A stem cell transplant, also known as a bone marrow transplant, is a procedure that infuses healthy stem cells into the body to replace damaged or diseased bone marrow. Stem cells are the basic cells of the body that have the potential to develop into many different cell types during early life and growth. In certain cases, they can also repair the body by dividing indefinitely to replenish cells.

Stem cell transplants are used to treat conditions where the bone marrow doesn't work and can't produce healthy blood cells. This could be due to disease (like certain types of leukemia or lymphoma), congenital conditions (like sickle cell anemia or thalassemia), or specific high-dose cancer treatments that destroy the bone marrow along with the cancer cells.

There are two main types of stem cell transplants:

1. Autologous Transplant: This involves using the patient's own stem cells. The cells are harvested, or removed, from the patient before they undergo a treatment (like chemotherapy or radiation) that destroys their bone marrow. After treatment, the patient's stem cells are returned to their body to help their bone marrow start to produce new blood cells.

2. Allogeneic Transplant: This involves using stem cells from a donor. The donor could be a family member, an unrelated donor, or umbilical cord blood. For the transplant to be successful, certain markers on the donated cells (called human leukocyte antigen (HLA) markers) must match the markers on the patient's cells as closely as possible to minimize the risk of complications.

One potential risk in allogeneic transplants is a condition called graft-versus-host disease (GVHD), which occurs when the donor cells attack the patient's own cells as they would a foreign substance.

Recovery from a stem cell transplant can take several months, and the patient will need to be closely monitored for complications. The success of the transplant depends on a range of factors, including the type of disease being treated, the type of transplant, the patient's overall health, and how closely the donor's HLA markers match those of the patient.

Radiation therapy, also known as radiotherapy, is a common method used to treat cancer. It uses high-energy particles or waves, such as X-rays, gamma rays, or proton beams, to destroy or damage cancer cells. It can be used alone or in conjunction with other treatments like surgery and chemotherapy.

Radiation therapy works by damaging the DNA within cancer cells, making them unable to divide and grow. While it aims to target cancer cells, it can also harm nearby healthy cells. However, normal cells can usually repair much of the radiation-induced damage, whereas cancer cells cannot.

There are two main types of radiation therapy:

1. External Beam Radiation Therapy (EBRT): This is the most common type and involves radiation being delivered from a machine positioned outside the body. The machine rotates around the patient, delivering radiation to the tumor site from several angles. The treatment is usually delivered over several sessions, with each session lasting just a few minutes.

2. Internal Radiation Therapy (Brachytherapy): This involves placing a small radioactive source directly into or near the tumor. The radioactive source can be left temporarily or permanently, depending on the type and stage of cancer being treated.

Radiation therapy can also be used in different contexts: - 

Curative Treatment: It can be used with the intent to cure the cancer, either as a standalone treatment or in conjunction with other therapies.

Adjuvant Treatment: After the primary treatment like surgery, radiation therapy can be used to kill any remaining cancer cells. 

Neoadjuvant Treatment: Radiation can be given before the primary treatment to shrink a tumor and make it easier to remove or treat.

Palliative Treatment: For advanced cancers, radiation therapy can be used to relieve symptoms caused by the cancer, such as pain or pressure. 

The use of radiation therapy has to be carefully planned to maximize its effects on cancer cells and minimize damage to healthy cells. This involves detailed imaging studies to identify the exact location of the tumor and sophisticated computer programs to plan the radiation doses.

Side effects of radiation therapy can include fatigue, mild skin reactions, upset stomach, and loose bowel movements. Most side effects go away a few weeks after treatment is completed. Long-term side effects, which are rare, can include fibrosis, damage to the bowels, or secondary cancers.