Primary myelofibrosis (PMF) is a rare, chronic blood cancer in which scar tissue (fibrosis) builds up in the bone marrow, crowding out the cells that produce blood. This leads to anemia, fatigue, and an enlarged spleen. “Primary” means the disease develops on its own, rather than progressing from another myeloproliferative neoplasm.
Myelofibrosis belongs to a group of blood cancers called myeloproliferative neoplasms (MPNs), alongside polycythemia vera and essential thrombocythemia. It shares the same driver mutations as those diseases, most commonly JAK2, CALR, and MPL.
Primary vs. Secondary Myelofibrosis
When myelofibrosis develops spontaneously, it is called primary myelofibrosis. When it evolves from an existing MPN, it is called secondary myelofibrosis, and is described as post-polycythemia vera MF or post-essential thrombocythemia MF depending on the disease that preceded it. Although they develop differently, primary and secondary myelofibrosis produce similar symptoms and are generally treated the same way.
Myelofibrosis is generally the most serious of the three main MPNs, but outcomes vary widely. Risk scoring tools help predict the course of the disease, and treatment options have expanded significantly over the past decade.
What Happens in the Bone Marrow
Myelofibrosis produces more whole-body (constitutional) symptoms than the other MPNs. Patients may present with elevated or decreased platelets and white blood cells, and by the time the disease is diagnosed, red blood cell counts are often low to normal.
As fibrosis increases, the marrow becomes less able to produce blood cells effectively, and the cells it does produce are often irregular in shape and function poorly. Over time this can lead to severe anemia, increased risk of infection, and bleeding problems.
When the marrow can no longer keep up, blood production migrates to other organs, a process called extramedullary hematopoiesis. This most often occurs in the spleen and liver, causing the organ enlargement that many myelofibrosis patients experience.
What Causes Myelofibrosis?
As with other MPNs, the underlying trigger for myelofibrosis is not fully understood. The large majority of patients carry one of three driver mutations: JAK2 (Janus kinase 2), CALR (calreticulin), or MPL. A small percentage have no identifiable driver mutation, sometimes described as “triple negative” disease.
These mutations are acquired, meaning you are not born with them and they are not passed directly to children. There does appear to be a genetic predisposition toward acquiring them, and clusters of MPNs within families have been documented, but inherited myelofibrosis is uncommon.
At the cellular level, mutated stem cells in the bone marrow stimulate fibroblast cells to overproduce collagen. That excess collagen is the scarring that defines the disease, and it progressively alters both the number and the shape of every blood cell type.
Risk Factors for Myelofibrosis
- Age over 60, with a median age at diagnosis around 65, though myelofibrosis can occur at any age
- Slightly more common in men
- A prior MPN diagnosis, particularly polycythemia vera or essential thrombocythemia
- Exposure to industrial chemicals such as benzene or toluene
- Prior radiation exposure
- A genetic predisposition
Common Symptoms of Myelofibrosis
Some patients have no symptoms early in the disease and are diagnosed after a routine blood test. As myelofibrosis progresses, symptoms typically develop and increase:
- Fatigue, weakness, and shortness of breath, worsening as anemia progresses
- Pain or fullness in the upper left abdomen, especially after eating, caused by an enlarged spleen
- Feeling full quickly (early satiety) and reduced appetite
- Unexplained weight loss and muscle wasting
- Bone and joint pain
- Night sweats or low-grade fever
- Easy bruising or bleeding from low or abnormal platelets
- Itching (pruritus)
- Abdominal swelling from portal hypertension, meaning increased pressure in the vein leading to the liver
- Frequent infections when white blood cell counts are low
- Difficulty concentrating, often described as brain fog
Symptom burden varies considerably between patients, and research shows that doctors frequently underestimate how severely MPN symptoms affect their patients. Tracking symptoms and reporting them specifically at appointments is one of the most useful things a patient can do.
Diagnosis of Myelofibrosis
Diagnosing myelofibrosis requires several tests, since no single result confirms the disease on its own:
- Physical examination, including palpation of the spleen and lymph nodes
- Complete blood count (CBC) to measure red cells, white cells, and platelets
- Molecular testing of the blood for the JAK2, CALR, and MPL mutations
- Bone marrow biopsy, which is required to grade the extent of fibrosis and check for chromosomal abnormalities
- Next-generation sequencing to detect additional mutations that may indicate higher risk of progression
- Imaging, such as ultrasound, to assess spleen and liver size
Because myelofibrosis is rare and can resemble other conditions, particularly in its early stages, the reading of a bone marrow biopsy matters enormously. Early myelofibrosis and essential thrombocythemia can be difficult to distinguish, and the treatment paths differ. A second opinion from an MPN specialist is worth pursuing whenever the diagnosis is uncertain.
Risk Scoring and Prognosis
Prognosis in myelofibrosis varies widely, which is why risk stratification drives nearly every treatment decision. Several validated scoring systems are used:
- IPSS (International Prognostic Scoring System), applied at the time of diagnosis
- DIPSS (Dynamic IPSS), which can be applied at any point during the disease course
- DIPSS-Plus, which adds chromosomal abnormalities, platelet count, and transfusion status
- MIPSS70 and MIPSS70+, newer systems that incorporate molecular data and are used particularly when evaluating patients for stem cell transplant
These tools sort patients into low, intermediate-1, intermediate-2, and high-risk categories. Patients with low-risk myelofibrosis can have a life expectancy exceeding 10 years, particularly those without anemia, without constitutional symptoms, with normal white counts, no circulating blasts, and under age 65. Intermediate and high-risk disease carries a considerably shorter prognosis.
Ask your hematologist which scoring system they used and where you fall within it. Risk category determines whether watchful waiting is appropriate, which medications are indicated, and whether stem cell transplant should be considered.
Treatment of Myelofibrosis
Treatment depends on risk category, symptom burden, and blood counts. Goals include stabilizing blood counts, reducing spleen size, easing constitutional symptoms, and in some cases slowing or reversing fibrosis. Low-risk patients with minimal symptoms may need no active treatment beyond regular monitoring.
Stem Cell Transplant
Allogeneic stem cell transplant is the only potentially curative treatment for myelofibrosis. It also carries substantial risk, and many patients do not qualify due to age, other health conditions, or advanced disease. Transplant timing is one of the most consequential decisions in myelofibrosis care and is best made with an MPN specialist using molecular risk scoring.
JAK Inhibitors
JAK inhibitors target the overactive JAK-STAT pathway that drives the disease. They can reduce spleen size and improve constitutional symptoms including itching, night sweats, fatigue, and bone pain.
- Jakafi (ruxolitinib), the first JAK inhibitor approved for myelofibrosis
- Inrebic (fedratinib), an option for patients previously treated with ruxolitinib
- Vonjo (pacritinib), indicated for intermediate or high-risk myelofibrosis with thrombocytopenia (low platelets)
- Ojjaara (momelotinib), approved for myelofibrosis with anemia
Other Treatments for Symptoms and Spleen Size
- Pegylated interferons, which may reduce spleen size, relieve itching and bone pain, and possibly reduce fibrosis
- Hydroxyurea, which can reduce spleen size and help control counts
- Splenic radiation, to reduce extramedullary hematopoiesis and organ enlargement
- Splenectomy (surgical spleen removal), reserved for extreme cases given the significant risks
- Palliative and supportive care, which should be part of the plan at any stage, not only late in the disease
Treatments for Anemia
- Momelotinib, which addresses anemia alongside spleen and symptom control
- Androgens such as danazol
- Thalidomide or lenalidomide, often combined with steroids
- Blood transfusions
Investigational Treatments
Myelofibrosis has one of the most active research pipelines in hematology. Agents under study, alone or in combination with ruxolitinib, have included pelabresib, navtemadlin, navitoclax, selinexor, bomedemstat, imetelstat, and pomalidomide, along with epigenetic drugs such as HDAC inhibitors.
Trial status changes frequently. Our MPN Clinical Trial Finder lets you search current studies by diagnosis and location.
These research programs reflect ongoing investigation and are not treatment recommendations. Discuss any treatment decisions with your hematologist or MPN specialist.
Complications of Myelofibrosis
- Excessive bleeding from low platelet counts
- Infection related to low white blood cell counts
- Splenic infarction or, rarely, splenic rupture from severe enlargement
- Portal hypertension from increased blood flow to the liver
- Progression to acute myeloid leukemia (AML), which occurs in a minority of patients but is the most serious risk associated with the disease
What This Means for You
A myelofibrosis diagnosis is serious, and it is also more variable than most patients realize when they first read about it online. Low-risk patients can live well for a decade or longer, and the treatment landscape has changed substantially since the first JAK inhibitor was approved.
Two priorities matter most. The first is knowing your risk score, since it drives every treatment decision including transplant timing. The second is being treated by someone who sees myelofibrosis regularly. Community hematologists may encounter only a few MPN patients in a career, and myelofibrosis is the MPN where that experience gap matters most.
Resources to Help You Take the Next Step
- MPN Specialist Directory: find a verified MPN specialist by state
- MPN Clinical Trial Finder: search open studies by diagnosis
- Financial Assistance Programs: help with treatment costs
- MPN Cancer Dictionary: plain-language definitions of terms you will hear
- Understanding MPNs: how myelofibrosis compares to PV and ET
Frequently Asked Questions About Primary Myelofibrosis
What is the difference between primary and secondary myelofibrosis?
Primary myelofibrosis develops spontaneously, with no preceding blood disorder. Secondary myelofibrosis evolves from an existing MPN and is described as post-polycythemia vera myelofibrosis or post-essential thrombocythemia myelofibrosis. Despite the different origins, symptoms and treatment approaches are largely the same.
What is the life expectancy with primary myelofibrosis?
Life expectancy depends heavily on risk category. Patients with low-risk disease, meaning no anemia, no constitutional symptoms, normal white counts, no circulating blasts, and age under 65, can have a life expectancy exceeding 10 years. Intermediate and high-risk disease carries a shorter prognosis. Risk scoring systems such as DIPSS and MIPSS70 give a more individualized estimate than general statistics.
Is myelofibrosis curable?
Allogeneic stem cell transplant is the only potentially curative treatment for myelofibrosis. It carries significant risks and is generally reserved for younger patients with intermediate-2 or high-risk disease. For most patients, treatment focuses on controlling symptoms, managing blood counts, and maintaining quality of life rather than cure.
What are the first signs of myelofibrosis?
Many patients have no symptoms early on and are diagnosed through routine bloodwork. When symptoms do appear, the most common early signs are persistent fatigue, shortness of breath from developing anemia, and fullness or discomfort in the upper left abdomen caused by an enlarged spleen.
Does myelofibrosis turn into leukemia?
A minority of myelofibrosis patients progress to acute myeloid leukemia. Progression is not inevitable, and risk varies based on mutation profile, blast count, and risk category. Regular monitoring allows the care team to identify changes early.
Is myelofibrosis hereditary?
Myelofibrosis is generally not inherited. The JAK2, CALR, and MPL driver mutations are acquired during a person’s lifetime rather than passed from parent to child. Some families do show a higher predisposition toward developing MPNs, but direct inheritance of the disease is uncommon.
