BONE MARROW AND STEM CELL TRANSPLANTATION

Bone marrow, the spongy material found inside our bones, contains immature, "stem" cells. Stem cells develop into three types of cellular elements found in the blood: red blood cells that deliver oxygen to all parts of the body and take away the waste product carbon dioxide; white blood cells that protect the body from infection; and platelets that help blood clot.

Very high doses of chemotherapy or radiation may be needed to destroy cancer cells. During transplantation, normal bone marrow is destroyed. A transplant of bone marrow or stem cells can then be used to restore healthy bone marrow.

Since transplants place great strain on the body, it is not an option for everyone. Among the things to consider are age, medical history, cancer stage, response to previous therapy, and chances for a successful transplant. Most transplants are now performed using stem cells rather than bone marrow. Compared with bone marrow transplantation, the stem cell procedure is easier to perform and the body is able to regenerate cells quicker.

The sources of bone marrow or stem cells
A key to the success of a transplant is the availability of stem cells. There are two sources of stem cells: allogeneic sources, in which patients receive bone marrow or stem cells donated by another person, often a sibling, and autologous sources , in which patients receive their own cells. Autologous bone marrow transplantation is far more common than allogeneic because it is much less toxic.

Allogeneic transplant:
Finding a suitable donor is important because your body will reject stem cells if they are too unlike your own or the new cells may react against your own cells, a condition called graft-vs-host disease (GVHD) Several different blood tests may be used to see if someone is a suitable transplant donor. The best candidates are brothers or sisters. One potential advantage of an allogeneic transplant is that the donor’s immune cells may fight against and destroy the lymphoma cells.

Autologous transplant:
In this procedure, patients receive their own stem cells. The advantage to this type of transplant is that there is no danger that the body will reject the bone marrow or stem cells, or that the stem cells will react against the body.

Harvesting is the procedure by which stem cells are obtained from the bone marrow or blood for use in repopulating the body's cells after high-dose chemotherapy. The cells used in autologous transplantation can be obtained either by harvesting bone marrow or stem cells. A bone marrow harvest involves removing bone marrow, which is a thick red liquid, from the hip bone with a needle. This procedure is done in an operating room and requires general anesthesia for comfort. Once the bone marrow is harvested, it is purified and stored until the day of the transplant. Multiple needle insertions are required to harvest enough bone marrow. This procedure is performed less commonly now, but was frequently performed in the past. Although the greatest number of stem cells are found in the bone marrow, these cells are also present in the blood, especially when mobilized by chemotherapy or growth factors. Another way to collect cells for a transplant is by harvesting them from the blood. This is often preferred, since it is an out-patient procedure and is currently most commonly employed. It involves the withdrawl of blood in a process called apheresis (which means to remove or take away). In this case, stem cells are removed, and the rest of the blood is returned to the patient.

The procedure for a bone marrow or stem cell transplant
Bone marrow and stem cell transplants are performed in four steps:

Step 1: Harvesting stem cells or bone marrow
Stem cells are usually filtered from the blood in a process called apheresis; the other components of the blood are then returned immediately to the patient. Bone marrow (which contains stem cells) is usually harvested from bones in the hip.

Step 2: Processing or preserving the stem cells or bone marrow
Machines are sometimes used to purge any malignant cells, filter out unwanted substances, and freeze and store the pure transplant material, including cells.

Step 3: Administering therapy
High-dose chemotherapy is then administered, with or without radiation, to eliminate the cancer cells. This high-dose treatment also wipes out healthy blood and immune cells. The type of chemotherapy will depend on the stage and type of disease, whether it is a recurrence, the condition of the patient, and the treatment facility's preferences and experiences.

Step 4: Reinfusing harvested stem cells or bone marrow
As the harvested stem cells or marrow are returned to your body (transplanted), they travel through the circulatory system to your bone marrow, where they implant themselves (like seeds in a garden) and slowly begin to make healthy new cells. Over time, the bone marrow produces enough healthy cells to completely repopulate the blood and immune systems. In the period before these new cells are created, patients are more susceptible to infection.

Mini-bone marrow or mini-stem (non-myeloablative) cell transplant
Mini-bone marrow or mini-stem cell transplants are procedures in which bone marrow or stem cells are received from an allogeneic donor. These procedures are quite new and are therefore still considered investigational. Unlike a normal bone marrow or stem cell transplant in which high-dose chemotherapy is administered, in these procedures the patient receives lower doses of chemotherapy, just enough to allow the body to accept the new cells. This approach is used to take advantage of the graft-versus-disease effect, in which the transplanted cells recognize the tumor as a foreign entity and activate T-cells to destroy the cancer. Patients who experience a graft versus disease effect may remain in remission for a longer period. Also, because patients receive lower doses of chemotherapy, they may avoid some of the toxicities seen with higher dose chemotherapy. graft-vs-host disease (GVHD), however, is a frequent and sometimes serious side effect (toxicity). GVHD is a potentially serious complication of allogeneic bone marrow transplantation.