Radiation-induced apoptosis in the rat spinal cord: lack of equal effect per fraction

This study assessed the apoptotic response in the rat spinal cord following split-dose and fractionated irradiation. The cervical spinal cord (C2-T2) of adult female Fisher 344 rats was irradiated with a single, two, three or four daily fractions of 8 Gy, or two doses of 8 Gy separated by an interval of 0, 1, 2 or 4 h, or 1, 2, 3, 6, 10, 14, 28 or 63 days between two doses. Animals were sacrificed at 0, 4, 8, 12 and 24 h after each final 8-Gy dose, and the percentage of apoptotic glial cells was scored histologically according to specific morphological features of apoptosis. After a single dose of 8 Gy, the percent of apoptotic glial cells increased at 4 h (0.29 +/- 0.02%), peaked at 8 h (1.04 +/- 0.06%) and returned to the baseline by 24 h (0.05 +/- 0.01%). The total apoptotic yield (TAY) per spinal cord section over a 24-h period after a single 8-Gy dose was 9.04 +/- 0.34%. The second, third and fourth 8-Gy fractions given once daily gave TAY = 3.66 +/- 0.22, 2.10 +/- 0.16 and 1.42 +/- 0.17% respectively, an apoptotic response that was significantly decreased compared with the initial 8-Gy fraction (p < or = 0.01). Irrespective of the interfraction interval, two fractions of 8 Gy always induced a greater apoptotic response than that from a single 16 Gy (8 Gy-0 h-8 Gy, p < or = 0.008). TAY from the second 8-Gy dose given at 0, 1, 2, 4, 24 and 48 h was 0.93 +/- 0.03, 2.32 +/- 0.10, 3.26 +/- 0.10, 2.57 +/- 0.11, 3.67 +/- 0.22 and 3.48 +/- 0.13% respectively versus 9.04 +/- 0.34% after the initial 8 Gy (p < or = 0.001). TAY = 7.01 +/- 0.33% from the second 8-Gy dose given at an interval of 3 days was significantly greater than that given at 1 or 2 days (p < 0.0001), but significantly less than that of 9.84 +/- 0.33% given at 14 days (p = 0.0003), suggesting recovery of the apoptotic response occurring at 3 days and completed at 14 days after the initial 8 Gy dose. For radiation-induced apoptosis in the rat spinal cord, we conclude that the concept of equal effect per fraction is not applicable, and that split doses are more effective than the same dose given as a single fraction.