Stem Cell Transplantation – Cancer Treatment

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:

• Autologous transplants use the child’s own stem cells. Healthy stem cells are harvested before treatment and returned to the child after treatment. Autologous transplants are generally used for children with solid tumours

• Allogeneic transplants (also known as allografts) use stem cells from a donor. This is a more complicated, more risky procedure. Recovery may take several months or more. Allogeneic transplants are used in the treatment of leukaemia.

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.