Myeloid/lymphoid neoplasm with FGFR1 rearrangement

This is a rare blood cancer that is thought to start in an early blood-forming stem cell. It can involve myeloid cells, such as neutrophils and eosinophils, and lymphoid cells, such as T cells and B cells. So it can show up in either branch of the blood, or in both at once. That is why it is called a myeloid/lymphoid neoplasm.

It used to be called 8p11 myeloproliferative syndrome, after the spot on chromosome 8 (8p11) where the FGFR1 gene sits. The World Health Organization (WHO) now places it in a family called myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions. Eosinophils are a type of white blood cell. Most people with this disease have too many of them, though not everyone does.

At first it can look like other blood cancers, such as a myeloproliferative neoplasm, acute myeloid leukemia, or T-cell or B-cell lymphoblastic leukemia or lymphoma. Finding the FGFR1 change gives the disease its name, and it changes the treatment plan.

The cause is a rearrangement of chromosomes. The part of chromosome 8 that holds the FGFR1 gene breaks and joins another chromosome (a translocation). FGFR1 fuses with a partner gene. The fused gene makes an FGFR1 protein that is always switched on, telling cells to grow and divide without stopping.

The most common partner gene is ZMYM2 on chromosome 13, from a swap called t(8;13). Another common partner is BCR on chromosome 22. More than a dozen partner genes have been found, and different partners have been linked with somewhat different patterns of disease.

The change happens during a person’s life in blood-forming cells (a somatic change). MedlinePlus Genetics says the condition is generally not inherited.

Most people have symptoms such as tiredness or night sweats. Some have none, and the disease is found through a routine blood test. The spleen and liver often grow larger, and lymph nodes may swell. Blood tests usually show a high white count, often with many eosinophils.

The disease is known for changing quickly. In a 2018 review of published cases, nearly half of the people who started with a chronic form had moved into blast phase within 12 months. Blast phase means an acute leukemia or a lymphoblastic lymphoma. When it is a lymphoma, it most often involves T cells.

Sometimes the chronic form and a fast-growing form appear together, such as a myeloproliferative neoplasm in the marrow with a lymphoma in the lymph nodes. Disease outside the bone marrow (extramedullary disease) is common.

It is often first suspected when blood tests show a high white count with extra eosinophils, or when a leukemia or lymphoma has unusual features. The work-up includes a bone marrow biopsy, and a lymph node biopsy if nodes are swollen. Flow cytometry, a test that sorts cells by the markers on their surface, shows whether myeloid cells, T cells or B cells are involved.

The key test is a chromosome study (karyotype) of the marrow, which can show a break at 8p11. A FISH test, which uses glowing probes, then confirms that the FGFR1 gene is split. RNA-based gene fusion panels can find hidden or unusual partner genes when other tests are unclear. In the US, there is no FDA-approved test for choosing patients for pemigatinib.

The team also rules out other causes of high eosinophils, such as reactive causes from other illnesses, and other blood cancers. Scans may be used to look for disease outside the bone marrow.

Treatment depends on the phase of the disease and the person’s health. Imatinib works well for some related diseases with PDGFRA or PDGFRB changes, but it does not work for this one. Intensive chemotherapy, like that used for acute leukemia or lymphoma, can bring remission, but the disease nearly always comes back.

Pemigatinib is a pill that blocks FGFR proteins. In August 2022, the US Food and Drug Administration (FDA) approved it for adults with this disease. It is for disease that has come back (relapsed) or has not responded to treatment (refractory). In its main study, FIGHT-203, 23 of 24 people in chronic phase and 8 of 18 in blast phase had a complete response. Common side effects include high phosphate levels, nail changes, hair loss, mouth sores, dry eyes and fluid under the retina. It is not approved for this disease in the European Union. In March 2025, the European Medicines Agency recommended refusing this use. It said the study was small and had no comparison group.

A donor (allogeneic) stem cell transplant is the treatment known to offer a lasting cure. Care teams often use pemigatinib or chemotherapy first to get the disease into remission, then move to transplant. Experts say a transplant remains essential for accelerated-phase or blast-phase disease. In a European registry study of 22 people, about 74 in 100 were alive 5 years after transplant.

Getting the right diagnosis can take time, because the disease can look like several other blood cancers. The key clue is often a chromosome test that shows a change at 8p11, confirmed by a FISH test for FGFR1.

People taking pemigatinib have regular blood tests, including phosphate levels, and eye exams every few months, because of its side effects. Many need their dose paused or lowered at some point. In FIGHT-203, about 6 in 10 people had a dose reduction.

For people who go on to a transplant, there may be weeks of staying in or near the hospital. After a donor transplant, the immune system takes 1 to 2 years to fully recover. Researchers suggest a targeted medicine after transplant may help when tests still find the FGFR1 change.

A donor transplant for this disease uses healthy blood-forming cells from another person. As with other donor transplants, the donor may be a relative or someone who is not related, such as a registry volunteer. Donated cord blood has also been used.

Timing matters, because the disease can move into blast phase within months. European transplant experts advise identifying people who could have a transplant early, and pemigatinib can help people reach remission before transplant.

The published transplant studies for this disease are small and do not report how often an unrelated donor was used. A 2026 expert review notes that transplant use is limited by age, other illnesses and whether a suitable donor can be found.

When a transplant from a donor is planned, the team usually tests brothers and sisters first. Each full sibling has about a one in four chance of being a full match.

Most patients do not have a matched relative. In the words of NMDP, the U.S. registry, “75% of patients don’t have a fully matched donor in their own family.” The team then searches registries of volunteer donors around the world and banks of donated cord blood. In some transplants, a half-matched parent, child or sibling can also be the donor.

Matching depends on inherited tissue markers called HLA, so a patient is most likely to match someone who shares their ancestry. Every person who joins makes the search a little more likely to succeed, especially for patients from groups that are underrepresented on registries.

Before targeted treatment, the outlook was poor. In a 2018 review of 45 cases, mostly from published reports, fewer than half of people were alive 1 year after diagnosis. People whose blast-phase disease went into complete remission early lived longer.

Pemigatinib has changed the picture for many people. In FIGHT-203, almost everyone in chronic phase had a complete response. When results were reported, the typical (median) length of these responses had not yet been reached. Responses were lower in blast phase. The disease can become resistant to pemigatinib, and experts still see a transplant as essential for accelerated-phase or blast-phase disease.

In a European registry study of people who had a donor transplant, about 3 in 4 were alive 5 years later. About 1 in 4 had the disease come back. That study included only 22 people, all selected and fit for transplant. No figure can say what will happen to one person.