Friday July 11 2014
The increase of cancer risk, in absolute terms, was small
“Men who have the snip increase their risk of suffering fatal prostate cancer, according to research,” the Daily Mail reports. However, while the increase in risk was found to be statistically significant, it was small in absolute terms.
The newspaper reports on a US study that followed 49,405 men over 24 years, a quarter of whom had had a vasectomy.
It compared the risk of prostate cancer in men who had a vasectomy to the men who hadn’t.
During the 24 years of this study, 12.4% of those who had had a vasectomy developed prostate cancer, compared with 12.1% of those who hadn’t.
They also found vasectomy to be associated with a 19% increased risk of prostate cancer that had spread to other organs (metastatic) or that caused death.
However, it’s important to note that these increases in relative risk relate to a small increase in terms of absolute risk (a 0.3% absolute difference in incidence rate).
This type of study also cannot show that vasectomies cause prostate cancer, as there could have been differences in the men that opted for vasectomy that the researchers did not adjust for.
Overall, though the study findings are worthy of further research, men should not be overly concerned by these reports.
Where did the story come from?
The study was carried out by researchers from Brigham and Women’s Hospital, Harvard School of Public Health, the Dana Farber Cancer Institute and the University of Massachusetts Medical School. It was funded by the US National Cancer Institute/National Institutes of Health.
The study was published in the peer-reviewed Journal of Clinical Oncology.
The results of the research were mainly well reported. To give credit to the UK media, some of the news sources which covered the study made it clear that the increase in absolute risk is small (something that is often not made clear in health reporting).
One point to mention is that The Guardian and The Daily Telegraph both said that men who had vasectomies at a younger age were at the greatest risk, though this is not supported by the results of the study.
It was suggested in the research paper that the increased risk was more pronounced among men who were younger at the time of vasectomy. However, this association was not statistically significant, so it could have been due to chance.
What kind of research was this?
This was a cohort study that aimed to investigate the association between vasectomy and prostate cancer risk.
A cohort study is the ideal study design to address this question. However, cohort studies cannot show causation, as there is the potential for confounders (other variables that explain the association).
What did the research involve?
The researchers studied 49,405 men who were part of the Health Professionals Follow-Up Study, which is an ongoing cohort study conducted by Harvard University.
The men were aged between 40 and 75 years old at the start of the study in 1986. They were followed-up for 24 years, until 2010. Around a quarter of the men (12,321) had vasectomies.
During the follow-up period, 6,023 men were diagnosed with prostate cancer, and 811 men died from prostate cancer.
The researchers compared the risk of developing prostate cancer in men with a vasectomy to the risk of prostate cancer in men without a vasectomy.
This was to see if having a vasectomy was associated with an increased risk of prostate cancer.
The researchers adjusted their analyses for a number of confounders, including:
- body mass index (BMI)
- amount of vigorous physical activity
- smoking status
- whether the men had a family history of prostate cancer
- multivitamin use
- vitamin E supplement use
- alcohol intake
- history of prostate-specific antigen (PSA) testing
PSA is a protein produced by normal cells in the prostate and also by prostate cancer cells, and elevated levels can indicate a variety of prostate problems (for example, levels are raised with cancer, but also benign enlargement, inflammation and infection).
What were the basic results?
During the study 12.4% of those who’d had a vasectomy developed prostate cancer (1,524 cases out of 12,321 who’d had a vasectomy) compared with 12.1% of those who hadn’t (4,499 cases out of 37,804 who hadn’t had a vasectomy).
The researchers found that vasectomy was associated with:
- A 10% increase in risk of prostate cancer (relative risk [RR] 1.10, 95% confidence interval [CI] 1.04 to 1.17).
- A 22% increase in risk of “high-grade” cancer (more aggressive cancer with poorer prognosis) (RR 1.22, 95% CI 1.03 to 1.45). High-grade cancer was defined as having a Gleeson score of 8 to 10 at diagnosis.
- A 20% increase in risk of “advanced prostate cancer” (lethal or stage T3b [cancer had spread to the seminal vesicles], T4 [cancer has spread to nearby organs], N1 [lymph nodes contain cancer cells] or M1 [cancer has spread to other parts of the body]) (RR 1.20, 95% CI 1.03 to 1.40).
- A 19% increase in risk of prostate cancer with distant metastasis (where the cancer has spread to another part of the body) or that causes death (RR 1.19, 95% CI 1.00 to 1.43).
The researchers noted that men who had a vasectomy reported more PSA testing than men without vasectomy.
Although the researchers adjusted for frequency of testing in their analyses, they were concerned the results could be due to men with vasectomy being diagnosed with prostate cancer because they had PSA testing more frequently, rather than because they were more likely to have prostate cancer.
They then performed an analysis of “highly screened” men (who reported PSA screening in 1994 and 1996; note this is a US study and there is no national PSA screening campaign in the UK).
In this subcohort, having a vasectomy was not associated with increased risk of prostate cancer overall, but the association with cancer with distant metastasis or that causes death remained.
How did the researchers interpret the results?
The researchers concluded that their data “support the hypothesis that vasectomy is associated with a modest increased incidence of lethal prostate cancer.”
This 24-year cohort study found that men with a vasectomy had a 10% increased risk of prostate cancer and a 19% increased risk of prostate cancer that had spread to other organs, or that caused death.
However, it’s important to note that there are only tiny increases in absolute risk; during the 24 years of this study, 12.4% of those who’d had a vasectomy developed prostate cancer, compared with 12.1% of those who hadn’t.
The strengths of this study are its large size, its long follow-up period, and the collection of data on and adjustment for a large number of factors that could affect the association (confounders). However, as this is a cohort study, it cannot show causation, as the potential for other confounders remains.
Given that the 0.3% absolute difference in cancer incidence is small, there may be other factors differing between those who had a vasectomy and those that didn’t that could account for the differences.
Overall, though the study finding is worthy of further research, men should not be overly concerned by these findings.
As the researchers say, “the decision to opt for a vasectomy remains a highly personal one in which the potential risks and benefits must be considered.”
There are also less drastic steps you can take if you don’t want to have any children.
If used correctly, condoms are 98% effective. They also have the advantage of protecting you against sexually transmitted infections (STIs).
And there is always the possibility that you may change your mind about having children. Vasectomy reversal is expensive (it is rarely available on the NHS) and has a patchy success rate, ranging from 25% to 55%.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.