Hairy cell leukemia is a rare form of cancer, comprising only about 2% of leukemia cases in adults each year. About 700 people are diagnosed with hairy cell leukemia annually. It is five times more common in men than in women and the vast majority (90%) of patients are of European ancestry. People are typically diagnosed during middle age, but it is occasionally found in people in their 20s, as well as in older adults. 

Hairy cell leukemia can be slow growing, so some people with the disease don’t require immediate treatment. When treatment becomes necessary, this form of leukemia usually responds well to therapy, leading to remission in the majority of people. The cancer may recur, but treatments often help the cancer go back into remission.  

“Hairy cell leukemia is an uncommon diagnosis and can occasionally be mimicked by other B cell cancers,” says Smilow Cancer Hospital oncologist Scott Huntington, MD, MPH, MSc. “We now have excellent molecular testing—as in BRAF mutation testing—that not only helps with making the diagnosis—but can also inform treatment selection. Fortunately, available treatments for hairy cell leukemia are highly effective and the selection of the optimal therapy for a given patient requires discussion of individual preferences and potential side effects.“

Hairy cell leukemia is an uncommon type of cancer of the blood (leukemia) that affects the B cells, a type of white blood cell that makes antibodies to help the body fight infections. It gets its name from the unusual shape that the B cells take on when someone has this form of cancer: Each affected B cell appears to have small hairs protruding from its sides, when observed under a microscope.  

In healthy people, the bone marrow produces blood stem cells, which mature into healthy red blood cells, platelets and a variety of white blood cells. When someone has hairy cell leukemia, their bone marrow produces too many immature B cells. These immature cells don’t mature into healthy B cells. Instead, they multiply and crowd other types of blood cells out of the blood. This may lead to lower levels of red blood cells (anemia) or low levels of all three types of blood cells (red blood cells, white blood cells and platelets). Insufficient white blood cells make it harder for the body to fight off infections.  

Hairy cells congregate in the blood, spleen, bone marrow, and liver. They often cause the spleen to enlarge—and sometimes the liver as well. Enlargement of the spleen may cause pain or discomfort in the upper left portion of the abdomen, where that organ is located.

Doctors aren’t sure what causes hairy cell leukemia. It is sometimes linked to chemical exposure. Other people may have a genetic mutation that is linked to hairy cell leukemia. 

The most commonly associated genetic mutation is BRAF but another, CDKN1B, has also been identified in some cases. A gene mutation may cause someone’s B cells to mutate, become “hairy” and multiply uncontrollably, leading to cancer.

People who have hairy cell leukemia may:

• Bruise easily
• Experience night sweats
• Feel fatigued or weak
• Feel full after eating small servings of food
• Have pain or discomfort in the upper-left abdomen
• Become ill often with a fever and infection
• Lose weight unintentionally
• Experience swollen lymph nodes
• Have bone pain
  

Doctors are able to diagnose hairy cell leukemia after learning about a patient’s medical history, performing a physical exam and offering different tests.  

A doctor may suspect hairy cell leukemia if someone visits the doctor with fatigue, weakness and/or pain in the area near the spleen. The doctor may ask if the person has been sick frequently, if they feel full before finishing their meals, or if they have lost weight without trying to do so.  

If someone complains of abdominal pain, doctors should examine the abdomen; they may be able to feel an enlarged spleen. The doctor may also look for bruises on the skin and feel for enlarged lymph nodes.  

Sometimes, imaging tests, such as a CT scan or ultrasound, may help determine the cause of abdominal swelling, but these imaging tests can’t diagnose hairy cell leukemia.  

A simple blood test that looks at the complete blood count can also provide information useful in diagnosing hairy cell leukemia. Signs might include lower-than-normal levels of red blood cells, white blood cells and platelets. Additional blood work would look for a higher-than-expected proportion of hairy cells in the blood and lower-than-expected numbers of other white blood cells.  

To confirm the diagnosis, doctors typically perform a bone marrow biopsy to check for the presence of hairy cell leukemia in the bone marrow. BRAF mutation testing can help confirm a Hairy cell diagnosis, helping to differentiate the disease from variant Hairy cell or other non-Hodgkin lymphoma.

For some patients, doctors might recommend a watch-and-wait approach. If symptoms are present—such as discomfort from an enlarged spleen, frequent infections, or abnormally low levels of blood—treatment may be required. Some patients need blood transfusions in response to low blood levels before needing other treatment.  

However, most people diagnosed with hairy cell leukemia receive immediate treatment, typically one of two chemotherapy drugs: either cladribine or pentostatin. Between 70 and 95% of people experience remission following chemotherapy. For some, the cancer never returns, while others may stay in remission for 5 to 8 years or longer.  

When the chemotherapy drugs cladribine and pentostatin aren’t effective against hairy cell leukemia, doctors may prescribe different treatments, including:

• Vemurafenib, a BRAF inhibitor, which may also be used as a treatment if the cancer returns
• Interferon-alfa, a biologic therapy
• Moxetumomab pasudotox, a monoclonal antibody

If the spleen is a source of pain, it may be removed. This isn’t an effective cancer treatment, but it may alleviate some of the pain and discomfort that’s associated with hairy cell leukemia.  

If hairy cell leukemia returns, doctors may prescribe the same treatment that worked in the past, or they may try an alternate treatment. If cladribine or pentostatin isn’t effective on its own this time around, doctors may add the monoclonal antibody rituximab, which often provides better results.

Because this form of cancer responds well to treatment, more than 95% of people who are diagnosed survive 10 years or longer. The majority of people experience complete remission after treatment.  

Although 30 to 40% of people who go into remission may experience relapse, the treatments for hairy cell leukemia are so effective, people are often able to lead normal lives between diagnoses.

“As a rare condition, having both expert input from specialized pathologists and expert leukemia/lymphoma clinicians can help confirm the diagnosis and develop a personalized treatment plan,” says Dr. Huntington. “Smilow Cancer Center and its network of sites often allows patients to receive a highly specialized consultation, with the eventual treatment provided locally at their community site.”