Total body irradiation used before some bone marrow transplants often causes permanent infertility.
If the testicles get a mild dose of radiation, a man's fertility may drop but can then recover over the next one to four years.
If the radiation dose to the testicles is high, sperm production may stop forever. This happens because the spermatogonia are destroyed. These are the stem cells in the testicles that divide and grow to produce mature sperm.
Radiation damage to the part of the brain that controls hormone production can sometimes prevent the hormone messages from getting to the testicles.