From 2 Gy to 1 Gy per fraction: sparing effect in rat spinal cord?

Recently published results, from this group, on rat cervical spinal cord, a late responding tissue, indicated no further sparing with lowering the fraction size from 2 to 1.8, 1.5, and 1.3 Gy. In the present experiments a small but probably significant rise in tolerance is suggested, when the dose per fraction was decreased from 2 Gy down to 1 Gy. This rise would however still be much less than what is predicted by the linear quadratic model, based on the experimental data obtained with fraction sizes larger than 2 Gy.