Adult T-cell leukemia/lymphoma

It is a cancer of mature T cells — white blood cells that are part of the immune system — that develops only in people with long-standing infection by a virus called HTLV-1.

It is rare in the UK, at around 50 diagnoses a year. It is far more common in the regions where the virus itself is common: southwest Japan, the Caribbean, sub-Saharan Africa, South America and parts of the Pacific.

It is not one disease. Four forms are recognised, and they behave differently enough that treatment differs completely between them. Two are aggressive — the acute form and the lymphoma form. Two are slow-moving — the chronic form and the smouldering form.

The lymphoma form presents with enlarged lymph nodes without a high count of abnormal cells circulating in the blood. The acute form presents with those cells in the blood.

Which of the four someone has is the single most important thing on this page, because it decides whether a transplant is part of the conversation at all.

This is the one condition in this part of the library with an established viral cause. It develops from chronic infection with HTLV-1, which inserts itself into a type of T cell and stays there.

And the most important thing to say about that is what it does not mean. WHO estimates the lifetime risk of developing this cancer among people with HTLV-1 at about 5% — so roughly 95 in every 100 people who carry the virus never develop it. The delay between infection and cancer, in those who do, is measured in decades.

You may encounter a much higher figure. It applies specifically to carriers found on testing to have a high proportion of their blood cells carrying the virus. It does not describe carriers generally, and someone who has never had that test should not read it as being about them.

This cancer is also not the only outcome associated with the virus. WHO puts the lifetime risk of an associated neurological condition at about 2%.

WHO estimates 5 to 10 million people worldwide were living with HTLV-1 based on 2012 data, and notes that data are missing from many regions — so that is a range and probably a floor rather than a count.

The virus passes from mother to child, principally through breastfeeding, with a transmission rate WHO puts at 20% to 30%. It can also pass through sexual contact, through sharing needles, and through unscreened blood transfusion or organ transplantation. There is no vaccine, and WHO lists prevention approaches including screening blood donations and, in some circumstances, feeding infants formula instead of breast milk.

That last one is a clinical decision made with a doctor, weighed against local conditions. We are describing the route the virus takes, not telling anyone how to feed a baby.

The four forms behave so differently that describing them together would misinform most readers.

The acute and lymphoma forms progress rapidly and are treated urgently.

The chronic and smouldering forms are slow-moving and may be managed for extended periods without intensive chemotherapy.

The median age at presentation is 60 to 70 in Japan, and one to two decades earlier elsewhere.

We have not published survival figures by subtype. The review we used gives ranges but does not state how many people sit behind each one, and a range without a denominator is not something to put in front of someone reading about their own diagnosis.

Treatment is chosen by which of the four forms someone has, and the two paths barely resemble each other.

The slow-moving forms — smouldering and chronic — are commonly managed with antiviral treatment: zidovudine combined with interferon alfa, rather than intensive chemotherapy or a transplant.

The aggressive forms — acute and lymphoma type — are treated with combination chemotherapy. Different regimens are standard in Japan and in North America.

Two newer drugs are approved in Japan for disease that has come back or not responded: an antibody against a receptor these cells carry, and a drug that blocks a pair of enzymes the cells depend on.

One piece of timing is worth knowing if a transplant is being planned. The antibody drug increases the risk of severe graft-versus-host disease if it is given shortly before a donor transplant, and a gap of at least 50 days between them is recommended.

For the aggressive forms, a donor transplant is described as the treatment able to offer a lasting remission, and is recommended in first or a later remission for everyone eligible. That is covered properly in the next section.

Many people learn they carry HTLV-1 only at the point of this diagnosis. That raises immediate questions about family — particularly children who were breastfed, and partners. Testing and counselling for relatives is a real part of what follows, and it lands on top of a cancer diagnosis.

Two people told they have "the same" condition can be on entirely different paths. One may be starting urgent chemotherapy with a transplant being planned; another may be on antiviral treatment and being monitored. That is not a difference in how seriously they are being taken.

It is rare outside the endemic regions, which means referral to a specialist centre and, for many people, being the only case their local team has seen.

For those going to a transplant, there is the donor search, the conditioning treatment, and a recovery that the published outcomes show is genuinely dangerous.

For the aggressive forms of this disease, a donor transplant matters as much as it does anywhere in this library. It is described as the one approach able to offer a lasting remission, and it is recommended in first or a later remission for everyone eligible for it. A transplant using the person’s own cells is not discussed as an option at all.

EBMT’s 2025 recommendations likewise place a donor transplant in first-line therapy for this disease, and add that people whose disease is stable or progressing should still be considered — because the other treatments do not usually produce long-term remission.

That applies to the aggressive forms only. People with the smouldering or chronic forms are commonly managed with antiviral treatment, and roughly half the people with this diagnosis should not be told they need a donor.

We are going to state the outcomes rather than let the phrase "able to offer a lasting remission" stand on its own, because it promises more than the evidence delivers.

Where a donor transplant is the right route and no relative matches, that donor comes from a registry. For someone with an aggressive form of this disease, a stranger joining a registry is a real and direct link to whether they have that option.