The thresholds for permanent sterility in men for an acute exposure and prolonged exposure conditions are
6 Gy and 2.5-3 Gy respectively
2.5 to 3 Gy and 1.5 Gy respectively
6 Gy and 2-5 Gy respectively
7.5 to 10 Gy and 6 Gy respectively
Studies of patients receiving radiation therapy indicate that permanent sterility can result from 2.5 to 3 Gy (250-300 rad) in a fractionated regime over 2 to 4 weeks. The induction of sterility by radiation in human males does not produce significant changes in hormone balance, libido, or physical capability. The original single-dose data came from the irradiation of prisoners, which showed that a dose in excess of 6 Gy (600 rad) is needed to result in permanent sterility. In contrast to most organ systems, where fractionation of dose results in sparing, fractionated courses cause more gonadal damage than a single doseReference: Eric J Hall Textbook of Radiobiology for Radiologist; 5th edition
Radiotherapy