Objective: Complications of pelvic irradiation for rectal cancer have gained more attention because of increased survival of patients. The aim of this study was to compare testes doses when pelvis is irradiated using Cobalt 60 (Co60) for rectal cancer in comparison with linear accelerator (LINAC) and its effect on sex hormones levels.

Materials and Methods: In a cohort study, 28 rectal cancer patients that were candidate to receive pelvic radiotherapy were recruited in the study consecutively. They were sequentially assigned to receive radiotherapy using Co60 teletherapy or LINAC. Serum sex hormones levels were measured before and 3–6 weeks after irradiation. Testes absorption doses were measured three times during whole course of irradiation in nine patients.

Results: Testes doses in LINAC group were significantly lower than Co60 group (p < 0.001). Serum follicular-stimulating hormone (FSH) and luteinising hormone (LH ) levels increased after irradiation in both groups and there was not a significant relation between FSH and LH levels with treatment machine (p < 0.2 and p < 0.6, respectively). Serum testosterone level decreased significantly in Co60 group (p < 0.05) but not in LINAC group (p < 0.3).

Discussion: It seems using LINAC for pelvic irradiation in patient with rectal cancer cannot prevent hormonal changes and we suggest using extra shield to decrease testes doses below the toxic levels.