RESUMEN
We investigated the radioprotective effect of melatonin (MEL) against thyroid gland damage in rats caused by flattening filter (FF) and flattening filter free (FFF) single dose X-ray beams. We used 48 female rats divided into six groups of eight: group 1, untreated control group; group 2, MEL treated group; group 3, FF-low dose rate radiotherapy (FF-LDR) group; group 4, FF-LDR + MEL group; group 5, FFF-high dose rate radiotherapy (FFF-HDR) group; group 6, FFF-HDR + MEL group. Groups 2, 4 and 6 rats were injected intraperitoneally (i.p.) with 10 mg/kg MEL 15 min before exposure to radiation. The head and neck regions of each rat in groups 3 and 5 and groups 4 and 6 were irradiated with 16 Gy at 6 MV X-ray in FF and FFF beam modes. The histopathology of the thyroid gland and salient biochemical parameters were assessed in all rats 10 days after radiotherapy. We found increased inflammation, vacuolization, degradation, swelling and necrosis, and M30 apoptosis and M65 necrosis indicators in groups 3 and 5 compared to group 1; however, we found significant reductions in histopathological and biochemical parameters following application of MEL. MEL treatment before FF-LDR and FFF-HDR radiotherapy minimized thyroid gland injury due to irradiation.