What is myelofibrosis?

Myelofibrosis is a rare blood cancer. It causes scarring of the bone marrow. It's pronounced as my-eh-lo-fy-bro-sis. Myelo means bone marrow and fibrosis relates to the development of fibrous or scar tissue. The scar tissue builds up inside your bone marrow and blood cells can't develop properly. 

Myelofibrosis can affect people of any age. But it usually affects middle-aged and older people. The average age at diagnosis is 65 years.

Pre-fibrotic or early stage primary mylofibrosis means you have changes to your bone marrow but you do not have any bone marrow scarring.

Primary myelofibrosis

People who have no history of problems with their bone marrow can get MF. This is called primary myelofibrosis (PMF) or sometimes chronic idiopathic myelofibrosis. 

Secondary myelofibrosis

Secondary myelofibrosis is where the condition develops in people who have other bone marrow disorders. For example, polycythaemia vera or essential thrombocythaemia.

Myeloproliferative neoplasms

Myelofibrosis is a type of blood disorder called a myeloproliferative neoplasm. These are conditions that cause an increase in the number of blood cells.

The World Health Organisation (WHO) classes all myeloproliferative neoplasms as blood cancers. This is because the bone marrow is producing blood cells in an uncontrolled way. But many people with myeloproliferative neoplasms feel well and only need gentle treatment. The disorders often develop slowly and progress slowly. Or they can remain stable for a while.

The bone marrow and blood cells

The bone marrow is the soft inner part of our bones that makes blood cells. All blood cells start from the same type of cell, called a stem cell. The stem cell makes immature blood cells.

The immature cells go through various stages of development before they become fully developed blood cells and are released into the blood as:

  • red blood cells to carry oxygen around our bodies

  • white blood cells to fight infection

  • platelets to help the blood clot

The diagram shows how the various types of cell develop from a single blood stem cell.

A simplified diagram showing how blood cells are made

What happens in myelofibrosis?

In people with myelofibrosis, the bone marrow is too active, then scar tissue builds up (fibrosis). Because of this, blood cells are not made properly. The bone marrow gradually produces fewer blood cells. As the number of new blood cells in the bone marrow falls, the liver and spleen Open a glossary item try to help by making more blood cells. But they are not as good at making them as the bone marrow. So you might have too few red blood cells (anaemia Open a glossary item).

The spleen can become bigger (enlarged). Instead of releasing the red blood cells into the blood it keeps hold of them. This might cause discomfort in your tummy or make you feel full when you eat. As myelofibrosis develops, the bone marrow also makes fewer white blood cells Open a glossary item and platelets Open a glossary item.

Myelofibrosis and acute myeloid leukaemia (AML)

How myelofibrosis develops in people can vary. Treatment can control myelofibrosis for many people for some time.

Some people develop more serious problems. For example, heart disease and infections that might be difficult to control.

Around 20 out of every 100 people (around  20%) go on to develop acute myeloid leukaemia (AML). AML is a type of blood cancer. 

In this situation, your doctor would talk to you about starting treatment for AML.

Risks and causes of myelofibrosis

We don’t know exactly what causes primary myelofibrosis. Some research suggests that exposure to the chemical benzene may increase your risk of developing myeloproliferative neoplasms. You are most likely to come into contact with benzene through work. It is used in the rubber industry and is one of the chemicals in petrol.

Genetic changes 

Changes in the JAK2, CALR, and MPL genes are seen in people with primary myelofibrosis. 

More than 55 in 100 people (more than 55%) with myelofibrosis have a change in a gene Open a glossary item called JAK2. The JAK2 gene makes a protein that controls how many blood cells the stem cells make. 

Up to 35 in 100 people (up to 35%) have a change in the CALR gene. And about 8 in 100 people (about 8%) have a change in the MLP gene. When the gene becomes mutated Open a glossary item your bone marrow may not function correctly. This means scar tissue can build up in your bone marrow.

About 10 in 100 people with primary myelofibrosis (around 10%) will not have any of these particular gene changes. 

Some people with myelofibrosis have a history of essential thrombocythaemia or polycythaemia vera.


Myelofibrosis usually develops slowly and doesn’t cause symptoms at first. Some people find out about their myelofibrosis when having a blood test for something else.

As the condition progresses it causes symptoms. These might include:

  • tiredness and shortness of breath - due to low numbers of red blood cells
  • bleeding and bruising easily - due to low numbers of platelets
  • pain and discomfort in the tummy (abdomen) due to enlarged spleen and liver
  • bone pain
  • gout - you might have painful, stiff or swollen joints
  • loss of appetite and weight loss
  • fever
  • night sweats
  • very itchy skin (pruritus)

Myelofibrosis is rare, so if you have any of these symptoms it is more likely to be something else. But it is still important to contact your doctor to find out what is causing them.

  • Myelofibrosis: Clinicopathologic Features, Prognosis, and Management

    JM O’Sullivan and CN Harrison

    Clinical Advances in Hematology and Oncology, 2018. Volume 16, Issue 2

  • The 5th edition of the World Health Organization Classification of Haematolymphoid Tumours: Myeloid and Histiocytic/Dendritic Neoplasms

    J D. Khoury and others

    Leukemia, 2022. Volume 36, Pages 1703–1719

  • Acute Myeloid Leukemia Evolving from Myeloproliferative Neoplasms: Many Sides of a Challenging Disease

    F Mannelli

    Journal of Clinical Medicine 2021. Volume 10, Issue 4, Page 436.

  • Myeloproliferative Neoplasms, NCCN Clinical Practice Guidelines in Oncology

     A T Gerds and others

    Journal Of the National Comprehensive Cancer Network, 2022. Volume 20: Issue 9

  • Pan-London Haemato-Oncology Clinical Guidelines, Acute Leukaemias and Myeloid Neoplasms Part 4: Myeloproliferative Neoplasms

    RM Partners, South East London Cancer Alliance, North Central and East London Cancer Alliance, 2020

Last reviewed: 
07 Nov 2023
Next review due: 
07 Nov 2026

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