An Experimental Animal Model of Re-Irradiation in Terms of Radiation-Induced Damages in Normal Intestine

Abstract

Re-irradiation for various tumors has recently attracted increasing attention as a salvage modality, which focuses on the clinical outcomes. However, pathological changes after re-irradiation are poorly understood. The purpose of the present study was to report the pathological findings following re-irradiation in preclinical model. Male C57BL/6J mice (8-weeks-old) were used in the present study. Mice were divided into five groups as follows; mice received sham-irradiation (Sham-IR), mice received a single irradiation alone (Single IR), mice received the second irradiation 7 days after the first irradiation (Acute re-IR), mice were sacrificed 12 weeks after the first irradiation (Late change) and mice received the second irradiation 12 weeks after the first irradiation (Late re-IR). At the first irradiation, mice in the Acute re-IR, the Late change and the Late re-IR groups received abdominal irradiation at 15 Gy in a single fraction. For the first irradiation, lead shielding was used to cover the mice except for the whole abdomen. At the re-irradiation (second irradiation), mice were irradiated with a total body irradiation (TBI) of 10 Gy in a single fraction after 7 days and 12 weeks in the Acute re-IR and Late re-IR groups, respectively. 10 Gy-TBI was also given in mice in the Single IR group. Mice were sacrificed 6 hours after the second irradiation and intestine was harvested. The apoptotic index was defined the percentage of terminal deoxynucleotidyl transferase (TdT)-mediated d-UTP nick end-labeling (TUNEL) positive cells in each crypt. In some experiments, mice were sacrificed 72 hours after the second irradiation and 1.5 hours after administration of bromodeoxyuridine (BrdU). Submucosal fibrosis was observed 12 weeks after the first irradiation in the Late change group. No significant differences were observed in the incidence of crypt with TUNEL positive cells between the Sham-IR and Late change groups (p = 0.5090). The Late re-IR group showed a significantly less incidence of crypt with TUNEL positive cells than the Single IR group (p = 0.006). In the crypts with TUNEL positive cells, the Acute re-IR group showed significantly higher apoptotic index than the Single IR and Late re-IR groups (p < 0.0001 and < 0.0001, respectively). However, no significant differences were observed between the Single IR and Late re-IR groups (p = 0.3247). Moreover, there were no significant differences between the single IR and late re-IR groups in the number of BrdU positive cells in each crypt (p = 0.6597). Re-irradiation12 weeks after the first irradiation produced similar cell death and cell proliferation patterns in normal intestine in comparison with a single irradiation in an experimental animal model. Re-irradiation with a long break after the first irradiation might be an acceptable with equivalent biological effects on normal intestine in terms of an acute reaction.