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Bone Marrow/Stem Cell Transplant Frequently Asked Questions
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3. What is a bone marrow/stem cell transplant?

A transplant, at its most basic level, is a procedure where an individual receives healthy stem cells to replace stem cells that were damaged either because of disease or treatment. Stem cells are the source of the different types of blood cells in our body, including the white blood cells, which fight infection. As such, stem cells play a key role in the functioning of our immune system. Stem cells also give rise to the red blood cells, which carry oxygen, and the platelets, which aid in blood clotting.

Stem cells are found in the spongy center of our bones but can be collected from different places in the body. The type of transplant a person receives depends in part on where the stem cells are collected.

  • If the stem cells are collected from the bone marrow, which is the spongy tissue found in the cavities of the body's bones, the procedure is referred to as a bone marrow transplant.
  • If the stem cells are collected from the circulating blood, instead of from the bone marrow, the procedure is referred to as a peripheral blood stem cell (PBSC) transplant.
  • If the stem cells are collected from the umbilical cord of a baby immediately after it is born, the procedure is referred to as a cord blood transplant.

The type of transplant also depends on who donates the stem cells:

  • In an autologous transplant, the patient donates his/her own stem cells prior to treatment for reinfusion later.
  • In an allogeneic related transplant, the person donating the stem cells is a biologically-related family member (usually a brother or sister).
  • In an allogeneic unrelated transplant, the person donating the stem cells is not related to the patient but has been adequately tissue-matched.
  • In a syngeneic transplant, the person donating the stem cells is an identical twin.

Overview of the Transplantation Process

Autologous transplant—The autologous transplant is not necessarily performed because there is something wrong with the bone marrow or stem cells. It is performed because the dose of chemotherapy and radiation needed to treat the cancer in the body is so high that it will destroy the patient's existing stem cells in the bone marrow. And without stem cells, the body cannot produce blood cells or an immune system. So, in cases where the patient's stem cells are healthy, they are collected prior to the high dose chemotherapy/radiation and are stored for safekeeping. After receiving high doses of chemotherapy and/or radiation to treat the cancer, the stem cells are then re-infused into the patient's body through a vein, just as in the case of a blood transfusion. The patient's own cells thus "rescue" or restore blood cell production and the immune system.

In most cases, it is recommended that the patient be in a full remission for the autologous transplant to be successful. However, in the case of some malignancies, patients with minimal disease can also be transplanted with their own stem cells. The chemotherapy given for transplant often destroys any remaining diseased cells.

Even when the patient is in remission, it's possible for some cancer cells to remain in the bone marrow or peripheral blood. In order to deal with this, some transplant centers have developed methods of purging or cleansing the stem cell product in an attempt to remove any remaining cancerous cells. The goal is to destroy enough diseased cells so that the body's defense system will be able to destroy them after re-infusion. Purging is done differently from center to center, and many centers don't do it at all. Bring up any questions you have about this procedure and the different methods used at different hospitals with your doctor and other medical professionals.

Allogeneic transplant—An allogeneic transplant is usually done when a patient has a disease or condition that affects their stem cells, such as leukemia or aplastic anemia or some genetic conditions. In these cases, the healthy stem cells of a donor are used to replace the patient's damaged stem cells. Donors are all carefully screened to make sure that they are a good genetic match. Donors can be identical twins, siblings, or unrelated strangers.

Once infused into the patient's body, the new healthy stem cells from the donor will then migrate to the spongy tissue in the bone and generate new blood cells, including a new immune system. This can be important in preventing relapse after transplant because the donor's immune cells can destroy any remaining cancer cells in the patient, thus reducing the risk of relapse. This is referred to as graft-versus-tumor (GVT) or graft-versusleukemia (GVL) effect. This graft-versus-tumor effect is one of the important treatment benefits of allogeneic stem cell transplantation.

To prepare for an allogeneic transplant, patients receive a conditioning treatment which involves chemotherapy with or without radiation. The purpose of conditioning treatment is to destroy any remaining cancerous cells in the body and also to weaken the patient's immune system so that the new donor cells are not rejected and can grow and reproduce in the body. The duration and intensity of the conditioning treatment varies depending on the disease being treated and the age of the patient. Patients who are older or who have certain immune deficiencies may not require as intensive a conditioning regimen prior to transplant.

  • Myeloablative transplant: In cases where the conditioning treatment involves high doses of chemotherapy and/or radiation and destroys the stem cells in the body, the process is referred to as myeloablative (marrow destroying). In this type of transplant, the patient receives high doses of chemotherapy and/or radiation that will destroy the host immune system along with as many remaining cancer cells as possible. The process of destroying the stem cells in the marrow creates "marrow space" for the new donor stem cells to grow in. This type of transplant is associated with greater side effects because of the high-dose chemotherapy and/or radiation. However, it can lead to a lower risk of relapse (the cancer coming back).
  • Non-myeloablative transplant/reduced intensity transplant: In cases where the conditioning is less intensive and only weakens the immune system, the conditioning treatment is called non-myeloablative (non-marrow destroying). In this type of transplant, the patient receives lower doses of chemotherapy and/or radiation that will not destroy his or her marrow completely. Instead, the treatment weakens the patient's immune system just enough so that the donor's stem cells are not rejected by the existing immune system. After this type of transplant, both the donor's stem cells and the patient's own stem cells can coexist (also called "mixed chimerism"). In a successful transplant, the donor cells will gradually fight off the patient's blood and immune cells and establish a new donor immune system.

Graft versus Host Disease (GVHD)

In addition to the treatment benefits of high-dose chemotherapy and radiation therapy, allogeneic transplantation can provide the additional benefit of a strong, new immune system. However that new donor immune system can also attack the patient's healthy tissues and organs, including the skin, gut, and liver. This reaction is called graft versus host disease (GVHD). The medical team will try to control the severity of GVHD through the use of immunosuppressant medications that will allow the new donor immune system to accommodate to its new environment in the patient's body. Over time, the new immune system will learn to recognize the patient as "self" and will develop tolerance to the genetic differences in the tissues of its new environment. However, for the first several years after an allogeneic transplant, many patients do experience signs and symptoms of acute and then chronic GVHD. This can include symptoms such as dry eyes and mouth, discoloration in the skin, rashes, and fatigue, among others.

However, graft versus host disease can also be beneficial and reduce the risk of recurrence through the "graft versus malignancy" effect discussed earlier.

For additional information about graft versus host disease, please see webcasts: "Chronic Graft versus Host Disease in Adults" and "Graft versus Host Disease in Children and Adolescents" available at www.nbmtlink.org. Or call the nbmtLINK at 800-LINK-BMT (800- 546-5268) or e-mail info@nbmtlink.org for additional information and resources.

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