When a child/adolescent is being considered for a bone marrow transplant (BMT), one of the first steps is to determine the best donor source for the stem cells needed for the procedure.

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Allogeneic: A donor other than self. If an allogeneic transplant is indicated, the next step is to find out whether a family member is a suitable donor. This involves special blood studies called tissue typing performed on the blood from the patient and his/her family.

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HLA typing: Human leukocyte antigen is a fingerprint (or DNA) that represents our body’s ability to identify another person’s cells as foreign. These tests help gauge how much the donor and recipient cells will recognize one another as the same or different. The greater the differences between the donor and recipient in the HLA typing, the greater the chance that the left over recipient immune system will reject the donor’s cells—resulting in rejection or graft failure, or that the donor immune cells will attack the recipient and cause a reaction called graft versus host disease (GVHD).

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Each person inherits two sets of chromosomes, one set from their father and one set from their mother which makes up their tissue type. There is a 1 in 4, or 25% chance that any brother or sister will have inherited the same two sets of HLA genes as the patient. For a parent to be matched with his or her child, both parents must by chance have some HLA genes in common with each other. There is only a 1 in 200 chance that a parent and child will be HLA matched. The best donor is a histocompatible (HLA) matched relative who is usually a sibling or, in rare cases, a parent with an identical HLA tissue type. It is very unlikely for two unrelated individuals to have the same HLA genes in common; about 1 in a million chance. Fortunately the National Marrow Donor Program (NMDP) or its equivalent in your country if you do not live in the US has a very large computer registry, representing millions of donors from all over the world. This computer registry is used to find the best HLA unrelated match for patient’s without related donors.

 

• Matched sibling donor (MSD): Brother or sister

• Matched related adult donor (MRD): Parent or even more rarely another relative

• Matched unrelated donor (MUD): Unrelated individual whose stem cells match

• NMDP: National Marrow Donor Registry (NMDP) When a related donor cannot be found and there is time to conduct a search, an alternative is to identify an unrelated donor, which is a healthy individual who is matched to the patient and is willing to donate bone marrow stem cells.

• Unrelated Umbilical Cord blood (UCB): Cord blood, which is normally disposed of after a baby is born, contains a relatively large number of stem cells. There are over 30 cord blood registries worldwide which process and store cord blood collections from healthy babies. Potential advantages include; UCB does not need to be a perfect HLA match with the recipient and the units are readily available for transplantation. Disadvantages include the limited number of cells in the collection leading to a slower recovery of marrow function post transplant and potential graft rejection and no potential for a second infusion if one was needed.

Autologous: Patient is a donor for self. For some types of cancer, it is possible to use the patient’s own marrow stem cells either from the bone marrow or peripheral blood for the transplant.  This is called an autologous transplant.  This is used when there has been such an intense treatment of chemotherapy and/or irradiations used that the patient’s marrow function will recover too slowly to prevent very serious and life threatening infections. The autologous cells are used to “rescue” the marrow function. Some examples of types of cancers which might be treated using autologous stem cells include but are not limited to:

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• Brain Tumors

• Neuroblastoma

• Lymphomas

• Sarcomas

• Wilm’s Tumor

• Germ Cell Tumor

• Primitive neuroectodermal tumors (PNET)

• Acute promyelocytic leukemia (APL)

Whenever possible, peripheral blood stem cells are used instead of bone marrow for autologous transplants.  Again, prior to admission for transplant, the patient’s stem cells are collected using a procedure called leukopheresis.  The procedure frequently requires the placement of a special intravenous line, called an apheresis catheter.  In preparation for the leukopheresis, the patient receives a drug called granulocyte colony stimulating factor (G-CSF), brand name Neupogen, for three to four days to stimulate stem cells to leave the bone marrow space and enter the blood stream.  During the stem cell collection or leukopheresis the patient’s blood is passed through a machine that collects the portion of white cells containing bone marrow stem cells.  The rest of the remaining white blood cells, red cells and platelets are given back to the patient.  Following the high dose chemotherapy, the stored stem cells are thawed and re-infused back into the patient.

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If marrow needs to be harvested for transplant it is done several weeks to months before the transplant. The harvest is done in the operating room under general anesthesia and the stem cells are frozen in liquid nitrogen until the preparative therapy is completed, and the cells are needed to be re-infused for marrow recovery.

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Advantages of an autologous transplant are that once stem cells are collected, they are readily available and there is an absence of the risk of graft versus host disease. Graft versus host disease (when the donor cells see the recipient as foreign and attack the host) is a side effect of allogeneic transplants only.   A potential disadvantage of an autologous bone marrow transplant is that there is a greater risk of relapse of the patient’s cancer.  Your physician will discuss which donor option is best for you and your child.