferative neoplasms, or MPNs.

Normally, your bone marrow makes the right number of blood cells to keep you healthy. With myelofibrosis, you can have either high or low numbers of blood cells (high or low blood counts) at different times.

If you have symptoms, it may be because of changes in your blood cell counts, inflammation in your bone marrow, a swollen spleen, or a combination of the three.

Prefibrotic myelofibrosis (MF) is where you have changes in the type of blood cells and their position in your bone marrow, but no significant scarring. It looks similar to another type of blood cancer called essential thrombocythaemia (ET). People with ET have a high level of platelets, and that’s common in prefibrotic MF too.

The similarities between ET and prefibrotic MF can make it hard to diagnose which condition you have. Doctors will look for certain changes in the cells of the bone marrow which show whether you have ET or MF, even if there is no scarring to be seen.

This information will help them decide on the best treatment for you. Some treatments are used for both conditions but in the longer term it is better to understand what your exact diagnosis is.

At this stage, doctors can see significant scarring in a sample of your bone marrow tissue. This is called overt myelofibrosis (MF)because the signs of MF are more obvious when doctors examine your bone marrow sample under the microscope.

As scar tissue builds up, your bone marrow may struggle to produce enough blood cells. When this happens, your blood counts will start to drop below the normal range.

We have some information about the normal range for blood counts, but check with your hospital team. What’s normal varies from person to person and also depends on the laboratory where the tests are done.

There is a chance that in its later stages, myelofibrosis (MF) will progress to another type of blood cancer called acute myeloid leukaemia (AML). This happens for 1 to 2 people out of every ten (10-20%). So most people will not progress to AML.

See our page on the prognosis for myelofibrosis (what is likely to happen in the future) for more information.

We know that myelofibrosis (MF) is caused by specific genetic changes that happen during someone’s lifetime although we don’t know why. You don’t normally inherit them from your parents or pass them on to your children.

MF is more common in people over 50 years old, although younger adults can also get it. It is slightly more common in men than women.

About a third of people with MF previously had another MPN – either ET or PV – which has developed into MF.

Genes are like a set of instructions that tell your cells how to behave, including how to divide to make new cells. Sometimes there’s a copying mistake as the cell divides and that causes a mutation.

People with myelofibrosis (MF) usually have a mutation in one of three specific genes:

• JAK2 – about 55 people in every 100 with MF (55%) have the JAK2 gene mutation
• CALR – about 25 people in every 100 with MF (25%) have the CALR mutation
• MPL – about 7 people in every 100 with MF (7%) have the MPL mutation

Knowing which genetic mutation you have gives doctors a bit more information about your prognosis (what’s likely to happen in the future). If you have previously had ET or PV, you may already be aware that you have a particular genetic mutation. However, you may have genetic tests done again to check for other mutations which might affect your prognosis.

For a few people, tests don’t show any of the three main genetic mutations. This might be because the MF is caused by another genetic mutation that can’t be identified easily. This is known as triple negative MF.

Whether or not doctors identify a specific gene mutation, you will receive the best treatment for you, based on your age and fitness, your symptoms, your medical history and your blood counts.

Your doctor may calculate your risk score – usually low, intermediate-1, intermediate-2 or high. This looks at a range of information including blood and genetic test results, age and overall health. It helps doctors decide whether you need treatment, and which treatment is best for you.

Find out more about risk scoring on our prognosis page. Read our treatment pages for information about treatment options.