By Nancy P. Mendenhall, MD
Definition and Rationale
When radiation is given in a way to cover the whole body it is called total body irradiation, or TBI. This is done generally for one of two reasons:
- To suppress the patient’s immune system and prevent rejection of donor bone marrow after a bone marrow transplant using donor marrow (from someone other than the patient).
- To kill abnormal cells that escape other therapies such as surgery, chemotherapy, or local irradiation and remain hidden in the body to regrow later.
Radiation can penetrate all areas of the body. This allows the treatment to reach cells even within scar tissue or deep recesses of the body that other agents such as chemotherapy drugs, which are injected into the blood or taken by mouth, cannot reach. The radiation effect is generally on cells that are rapidly growing and/or have poor repair function.
To take advantage of this, total body irradiation is generally given over several fractions, 2 to 3 times a day for 2 to 5 days. This is to allow the normal tissue cells such as lung to repair and tolerate the treatment while also increasing the chances that the abnormal cells will be affected and killed. Even so, the total dose of radiation that can be used is much less than if only a small area of the body was being treated. This is so that the ability of normal cells to repair injury from treatment is not overwhelmed. For this reason TBI alone is not used to kill large amounts of malignant or abnormal cells and is generally a part of a larger treatment plan that may include local irradiation, chemotherapy, or surgery.
Diseases in which this treatment has been used here at the University of Florida Radiation Oncology Department include:
- Aplastic anemia – to prevent rejection of donor marrow
- B thalassemia
- Chronic myeloid leukemia
- Acute myeloid and lymphoblastic leukemias
- Non-Hodgkin’s lymphoma
- Ewing’s sarcoma/unfavorable pediatric sarcomas
The most sensitive cells in the body are the blood cells, which include lymphocytes, neutrophils, platelets, and red blood cells. Treatment with standard or high dose TBI as part of bone marrow transplant destroys these cells or their precursor stem cells, which then must be transfused back using stored bone marrow or blood stem cells obtained from the patient before treatment or from another person (donor).
Low dose TBI is sometimes used to treat disorders of the blood cells such as low grade lymphoma and does not require bone marrow transplant or stem cells. Other sensitive tissues include the lungs, GI tract, skin, liver, kidneys, and lens of the eye. Partial blocking is sometimes used, depending on the TBI dose and disease being treated, to help prevent any lung damage. This blocking is prepared using special x-rays obtained at the time of treatment planning.
To deliver total body irradiation in a homogenous manner, patient measurements are obtained and special “tissue compensators” may be required to make up for differences in body thickness. Treatments are thus customized for the individual patient, but also for each treatment facility, and require a team of specialists including the radiation oncologist, physicist, dosimetrist, therapists, and support staff of nurses and other personnel knowledgeable about TBI. They will take into account the equipment being used and the physical setup of the treatment room as well as the specific disease process and patient characteristics (size, thickness, and lung volumes).
At the University of Florida we have participated in many treatment trials using TBI as part of protocols testing the newest therapeutic approaches for treatment of children and adults with high risk malignancies. To assure our ability to participate in these trials, our treatment techniques and setup have been reviewed and have met all standards for such studies. To cover the whole body in a radiation treatment field, the patient is positioned at some distance from the treatment machine on a platform, sitting as though on the edge of a chair or bicycle seat but with a backrest to lean against for comfort. Babies and very small children can be treated lying down since they will easily fit inside the field. The treatment often takes about 25 minutes and is given part from the front or side and part from the back or other side. Patients are always in view on a TV monitor, and there is an intercom for verbal communication. Patients may request music to help the time pass more quickly.
Treatment Toxicity or Side Effects
The use of TBI is complex and requires careful delivery and support for the patient. Some toxicities during treatment are expected, such as temporary hair loss, nausea and vomiting, diarrhea, and drop in blood cell counts with standard or high dose TBI. Measures are taken to control these symptoms, such as the routine use of antinausea medicine. Each patient and/or their parents (in the case of a child) are counseled regarding these and other possible toxicities. The toxicities are separated into acute (during or within 6 months of treatment) and chronic (more than 6 months after treatment), and both the likelihood of developing a specific toxicity and its severity are discussed at the time of informed consent before any treatment is given.
An example of acute toxicity is the diarrhea as described above. An example of a chronic toxicity would be development of early cataracts, which is seen in 5% to 30% of patients treated with TBI. Because children are actively growing, their normal tissues are often more sensitive to radiation, and the toxicity of TBI treatment can be different for them; it may even vary with the age of the child. This is reviewed at the time of informed consent.