A stem cell transplant is used to restore normal blood cell production in people whose bone marrow has been damaged.
A stem cell transplant (SCT) allows children to have much higher doses of chemotherapy than they could otherwise tolerate. This improves the chance of curing their cancer but has more side effects. Very high doses of chemotherapy destroy the blood stem cells in bone marrow, so after high-dose chemotherapy a child will be given stem cells via a drip. These make their way to the bone marrow and start producing blood cells again.
Stem cell transplantation is the new terminology for what used to be known as bone marrow transplantation. The new terminology reflects a different way of collecting the cells for transplantation.
Our bones contain a spongy tissue known as bone marrow. Contained within the bone marrow are blood stem cells, from which all our blood cells develop. The bone marrow of a healthy adult produces about three million red blood cells and 120,000 white blood cells every second.
Chemotherapy and radiotherapy affect normal tissues as well as cancer cells. Stem cells are particularly vulnerable.
When bone marrow has been badly damaged by high dose treatment, a stem cell transplant (SCT) can help to repair this damage and restore normal blood cell production. SCT therefore enables doctors to give much more intensive treatment than would otherwise be possible. There are two different types of transplant:
As well as enabling doctors to use high dose chemotherapy, SCT can play an important role in the cure of leukaemia through an effect known as graft versus leukaemia (GvL). In GvL, immune cells contained in the graft attack leukaemic cells remaining in the patient’s body. This makes SCT a very important treatment option for children for whom chemotherapy has failed.
Allogeneic stem cell transplantation is a high-risk procedure. Its use is usually restricted to children who have relapsed early in their treatment for leukaemia, or who have suffered more than one relapse.
In order to minimise the risk of complications, it is necessary to find a donor whose tissue type matches the patient’s as closely as possible. The preferred donor is a sibling with a closely matched tissue type. The next best choice is a matched family member. If not available, a matched unrelated donor will be sought from the national bone marrow register.
Transplants contain functioning cells from the donor’s immune system. This gives the patient important protection because their own immune system has been destroyed in preparation for the transplant. It also promotes GvL.
However, there is a risk that the immune cells will recognise the patient’s cells as foreign and attack them. This is called graft versus host disease (GvHD). Closely matching the tissue types of the donor and the patient minimises the risk of GvHD.
Cord blood is the blood that remains in the placenta and umbilical cord after a baby is born. It is rich in stem cells.
In England and Wales the NHS operates a cord blood bank. Parents can choose to donate the placenta and umbilical cord following the birth of their baby. This blood is then available to anyone who needs it.
The great advantage of cord blood, other than the fact that it is already banked and available for transplant, is that cord blood stem cells are immunologically naïve and so need not be as closely matched as stem cells from other sources.
The limiting factor, however, is the number of stem cells available from a single cord; this restricts its use to younger children who need a smaller ‘dose’ of stem cells.
Autologous stem cell transplants are relatively straightforward but allogeneic transplantation is an inherently risky procedure.
The child’s own bone marrow is destroyed before the transplant takes place; this effectively destroys their immune system. With no immune system, exposure to even a common infection could prove deadly. To minimise the risks, the child will have to remain in isolation until the graft has ‘taken’ – this usually takes several weeks. They will probably have to stay off school for several months until their immune system is stronger.
The transplant itself can cause all sorts of complications including the risk of graft rejection and graft versus host disease.
Sadly one of the most common long-term side effects is infertility because of the high-dose chemotherapy and radiotherapy used to destroy the child’s bone marrow before the graft.