Total body irradiation in modern radiation oncology has three goals:
- Eradication of the recipients bone marrow
- In combination with chemotherapy, eradication of tumour cells
- Preserve the bone marrow stroma to allow grafting of stem cells to occur
The D0 value for bone marrow stem cells is between 0.5 and 1.4 Gy, suggesting significant cell killing at low doses and therefore intrinsic radiosensitivity. Leukaemia cells also have a low D0 value. Bone marrow stromal cells have a D0 of 1.46 Gy.
Fraction size is also important in TBI. T-lymphocytes and lymphoblasts appear to have a shoulder on their cell survival curve, indicating that higher fraction sizes (or a single large fraction) are required. Stromal cells, which must survive to allow taking of the graft, are also sensitive to large fraction size.
Late effects in the lung limit dose, as do other solid organs. Two strategies to reduce the risk is to fractionate the dose or use a low dose rate method (2 – 3 hours).