title: The effect of small radiation doses on the rat spinal cord: the concept of partial tolerance.
abstract: To evaluate the tolerance of the rat spinal cord to small radiation doses per fraction, an increasing number of fractions is required for induction of paralysis. The assessment of doses of 1-2 Gy, as used in the clinic, would require that over 100 fractions be given. The validity of replacing part of a fractionated irradiation of the spinal cord by a single large dose has been tested. Fractionated irradiation doses with 18 MeV X rays were followed by a "top-up" dose of 15 Gy as a single treatment. This is the fraction size of a treatment with two irradiation doses leading to paralysis in 50% of the animals (ED 50). Fractionated treatments were carried out with 2, 5, 10 and 20 fractions followed by the top-up dose of 15 Gy. The isoeffect curve, as a function of the number of fractions, has the same slope as experiments performed without top-up dose. The results show that the quality and quantity of cellular repair is not modified when part of a multifractionated exposure is replaced by a larger top-up dose. An important consequence of this finding is, that in treatments with unequal fraction sizes, the partial tolerances can simply be added. Since a top-up dose can replace a sizable number of irradiation treatments, its application will allow investigations of the extent of sublethal damage repair for fraction sizes as low as 1 Gy.
mesh_terms: D000818:Animals; D004307:Dose-Response Relationship, Radiation; D004601:Elementary Particles; D008297:Male; D010243:Paralysis / Q000209:etiology; D010315:Particle Accelerators; D011836:Radiation Tolerance; D051381:Rats; D011919:Rats, Inbred Strains; D013047:Specific Pathogen-Free Organisms / Q000528:radiation effects; D013116:Spinal Cord / Q000528:radiation effects*; D013997:Time Factors