A bone marrow transplant may be needed if:
Your bone marrow is defective, containing either cancer cells or other abnormal types of blood cells (example – sickle cells)
Your bone marrow is not strong enough to survive the effects of high dose chemotherapy. For example, patients with tumors often require high doses of chemotherapy to kill their tumor cells. This chemotherapy may also be strong enough to destroy your blood and bone marrow cells. In this case, the bone marrow transplant is given as a rescue, to allow new bone marrow and blood cells to grow.