Re-irradiation tolerance of rat spinal cord to fractionated X-ray doses.

Experiments were performed to assess the re-irradiation fractionation sensitivity in the rat cervical spinal cord. Animals were given initially three daily fractions of 9 Gy representing 75% of tolerance at the ED50 level. After an interval of 20 weeks, they were re-irradiated with graded doses of X-ray in single, 2, 5, 10 and 20 daily fractions, or a single retreatment top-up dose of 12.8 Gy (equivalent to 80% of retreatment tolerance) followed by doses in 1, 2, 4, 10 and 20 daily fractions. The end-point was paralysis of the forelimbs secondary to white matter necrosis. Latent periods to paralysis ranged from 188 to 245 days from initial irradiation, or from 48 to 105 days from re-irradiation. For a given fractionated retreatment schedule, shorter latent times were observed in animals re-irradiated to higher total doses. The re-irradiation ED50 values for single, 2, 5, 10 and 20 fractions were 14.0 (95% CI 13.3, 14.2), 20.5 (19.9, 21.2), 29.1 (28.0, 30.1), 36.3 (35.1, 37.4) and 47.8 Gy (46.2, 48.3), respectively. For re-irradiations with a 12.8 Gy top-up dose followed by doses in single, 2, 4 and 20 fractions, the retreatment ED50 values excluding the 12.8 Gy top-up dose were 4.5 (95% CI 3.0, 5.4), 6.5 (5.6, 7.3), 7.0 (5.0, 8.1) and 10.9 Gy (8.9, 12.5), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)