Vasectomy may be associated with a small increase in absolute risk of prostate cancer, the findings from a large cohort study in the United States suggest.

A cohort of 49,405 men taking part in the Health Professionals Follow-Up Study, run by Harvard University, was followed up from 1986 to 2010. During this period 12,321 men (25%) had vasectomies. The men were aged between 40 and 75 at baseline.

The study authors, acknowledging the weakness of a cohort study (i.e. it is able to demonstrate association but not causation because of the potential presence of confounding factors), aimed to allow for a variety of potential confounders in their statistical analysis. These included age, BMI, exercise levels, smoking, diabetes, family history of prostate cancer and history of PSA testing.

During the study period, 6,023 men were diagnosed with prostate cancer and 811 died from the disease.

The risk of being diagnosed with prostate cancer was 12.4% in those who had vasectomies, compared with 12.1% in those who did not – this represents a 0.3% increase in absolute risk of prostate cancer in men who had a vasectomy, and a relative risk (RR) of 1.10 (95% CI:1.04-1.17).

The risk was also elevated for high-grade (Gleason 8-10) disease RR 1.22 (95% CI:1.03-1.45) and lethal disease (distant metastases or death) RR 1.19 (95% CI:1.00-1.43).

One of the strongest predictors of a prostate cancer diagnosis is whether the man has had a PSA test, either because of symptoms or opportunistically/via a screening programme. Therefore, the increased risk of prostate cancer in the vasectomy group might be explained if this group of men were more likely to engage with healthcare and therefore be more likely to have had a PSA test.

The authors therefore looked at a subcohort of 13,901 ‘highly screened’ men. In this group, 3,740 (27%) had had a vasectomy. Vasectomy was not associated with an increase in total prostate cancer incidence or with risk of low-grade or localised prostate cancer. It was, however, associated with an increase in high-grade and Gleason 7 cancers, although the difference did not reach statistical significance.

Lethal prostate cancer was seen in 56% more men who had undergone vasectomy (RR 1.56). However, because of the low incidence of lethal prostate cancer, 1.6% over 24 years, this increase in relative risk represents only a very small increase in absolute risk.

Overall, the study found a 10% increase in relative risk of prostate cancer, a 22% increase in risk of high-grade cancer (Gleason 8 to 10 at diagnosis), a 20% increase in risk of advanced prostate cancer and a 19% increase in risk of prostate cancer with distant metastasis or that causes death.

This is a high quality study, well designed, with large numbers of men followed up for a long period. It is not, however, a prospective, randomised, controlled trial and therefore cannot demonstrate causation. The absolute risk of developing prostate cancer, even if the study is correct, is small, but not insignificant. The finding that more men undergoing vasectomy had PSA tests does raise the question of whether this influenced results, even allowing for the statistical analysis of this.

The authors conclude: ‘The decision to opt for a vasectomy remains a highly personal one in which the risks and benefits should be considered.’

This paper attracted a huge amount of interest in the lay press and will undoubtedly lead to GPs being questioned by men considering a vasectomy in the future and also by those who have had the procedure in the past.

GPs need to be aware of this small increase in absolute risk over a 24-year follow-up and arm patients with the facts to help them make a decision as to whether they wish to proceed with a vasectomy.

Dr Jon Rees