Radiobiological effects of total body irradiation on the spinal cord.

Total body Irradiation (TBI) is often used for conditioning, prior to bone marrow transplantation. Doses of 8-14 Gy in 1-8 fractions over 1-4 days are administered using low dose rate external beam radiotherapy (EBRT). When necessary, consolidation EBRT using conventional doses, fractionation and dose rate is given. The irradiated volume usually contains critical organs such as spinal cord. The purpose of this study was to assess the biologic effect of TBI on the spinal cord in terms of EQD(2) (equivalent dose given in fractions of 2 Gy). EQD(2) values were calculated using the linear-quadratic generalized incomplete repair (IR) model that incorporates IR between fractions and low dose rate irradiation corrections and accounts for mono and bi-exponential repair. Three fractionation schemes were studied as function of dose rate: 8 Gy in 1 and 2 fractions and 12 Gy in 8 fractions. For the 12 Gy in 8 fractions scheme, the influence of dose rate on EQD(2) was limited because the effect of IR between fractions dominates. For the 8 Gy in 1 fraction scheme, significant sparing of the spinal cord may be achieved for low dose rate (5-20 cGy/min). The extent of effects depends on the parameters used. The IR model provides a useful mathematical framework for examination of the effects of fractionated treatments of varying dose rate. Reliable experimental data are needed for accurate assessment of radiation damage to the spinal cord following fractionated low dose rate TBI.