Canadian Health: Lonising Radiation in Men
Canadian Health: Lonising Radiation in Men
Speed of cell division. Rapidly dividing cells are the most radiosensitive and this has important clinical implications. The differentiating spermatogonia are the most radiosensitive whereas spermatids do not divide and are relatively radioresistant. Similarly Leydig and Sertoli cells are usually unaffected by moderate radiation doses. Following testicular irradiation, there are four phases in sperm response. 
For the first 2 months after irradiation, there is relatively little effect on sperm count as the more mature spermatids and spermatozoa continue to be ejaculated. In the second phase, there is a significant reduction in the sperm count. The magnitiude of the reduction depends on the testicular dose.
We also found after a dose of less than 20 rad, few men become azoospermic whereas after 100 rad 90% develop azoospermia. In the third phase, recovery of sperm production can occur, which may be followed by recovery to normal levels in the final phase. Reductions in normal morphology and motility accompany the changes in sperm count.
Fractionation of dose. It is a general principle of radiobiology that tissues recover more following multiple small doses than from a single large dose as the damaged tissues are able to repair themselves.
Unfortunately for the testis, the reverse is true as the stem cell response to irradiation is to proliferate, which makes it more radiosensitive to the next dose of radiation. Therefore 2 years following a single dose of less than
600 rad, nearly all azoospermic men will have recovered sperm production. If a dose of greater than 200 rad is given in fractionated doses, 85% will remain azoospermic after 2 years.