Total Body Irradiation (TBI) is variation of radiation therapy where the entire body is the target volume. The treatment is a challenge geometrically, dosimetrically, and logistically.
Malignant diseases originating from myeloid and lymphoid cells may be appropriate for TBI for two reasons. The neoplastic cells are usually relatively sensitive to radiation and must generally be considered a systemic disease.
The doses administered for TBI are very limited mainly due to acute toxicity of bone marrow and lungs, but this is usually surmounted by subsequent transplantation of hematopoietic stem cells. This limits the use of TBI considerably.
The treatment arrangement requires a high degree of cooperation from the patient. TBI in children requiring general anesthesia consitutes a special problem.
- TBI is used today mainly as a step in conditioning of hematopoietic stem cell tranplantation for myeloid and lymphoid neoplasias and related conditions. For autologous or syngeneic stem cell transplantation, the antitumor effect of radiation therapy is the intended effect, usually together with chemotherapy. Irradiation given in adequate doses for an allogeneic stem cell transplantation will also suppress the patients immune system such that the danger of rejection (host versus graft reaction, graft rejection) of the allogeneic stem cells is reduced. For a traditional stem cell transplant, doses higher that 9 Gy are given. Considering the danger of radiation pneumonitis, traditionally the most serious side effect of acute toxicity from TBI, it has been shown that fractionated treatment with 1.2-2 Gy per fraction given 1-2 times daily is preferred over single fractions above 9 Gy, and the total dose for fractionated treatment can be increased. The dose rate varies between 0.05–0.1 Gy/min (low dose rate) to 0.5–0.7 Gy/min (high dose rate) and also plays an important radiobiological role. The most common conditioning regimen utilizing TBI at Oslo University Hospital consists of 13 Gy with fractionation in 1.3 Gy x 10 with two fractions daily at a low dose rate followed by cyclophosphamide 3 g/m2 daily for two subsequent days (total 6 g/m2).
- TBI has a possible place in conditioning of allogenic stem cell transplantation with reduced conditioning (mini-allo transplantation). Here, TBI is given in lower doses, usually in one fraction but doses far under 8 Gy. The purpose of TBI for a mini-allo is mainly to suppress the patient's remaining healthy immune system to prevent rejection of the allogeneic stem cells. The most commonly used regimen containing TBI at Oslo University Hospital is the Seattle regimen where TBI is given in one fraction of 2 Gy together with fludarabine.
- Use of TBI as part of conditioning before a stem cell transplantation must be a considered individually. The most common indications for TBI today are part of conditioning for allogeneic stem cell transplantation for certain patients with acute leukemias, myelodysplastic syndrome and anaplastic anemia. TBI is used rarely today for autologous stem cell transplantations where conditioning regimens consisting of chemotherapy alone are mostly used.
- Low-dose TBI (less than 2 Gy) given as a single fraction or in fractions of 0.05–0.15 Gy/fraction given in 2–5 fractions per week have previously been used without hematopoietic stem cell support, especially for chronic lymphatic leukemia and indolent lymphomas. Bone marrow suppression, especially thrombocytopenia, is considerable. This from of treatment is not used in Norway today.