The prostate, part of the male reproductive system, is a small, walnut-sized gland that sits under the bladder, and in front of the rectum. It produces semen, the opaque fluid responsible to carrying sperm from the testicles through the penis during ejaculation. The prostate -- in addition to the seminal vesicles that sit just above the gland -- also helps produce zinc, citrate and fructose that give sperm the required energy to make their journey. So, any treatment that impairs the function of the prostate can have an adverse effect on fertility.
In early stage prostate cancer, the cancer cells are confined to the prostate (it is one of the slowest growing cancers, and can take 10 to 30 years for a tumor to grow large enough for detection) [source: National Cancer Institute]. However, given the prostate's role in sperm delivery, concerns of radiation treatment are well founded.
Any male patient considering brachytherapy can always opt for sperm banking to ensure an ample supply following treatment. However, a study conducted by the Temple University School of Medicine in Philadelphia "found no significant change in semen parameters post-therapy" for four young patients, and three were able "to father a child subsequently without any deleterious side-effects" [source: Mydio and Lebed].
Still, the same study recommended that brachytherapy patients "should wait for at least three to four months before trying to conceive," due to the half-life of the radioactive materials used in the treatments. Others suggest a much longer window -- up to 18 months -- which makes the sperm bank option more appealing [source: Prostate Cancer Treatment Guide].
In short, the greater concern is not the quality of the sperm, but the mode of transportation. Simply, the prostate gland will not produce the same amount of semen, or the same quality, immediately after treatment, and that will have an impact on sperm mobility.
According to the Prostate Cancer Foundation, roughly 10 percent men with prostate cancer will also have seminal vesicle invasion. In these patients, the cancer has either spread into the seminal vesicles or has spread around them. In this instance, provided the patient doesn't have a complete prostatectomy, the seminal vesicles will also be targeted during radiation therapy. This will also impair the ability of sperm to travel.
There is also the possibility that brachytherapy can adversely affect sperm counts. Low levels of scatter radiation can reach the testicles, decreasing sperm production. Though this reduction is usually temporary, permanent reduction in the sperm count or sterility has been reported [source: UCSF Medical Center].