Mast cell leukemia (MCL) is a very rare type of blood cancer. It often grows quickly and can lead to severe symptoms. MCL is slightly more likely to affect females than males. It can develop in people of all ages, including adults and children. MCL does not run in families and isn’t passed down from parents to children.
MCL develops from mast cells, which are a part of the immune system. Mast cells are made in the bone marrow (spongy tissue found inside certain bones) and are found in different organs throughout the body. Healthy levels of mast cells help protect the body from infection and are responsible for allergic reactions.
Mast cell leukemia is a type of acute myeloid leukemia (AML). It is also part of a group of conditions called mastocytosis. The symptoms of MCL are more similar to those of mastocytosis than they are to AML.
Different forms of mastocytosis are caused by high levels of mast cells in the body. The two main types of mastocytosis are cutaneous and systemic mastocytosis. Cutaneous mastocytosis develops in the skin, and systemic mastocytosis affects the whole body. The World Health Organization defines MCL as a type of aggressive (fast-growing) systemic mastocytosis.
In systemic mastocytosis, mast cells build up in different organs and cause allergic reactions or inflammation when the body is exposed to triggers like spicy foods, alcohol, or exercise. Symptoms can sometimes be severe and may lead to organ damage.
Milder types of systemic mastocytosis can transform into mast cell leukemia or mast cell sarcoma (cancer that affects connective tissues like muscle, fat, or blood vessels). About 7 percent of children and 30 percent of adults with systemic mastocytosis develop one of these mast cell cancers. Overall, less than 1 percent of all mastocytosis cases are MCL.
MCL may go by different names depending on a person’s signs and symptoms. People with MCL usually have mast cells in both the blood and the bone marrow. However, in some cases, people have low levels of mast cells in the blood. Doctors call this aleukemic MCL. Additionally, if MCL causes organ damage, it may be called acute MCL (aMCL). If there is no organ damage, doctors may call the condition chronic MCL (cMCL).
Mast cell leukemia sometimes occurs along with other types of leukemia. In this case, the disease is called MCL with an associated hematologic neoplasm (MCL-AHN). Blood cancers that may occur along with MCL include chronic myelomonocytic leukemia (CMML), myelodysplastic syndrome (MDS), myelodysplastic syndrome/myeloproliferative neoplasm unclassifiable (MDS/MPNu), and chronic eosinophilic leukemia (CEL).
If MCL develops in a person who previously had another form of mastocytosis, doctors refer to it as secondary MCL. When MCL occurs on its own, it is known as de novo MCL.
Many people with MCL experience symptoms of mast cell activation syndrome (MCAS). MCAS occurs when mast cells create an allergic response although there is no allergen (a substance that triggers an allergic reaction). Symptoms of MCAS include fever, flushing (warmth and reddening of the skin), diarrhea, fast heart rate, and malaise (generally feeling unwell).
MCL can also cause other signs and symptoms, including:
Your health care team may do several tests to confirm a diagnosis of MCL.
Blood and bone marrow tests are important in the diagnosis of mast cell leukemia. Diagnosis of MCL usually starts with bone marrow tests. A doctor will remove a small sample of cells from inside the bone using a procedure called a bone marrow biopsy. Fluid may also be removed by a procedure called bone marrow aspiration. Doctors usually remove these samples from the hip bone.
After bone marrow tests, the sample cells will be closely examined in the laboratory. Cells from blood samples may be analyzed as well. Doctors will usually check to see how many mast cells are present. A person is diagnosed with MCL if at least 20 percent of their bone marrow cells are mast cells, or if more than 10 percent of the cells in their blood are mast cells.
Blood tests may also be used to measure serum levels of an enzyme called tryptase. Most people with systemic mastocytosis have high levels of this enzyme, which is produced by mast cells.
Other laboratory tests examine the genes and proteins found within mast cells. MCL is often caused by mutations in a gene called KIT. People with MCL may need to undergo gene sequencing to determine which KIT mutation they have. Knowing which mutations are found in mast cells can help the doctor diagnose someone with systemic mastocytosis or help the doctor know which treatments to recommend or avoid.
Doctors also use immunophenotyping tests, such as flow cytometry, to study mast cells in the bone marrow and look for abnormalities. Flow cytometry measures which proteins are found on the outside surface of a cell. People with different types of systemic mastocytosis (including mast cell leukemia) often have mast cells that carry specific proteins on their surface, so immunophenotyping tests can help a doctor diagnose MCL.
Because MCL is so rare, it is difficult to study. This means that there is no standard therapy doctors recommend. However, researchers have identified several types of treatment that may help kill abnormal mast cells or reduce symptoms of MCL. Clinical trials, if available, may be a good option for people with this condition. People with mast cell leukemia may be able to access new treatment options by participating in a clinical trial.
Targeted therapies are medications that block specific genes or proteins found within cancer cells. The U.S. Food and Drug Administration has approved a targeted therapy drug called Rydapt (midostaurin) to treat MCL. Midostaurin is a type of medication called a tyrosine kinase inhibitor. It blocks enzymes that leukemic cells rely on to grow and divide.
Sometimes, doctors will also recommend other kinase inhibitors that are not approved for MCL, but that are used to treat other types of leukemia. These other targeted therapies, such as Gleevec (imatinib) and Sprycel (dasatinib), may help some people, but these medicines don’t work against mast cells that have developed certain gene mutations.
Researchers are continuing to study targeted therapies for MCL. In recent years, studies have found that drugs like Ayvakit (avapritinib) and Qinlock (ripretinib) may help people with advanced systemic mastocytosis including MCL. Mylotarg (gemtuzumab ozogamicin) may also help kill MCL cells that have certain gene and protein changes.
Chemotherapy may include the medications Intron A (interferon-alpha) or Mavenclad (cladribine). If someone has severe MCL that is getting worse quickly or is resistant to other treatments, doctors may recommend using aggressive chemotherapy regimens that are often used to treat AML. One possible regimen, called FLAG, consists of Fludara (fludarabine), Cytosar-U (cytarabine), and Neupogen (granulocyte colony-stimulating factor or G-CSF). A similar regimen, CLAG, consists of cladribine, cytarabine, and G-CSF. Hydroxyurea can also be used to lessen MCL symptoms.
Doctors often recommend treatment with corticosteroids for MCL. These drugs may decrease levels of mast cells and relieve MCL symptoms. However, steroids often only work in the short term and have side effects.
Allogeneic stem cell transplantation is an option for some people who are younger and don’t have many other health problems. This procedure provides a new supply of stem cells (cells that make new blood cells) when the old stem cells become damaged by medical treatments or crowded out by leukemia cells. During an allogeneic stem cell transplant, a person receives high-dose chemotherapy or radiation therapy to remove old blood cells and stem cells. They then receive new stem cells from a donor.
MCL can be a fast-growing disease, and treatments don’t always work. Many people with MCL have a poor prognosis (outlook). Research has found that people live for an average of six months after being diagnosed with MCL. However, the outlook for MCL is improving due to new treatments. In one study, people lived for an average of 17 months after diagnosis while being treated with midostaurin. Some people may live for several years with MCL.
Various factors can affect a person’s outlook. People who have chronic MCL (do not have organ damage) tend to have a better prognosis. People with mutations in the genes SRSF2, ASXL1, or RUNX1 are likely to have worse outcomes.
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