Chronic myeloid leukemia

CML is a cancer of the blood-forming cells in the bone marrow. The marrow makes far too many white blood cells, and they build up in the blood over months and years.

“Chronic” here means slow-growing. It does not mean mild, and it does not mean harmless — it describes the pace, which is the thing that separates CML from the acute leukemias that move over weeks.

Almost everyone with CML has the same underlying fault, called the Philadelphia chromosome. Part of one chromosome has swapped places with part of another, joining two genes — ABL1 and BCR — that were never meant to be joined. The fused gene, written BCR::ABL1, makes an enzyme called a tyrosine kinase. A tyrosine kinase is a switch that tells cells when to grow. In CML that switch is stuck on.

Almost the whole of modern CML treatment follows from that one sentence: if you know exactly which switch is stuck, you can make a drug that blocks it.

The BCR::ABL1 fault is acquired during a person’s life, inside a single blood-forming cell in the marrow. It is not inherited, and it is not something anyone did or failed to do.

The NCI states it plainly, and it is the sentence families ask about most: the Philadelphia chromosome is not passed from parent to child. A CML diagnosis is not a reason for relatives to have genetic testing.

Cancer Research UK is equally plain on the wider question: the cause of most cases of CML is not known. There is no evidence base pointing at diet, stress or lifestyle, and CML is not contagious.

The marrow overproduces white cells, so a blood test shows a white cell count that is much too high. That is how a great many people are diagnosed — the blood test was being done for something else entirely.

The NCI notes that in some cases CML causes no symptoms at all. Where there are symptoms they tend to be vague: fatigue, weight loss for no clear reason, drenching night sweats, fever. The spleen sits under the left ribs and often enlarges, which people feel as an ache, a fullness, or getting full after a small meal.

CML is described in three phases, defined by how many blast cells there are. A blast is an immature blood cell that has not matured into anything useful. In the chronic phase, blasts are under 10% of cells in the blood and marrow; in the accelerated phase, 10% to 19%; in the blastic phase, 20% or more.

Left uncontrolled, CML can move from the chronic phase toward the blastic phase, where it starts behaving like an acute leukemia. That progression is what treatment is designed to prevent. It is worth understanding as the reason for the monitoring, rather than as something that is coming.

First-line treatment is a tyrosine kinase inhibitor, usually shortened to TKI. It is a tablet that blocks the stuck switch. Several are available as a first treatment — imatinib, which is also sold as a generic, along with dasatinib, nilotinib and bosutinib.

Treatment is judged by the quantitative PCR result rather than by how someone feels, and there are milestones along the way. The European LeukemiaNet guidance treats a BCR::ABL1 level above 10% at three months, confirmed, as treatment failure — meaning the plan changes, not that the person has failed at anything.

Changing drugs is normal and expected. The two words that come up are resistant and intolerant. Resistant means the disease keeps growing despite the drug. Intolerant means the drug is working but the side effects cannot be lived with. Both are reasons to switch, and there are later-line options — ponatinib for disease carrying a change called T315I that blocks the older drugs, and asciminib, which grips a different part of the same protein, so some disease that has stopped responding still responds to it.

For a subset of people, stopping treatment becomes possible. This is called treatment-free remission, and it is real — but it is conditional and it is monitored closely. In the EURO-SKI trial, everyone had already taken a TKI for at least three years and held a very deep response for at least a year before stopping, and about half still lost that response afterward and restarted.

Many people arrive at this diagnosis sideways. A routine blood test comes back wrong, and within a week or two they are being told they have leukemia while feeling more or less fine. The shock is often out of all proportion to how ill they feel, and that gap is its own difficulty — it is hard to be frightened about something invisible, and hard to explain to other people.

Day to day, treatment is a tablet at home. For most people there is no admission, no infusion suite, and no course that ends on a set date.

Life instead becomes organised around blood tests. The PCR number becomes the number people live by, and waiting for each result is a recurring strain that does not really go away — a good result buys a few months of calm, and then the next test comes round.

Side effects are the usual reason a treatment changes. The problem is rarely one dramatic event; it is a low-grade daily burden — fatigue, nausea, cramps, rashes, swelling — that varies by drug and accumulates over years. Switching to a different TKI because of it is an ordinary part of care.

For most people diagnosed with CML today, a donor plays no part at all. That is the most important sentence on this page, and it is placed here deliberately rather than buried: a transplant is not the usual treatment, and it is not what most people with CML will ever face.

Where a donor still matters is a small minority — disease in the blastic or advanced phase, disease that has become resistant to several TKIs including ponatinib, and people who cannot tolerate any of the available drugs. For those people the donor question is entirely real.

CML used to be one of the classic reasons a person needed a bone marrow transplant, and older material still says so. It is worth knowing that this changed, and that it changed because a drug worked. Any activity statistic from before the TKI era overstates how much transplantation happens for CML now.