Stem cell transplantation, sometimes called a bone marrow transplant, is an intervention that replaces the diseased cells that make the blood with healthy cells. Stem cell transplantation allows physicians to give the patient high doses of chemotherapy or radiation therapy to increase the chances of eliminating cancer from the blood in the marrow and restoring normal blood cell production. Researchers continue to improve stem cell transplants, making it an option for more patients.
The basis of stem cell transplantation is that blood cells (red blood cells, white blood cells and platelets) and immune cells (lymphocytes) are derived from stem cells, which are present in bone marrow, peripheral blood and blood cord. Intense treatment with chemotherapy or radiation kills the patient’s stem cells. This prevents stem cells from making enough blood and immune cells.
The patient receives high dose chemotherapy or radiation therapy, or both, followed by a stem cell transplant. The stem cells of a donor are then transfused into the blood of the patient. The grafted stem cells move from the patient’s blood to his or her marrow.
The donor is usually a brother or sister of the patient who is compatible with him. If the patient does not have a compatible sibling, an unrelated person whose stem cells are compatible with the patient’s tissue type will be used. It is through banks or stem cell donor registries that compatible unrelated donors can be found.
The new stem cells develop and provide a supply of red blood cells, white blood cells (including immune cells) and platelets. Stem cells received from the donor make immune cells that are not fully compatible with the patient’s cells. The major types of tissues of patients and donors are compatible, but not minor types of tissue. This is why the immune cells of the donor could recognize the minor types of tissues of the patient’s cancer cells as foreign and kill the cancer cells. This is called the graft-versus-cancer effect.
Types of stem cell transplants
The doctor will recommend to the patient who is a candidate for a stem cell transplant one of three types of transplant:
- Autograft, where the stem cells come from the patient himself
- Allograft, where the stem cells come from a healthy person (the donor)
- Reduced intensity stem cell transplantation: as in allografting, stem cells come from a healthy donor, but administered chemotherapy is less intensive.
A fourth type of stem cell transplant, syngeneic graft or isograft, is much rarer than the other three. This is a transplant where the stem cells come from an identical twin. In addition, the donor twin and the twin receiving the transplant must have the same genetic background and the same type of tissue.
Who is a candidate for a stem cell transplant?
The doctor will consider several factors in deciding whether or not a patient is a candidate for stem cell transplantation. For the allograft, the doctor will consider:
- The possibility that the disease responds to the conditioning treatment
- The availability of an appropriate donor
- The general state of health of the patient
- His age and any medical condition.
In the case of an autograft, the doctor will consider:
- the possibility that the disease responds to other types of treatment
- patient diagnosis (eg, autograft is not commonly used to treat acute lymphocytic leukemia because of high relapse rates, nor is it common in myelodysplastic syndromes)
- if the patient has a sufficient number of stem cells that can be harvested
- his general state of health
- his age.
Stem cell allograft is more likely to succeed in younger patients than in older patients. About three-quarters of people who develop blood cancer are over 50 years old. In general, older people are more likely to:
- to present medical problems, a complicating factor
- to develop the graft-versus-host reaction
- to have a lower tolerance to the cumulative effects of intensive chemotherapy and the necessary radiotherapy treatments with the transplant.
However, these factors are only general considerations and there is no real age limit for a stem cell transplant.
Other factors and the underlying disease response to initial cancer treatment determine when the doctor will consider transplant options. Some patients receive a stem cell transplant during the first remission. For others, transplantation is recommended later as part of treatment for refractory or recurrent disease.
Before receiving a stem cell transplant, the patient will need a preparatory treatment, also called conditioning treatment. High dose chemotherapy or radiation therapy will be given to the patient to:
- reduce the risk of immune cells shedding donor cells (for standard allograft)
- to eliminate any disease that may still remain at the time of the transplant
- eliminate disordered lymphocytes (white blood cells) that attack developing blood cells, in some cases involving bone marrow deficiency.
Conditioning for a reduced-intensity allograft involves reduced doses of chemotherapy or radiation therapy compared to standard allograft.
Stem cell administration
The donor stem cells are transferred to the patient by infusion, a procedure similar to a blood transfusion. The blood is given to the patient by inserting a catheter, a thin and flexible tube, into a large blood vessel, usually in the chest.
Stem cell infusion usually takes several hours. The medical team monitors the patient for signs of fever, chills or hives, low blood pressure, or shortness of breath. The patient may experience side effects such as headache, nausea, redness and shortness of breath caused by the cryopreservative used to freeze the stem cells. These symptoms will be treated before continuing the infusion.