In a typical stem cell transplant very high doses of chemo are used, often along with radiation therapy, to try to destroy all of the cancer. This treatment also kills the stem cells in the bone marrow. Soon after treatment, stem cells are given to replace those that were destroyed. These stem cells are given into a vein, much like a blood transfusion. Over time they settle in the bone marrow and begin to grow and make healthy blood cells. This process is called engraftment.

There are 3 basic types of transplants. They are named based on where the stem cells come from.

• AUTOLOGOUS: The cells come from you
  
• ALLOGENEIC: The cells come from a matched related or unrelated donor
• SYNGENEIC: The cells come from your identical twin or triplet

Autologous stem cell transplant

In this type of transplant, your stem cells are taken before you get cancer treatment that destroys them. Your stem cells are removed, or harvested, from either your bone marrow or your blood and then frozen. After you get high doses of chemo and/or radiation the stem cells are thawed and given back to you. One 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 or reject your body. This kind of transplant is mainly used to treat leukemias, lymphomas, and multiple myeloma. It's sometimes used for other cancers, like testicular cancer and neuroblastoma, and certain cancers in children.

Tandem transplants

A tandem transplant is also called a double autologous transplant. In a tandem transplant, the patient gets 2 courses of high-dose chemo, each followed by a transplant of their own stem cells. All of the stem cells needed are collected before the first high-dose chemo treatment, and half of them are used for each transplant. Most often both courses of chemo are given within 6 months, with the second one given after the patient recovers from the first one. Tandem transplants are sometimes used to treat certain types of cancer, but doctors do not agree on when and how to use this type of transplant. For many people, the risk of serious outcomes is quite high.

Allogeneic stem cell transplant

In this type of transplant, the stem cells do not come from the patient, but from a donor whose tissue type closely matches the patient. The donor is often a family member, usually a brother or sister. If you do not have a good match in your family, a donor might be found from the general public through a national registry. This is sometimes called a MUD (matched unrelated donor) transplant. Blood taken from the placenta and umbilical cord of newborns is a newer source of stem cells for allogeneic transplant. Called cord blood, this small volume of blood has a high number of stem cells that tend to multiply quickly. But the number of stem cells in a unit of cord blood is often too low for large adults, so this source of stem cells is limited to small adults and children. An advantage of allogeneic stem cell transplant is that the donor stem cells make their own immune cells, which could help destroy any cancer cells that remain after high-dose treatment. This is called the graft-versus-cancer effect. Other possible advantages are that the donor can often be asked to donate more stem cells or even white blood cells if needed, and stem cells from healthy donors are free of cancer cells. Allogeneic transplant is most often used to treat certain types of leukemia, lymphomas, and other bone marrow disorders such as myelodysplasia.

Non-myeloablative or mini-transplants (allogeneic)

Some people have health conditions that would make it more risky to wipe out all of their bone marrow before a transplant. For those people, doctors can use a type of allogeneic transplant that's sometimes called a mini-transplant. Compared with a standard allogeneic transplant, this one uses less chemo and/or radiation to get the patient ready for the transplant. Your doctor may refer to it as a non-myeloablative transplant or mention reduced-intensity conditioning (RIC). 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. Unlike the standard allogeneic transplant, cells from both the donor and the patient 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 can then develop an immune response to the cancer and help kill off the patient's cancer cells — the graft-versus-cancer effect. One advantage of a mini-transplant is the lower doses of chemo and/or radiation. And because the stem cells aren't all killed, blood cell counts don't drop as low while waiting for the new stem cells to start making normal blood cells. This makes it especially useful in older patients and those with other health problems who aren't strong enough for a standard allogeneic stem cell transplant. It may rarely be used in patients who have already had a transplant. Mini-transplants treat some diseases better than others. They may not work well for patients with a lot of cancer in their body or those with fast-growing cancers. Also, the lowered immune response can still lead to graft-versus-host disease.

Syngeneic stem cell transplant

This is a special kind of allogeneic transplant that can only be done when the recipient has an identical twin or identical triplet donor — someone who will always have the same tissue type. An advantage of syngeneic stem cell transplant is that graft-versus-host disease will not be a problem. There are no cancer cells in the transplant, either, as there would be in an autologous transplant. A disadvantage is that this type of transplant won't help destroy any remaining cancer cells because the new immune system is so much like the recipient's immune system. Every effort must be made to destroy all the cancer cells before the transplant is done to help keep the cancer from relapsing.