ACS :: Types of Stem Cell Transplants

In a typical stem cell transplant high doses of chemotherapy are used, sometimes along with radiation therapy, to treat the cancer. This treatment also kills the stem cells in the bone marrow. In some diseases, like aplastic anemia, treatment is meant to kill the existing bone marrow to make room for new stem cells. Very soon after treatment, a stem cell transplant is done to provide new stem cells that will grow into healthy blood cells. These normal stem cells are given into a vein, much like a blood transfusion. Over time they will settle in the bone marrow and begin to grow and make blood cells. This process is called engraftment.

There are 3 basic types of stem cell transplants: autologous, allogeneic, and syngeneic. The type of transplant depends on where the stem cells come from.

In this type of transplant, you are your own donor, using stem cells from either your bone marrow or circulating blood. Your stem cells are removed or harvested before treatment and then frozen. After you get high doses of chemo and/or radiation the stem cells are thawed and given back to you.

An advantage of autologous stem cell transplant is that you are getting your own cells back. This means there is no risk that your immune system will reject the transplant or that the transplanted cells will attack your own body.

A possible disadvantage is that cancer cells may be harvested along with the stem cells and then put back into your body. To prevent this, doctors may treat your stem cells with anti-cancer drugs or other therapies to reduce the number of cancerous cells that may be present. This is called purging. Purging may damage some healthy stem cells, so extra cells are taken from the patient before the transplant to be sure that enough healthy stem cells will be left after purging.

This kind of transplant is mainly used to treat leukemias, lymphomas, and multiple myeloma, but it is sometimes used for other cancers.

A tandem transplant is a "double autologous transplant." In a tandem transplant, a patient gets 2 courses of high-dose chemo, each followed by a stem cell transplant. All of the stem cells needed are collected before the first high-dose chemo treatment and half of them are used for each procedure. Most often both courses are given within 6-months, with the second one done after the patient recovers from the first one. Researchers hope that this method can keep the cancer from coming back and are still studying how this method can best be used.

This type is being used for the treatment of certain types of cancer, including multiple myeloma, Hodgkin disease, and non-Hodgkin's lymphoma.

Here, the stem cells do not come from the patient, but from a donor whose tissue type (described below under "HLA matching") best matches the patient. The donor is most often a family member, usually a brother or sister. If you do not have a good match in the family, a donor may be found from the general public through a national registry. Blood taken from the placenta and umbilical cord of newborns is a newer source of stem cells. This small amount of blood has a high number of stem cells. Still, the numbers are often too low for large adults, so this source of stem cells is used mostly in small adults and children.

An advantage of allogeneic stem cell transplant is that the donor stem cells produce their own immune cells, which may help destroy any cancer cells that remain after high-dose treatment. Another possible advantage is that the donor can often be asked to donate more stem cells if needed. Stem cells from healthy donors are also free of cancer cells.

Still, there are many possible drawbacks to allogeneic stem cell transplant. The transplant, also known as a graft, may not "take" -- that is, the donor cells may be more likely to die or be destroyed by the patient's immune system before settling in the bone marrow. Another possibility is that the donor cells will make new immune cells that attack the recipient's body -- a condition known as graft-versus-host disease (described in the "Problems in the post-transplant period" section). There is also a very small risk of certain infections from the donor cells, although donors are always tested beforehand to minimize this risk.

This type of transplant is most often used to treat leukemias, lymphomas, and other bone marrow disorders.

Another type of allogeneic transplant is called a reduced-intensity transplant, non-myeloablative transplant, or mini-transplant. This transplant uses less intense chemo and/or radiation to get the patient ready for the transplant compared with a standard allogeneic transplant. The idea here is to kill some of the cancer cells, some of the bone marrow, and suppress the immune system just enough to allow donor stem cells to settle in the bone marrow. The new immune cells then begin to destroy the remaining cancer cells, in what is known as a "graft-versus-tumor" effect.

In this procedure, the patient is given low doses of chemo -- not enough to destroy all the cancer or all of the bone marrow, but enough to suppress the patient's immune system. After the chemo the donor stem cells are infused. Unlike the standard allogeneic transplant, cells from both the donor and the patient may exist together in the patient's body for some time after a mini-transplant. But slowly, over the course of months, the donor cells take over the bone marrow and replace the patient's own bone marrow cells. These new cells then develop an immune reaction to the cancer and kill off the patient's cancer cells.

The advantage of a mini-transplant is that you don't need high doses of chemo and/or radiation. This makes it especially useful in older patients, those with other health problems who aren't strong enough for a normal stem cell transplant, or patients who have already had a transplant.

Mini-transplants have been found to treat some diseases better than others. They may not work well for patients with a lot of disease in their body at the time of transplant or those with fast-growing disease. Also, the lowered immune response could still lead to graft-versus-host disease.

Although it is actively being studied, this procedure has only been in use since the late 1990's and long-term patient outcomes are not yet available. Ways to improve the procedure are still being studied.

Another possibility that is being studied is autologous transplant followed by non-myeloablative allogeneic transplant. This decreases the amount of cancer present so that a non-myeloablative conditioning regimen with an allogeneic transplant can be more effective.

This is a special kind of allogeneic transplant because the donor is an identical twin with identical tissue types. Since few people are identical twins, this type of transplant is very rare. An advantage of syngeneic stem cell transplant is that graft-versus-host disease will not be a problem. A disadvantage is that this type of transplant won't help destroy any remaining cancer cells. So every effort must be made to destroy all the cancer cells before the transplant is done.

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