Myelofibrosis is a cancer. Specifically, it is a blood cancer classified by the World Health Organization as a myeloproliferative neoplasm, a group of cancers in which the bone marrow produces abnormal blood cells. With an estimated U.S. incidence of about 1.56 per 100,000 people per year, it is rare, and the median age at diagnosis is 67.
Why Myelofibrosis Is Classified as Cancer
The word “neoplasm” means abnormal, uncontrolled cell growth, which is the defining feature of cancer. Myelofibrosis begins when a stem cell in the bone marrow acquires a genetic mutation and starts producing abnormal blood cells that crowd out healthy ones. These mutated cells don’t just misbehave locally. They trigger a cascade of damage that progressively scars the bone marrow, disrupts normal blood production, and can eventually transform into acute myeloid leukemia, a more aggressive blood cancer, in roughly 10 to 20% of cases within a decade.
The WHO groups myelofibrosis alongside two other “classical” myeloproliferative neoplasms: polycythemia vera (PV), which overproduces red blood cells, and essential thrombocythemia (ET), which overproduces platelets. Myelofibrosis can arise on its own (called primary myelofibrosis) or develop as a later stage of PV or ET. About 5% of ET patients and up to 23% of PV patients eventually progress to secondary myelofibrosis over the course of one to two decades.
What Happens Inside the Bone Marrow
Healthy bone marrow is soft, spongy tissue that manufactures red blood cells, white blood cells, and platelets. In myelofibrosis, mutated stem cells produce abnormal megakaryocytes, the large cells responsible for making platelets. These defective megakaryocytes release massive amounts of a signaling protein that stimulates nearby cells called fibroblasts to lay down scar tissue. Studies have found that the concentration of this protein inside the abnormal megakaryocytes is 5 to 10 times higher than normal.
As scar tissue accumulates, the marrow gradually loses its ability to make blood cells. The body compensates by shifting blood production to other organs, particularly the spleen and liver. The spleen swells dramatically as it takes on work it was never designed to handle. Among patients with an enlarged spleen, about 74% have a spleen that extends 10 to 20 centimeters below the rib cage, far larger than normal.
Genetic Mutations Behind the Disease
Three driver mutations account for the vast majority of cases. About 60 to 66% of patients carry a mutation in the JAK2 gene, which tells bone marrow cells to keep dividing when they should stop. Roughly 12 to 20% have a mutation in the CALR gene, and about 5% have a mutation in the MPL gene. The remaining 17% test negative for all three and are called “triple-negative,” a group that generally carries a worse prognosis.
These mutations don’t just identify the disease. They also influence how it behaves and how well it responds to treatment. Patients with CALR mutations, for example, tend to have longer survival than those with JAK2 mutations or triple-negative disease.
Symptoms and How They Affect Daily Life
Fatigue is the most common symptom, reported by roughly 85% of patients. This is not ordinary tiredness. It stems from anemia (too few red blood cells), the body’s inflammatory response to the cancer, and the metabolic drain of an overworked spleen. Weight loss, night sweats, and fever each affect 50% or more of patients, a cluster sometimes called constitutional symptoms because they reflect the body’s systemic response to the disease.
An enlarged spleen creates its own set of problems. It presses against the stomach, causing early fullness after eating, abdominal pain (especially on the left side), and general discomfort. In one large review, 85% of patients with splenomegaly reported left-side abdominal pain, 79% experienced early satiety, and 67% had generalized abdominal pain. Bone pain, muscle pain, itching, bruising, and shortness of breath each appear in more than 30% of patients.
How Prognosis Is Determined
Doctors use scoring systems that combine factors like age, blood counts, symptoms, genetic mutations, and chromosomal abnormalities to place patients into risk categories. One widely used tool, DIPSS Plus, divides patients into four groups with strikingly different median survival times: roughly 15 years for low-risk, 6.5 years for intermediate-1, about 3 years for intermediate-2, and around 16 months for high-risk. These are population-level medians, so individual outcomes vary considerably.
The risk category also guides treatment decisions, particularly whether a stem cell transplant should be considered.
Treatment: Managing Symptoms vs. Seeking a Cure
For most patients, treatment focuses on controlling symptoms rather than eliminating the disease. JAK inhibitors are the mainstay of drug therapy. These oral medications work by blocking the overactive signaling pathway driven by the JAK2 mutation (and related pathways), which helps shrink the spleen and reduce constitutional symptoms like night sweats, weight loss, and fatigue. They are approved for patients with intermediate or high-risk disease.
JAK inhibitors do not cure myelofibrosis or reverse bone marrow scarring. They improve quality of life, sometimes dramatically, but the underlying cancer persists.
The only treatment with curative potential is an allogeneic stem cell transplant, in which a donor’s healthy stem cells replace the patient’s diseased bone marrow. This is a high-risk procedure. Five-year survival after transplant ranges from about 83% for low-risk transplant candidates down to 22% for very high-risk patients, based on a scoring system that factors in age, fitness level, blood counts, donor match quality, and specific mutations.
Transplant eligibility depends more on overall health than on a strict age cutoff. Historically, it was reserved for patients under 60, but the shift toward less intensive conditioning regimens has extended eligibility to older patients. Heart, lung, kidney, and liver function all need to meet minimum thresholds. Because myelofibrosis is a disease of older adults and transplant carries significant risks, including treatment-related death, it is typically reserved for patients whose disease is intermediate-risk or higher and who are physically fit enough to tolerate the procedure.
Primary vs. Secondary Myelofibrosis
Primary myelofibrosis develops on its own, without a preceding blood cancer. The WHO further divides it into two stages: a prefibrotic/early stage, where scarring is minimal and blood counts may appear nearly normal, and an overt fibrotic stage, where significant scarring has already developed. Early-stage disease can sometimes be mistaken for essential thrombocythemia because both conditions produce elevated platelet counts, but the distinction matters because myelofibrosis carries a higher risk of progression.
Secondary myelofibrosis develops in patients who already have polycythemia vera or essential thrombocythemia. The transition can happen years or even decades after the original diagnosis, and it signals a more advanced phase of disease. Treatment options for secondary myelofibrosis are largely the same as for primary disease, including JAK inhibitors and, for eligible patients, stem cell transplant.