There are two main types of stem cell transplantations in the treatment of some blood cancers, and it comes down to whether the transplanted cells are yours, or someone else's.
When the cells are your own, it is called an autologous stem cell transplant. When the cells are someone else's, it is called an allogeneic stem cell transplant.
From there, stem cell transplants can be further divided a little bit depending on a couple of factors. One of those subtypes is called a tandem transplant.
In a tandem transplant, a patient undergoes two autologous stem cell transplants back to back, in a row. This usually involves two corresponding courses of high-dose chemotherapy with the stem cell transplant following the completion of each chemo course.
Prior to beginning the first transplant, the patient's own stem cells are harvested. For each of the two transplants, the patient receives half of the collected cells.
Because the high-dose chemotherapy is so tough on the body, the courses are separated by no more than six months. Just as the patient recovers from the first transplant, the second one begins.
According to the American Cancer Society, tandem transplants are:
" ... most often used to treat multiple myeloma and advanced testicular cancer, but doctors do not always agree that these are really better than a single transplant for certain cancers. Because this involves 2 transplants, the risk of serious outcomes is higher than for a single transplant. Tandem transplants are still being studied to find out when they might be best used."
The ACS also notes that sometimes, a tandem transplant refers to a patient undergoing both an autologous transplant and then an allogeneic transplant.
Source: American Cancer Society