Prostate cancer is a major cause of cancer-related deaths in men in the US, where vasectomy is a common form of contraception, with around 15% of American men having the minor procedure, which blocks the tubes that carry sperm from the testicles to the penis.
The researchers report their findings in the Journal of Clinical Oncology, where they note that the link was still evident among men who had regular PSA tests, suggesting the link with increased risk of lethal cancer cannot be due to diagnostic bias.
Co-author Lorelei Mucci, associate professor of epidemiology at Harvard School of Public Health (HSPH), says:
"This study follows our initial publication on vasectomy and prostate cancer in 1993, with 19 additional years of follow-up and tenfold greater number of cases. The results support the hypothesis that vasectomy is associated with an increased risk of advanced or lethal prostate cancer."
For the study, Prof. Mucci and colleagues analyzed data on 49,405 American men who were followed between 1986 and 2010 as participants of the Health Professionals Follow-up Study. The men were aged between 40 and 75 years at the start of the 24-year follow-up period.
In this study, 16 out of every 1,000 men developed lethal prostate cancer over 24 years of follow-up.
Over that time, 6,023 men were diagnosed with prostate cancer, including 811 who died of the disease. One in four of the participants reported having undergone a vasectomy.
When they analyzed the data, the team found a 10% overall increased risk of prostate cancer in those men who had a vasectomy.
However, further analysis found vasectomy was linked to a stronger increased risk of more aggressive forms of prostate cancer: a 19% higher risk for advanced cancer and a 20% higher risk of the lethal form.
And for a subgroup of men who were having regular PSA tests, the increased risk was 56%.
There was no significant link between vasectomy and risk of low-grade cancer.
Concerns have been raised before in connection with this type of study, that the links could be a result of bias, but the researchers say their analyses took into account diverse information that meant they could rule out potential biases. For example, one bias could be that men who have vasectomies are more likely to seek medical care, or undergo more PSA tests. They also ruled out bias due to the possibility of sexually transmitted infections.
It should be noted that the researchers are not suggesting in this study that simply undergoing regular PSA screening can raise the risk of prostate cancer. There could be other explanations for why the men having regular PSA screening showed the strongest raised risk for prostate cancer. For instance, if an initial screening is positive, then regular screening is recommended.
Relative risk does not mean absolute risk
Another point to note is that these numbers refer to relative risk - that is the extent to which the underlying or absolute risk of developing the disease is affected. So, for example, if a person's risk of getting a disease is 10%, then a relative risk of 10% means the absolute risk is only 10% of 10%, or 11%.
In this study, 16 out of every 1,000 men developed lethal prostate cancer over 24 years of follow-up. Thus, the result that vasectomy was linked to a 20% raised risk of lethal prostate cancer, is relative to that 16 out of 1,000.
Therefore, as the researchers point out, although the 20% was found to be statistically significant (that is unlikely due to chance), its effect is a relatively small increase in the absolute risk of developing prostate cancer.
"The decision to opt for a vasectomy as a form of birth control is a highly personal one and a man should discuss the risks and benefits with his physician," says co-author Kathryn Wilson, a research associate in HSPH's Department of Epidemiology.
Funds from the National Cancer Institute and the National Institutes of Health helped finance the study.
Meanwhile, Medical News Today reported how a study led by the University of Adelaide in Australia found that a test based on semen may be more accurate at diagnosing prostate cancer. The researchers found that testing semen samples for small molecules called microRNAs was "surprisingly accurate" at indicating which men had prostate cancer and how severe it was.