Does Circumcision Prevent Prostate Cancer?

A 2014 study by Spence was widely publicised in the media as “circumcision reduces risk of prostate cancer“.

In fact, the study found that overall there was no difference in the risk of prostate cancer of circumcised men compared to normal men (Odds Ratio 0.89, 95% CI 0.76–1.04).  The authors’ expectation bias is betrayed by the way they present this: “Circumcised men had a slightly lower risk, albeit not statistically significant, of developing prostate cancer than uncircumcised men”.

The authors went on to subdivide their study group and found a marginally significant “protective effect” in circumcised men aged 36 years or over (Odds Ratio 0.55, 95% CI 0.30–0.98).

They also claim a “weaker protective effect” for men circumcised within 1 year of birth (OR 0.86, 95% CI 0.72–1.04). This however does not meet the test of statistical significance and therefore should have been dismissed as a chance finding.

The authors go on to report that the “strongest protective effect of circumcision was recorded in Black men (OR 0.40, 95% CI 0.19–0.86, P-value for interaction 0.02)  but no association was found with other ancestral groups”.

This last finding is, according to the authors, a finding with no more than a 1 in 50 probability of happening by chance.  In fact, even this must be doubtful.

Medical research typically looks for statistical significance at the 95% level of probability.  This is equal to the standard of proof set in English law for a criminal trial but still leaves a 1 in 20 chance that a “significant” research finding will be found by chance.  A cartoon relates this to jelly beans causing acne.

Returning to the Spence study, the authors of the study hypothesised that circumcision prevents cancer.  They did not find this, but they subdivided their study population into 2 groups by aggressiveness of the cancer, 24 groups by characteristics of subjects and 5 groups by age at circumcision.  On average, out of the 31 subgroups in the research population, you would expect to find 1/20*31=1.55 “significant” findings.

As such, the authors’ conclusion of “novel evidence for a protective effect of circumcision against prostate cancer development, especially in those circumcised aged ≥36 years; although circumcision before the age of 1 year may also confer protection” is premature.

The Spence findings of one significant and 1 marginally significant result is to be expected by chance given the number of subgroups in the research.  Their claim of a protective effect for men circumcised under 1 year of age is unjustified and invokes the suspicion of advocacy research.

Abstract of the Spence Study:-

To investigate the possible association between circumcision and prostate cancer risk, to examine whether age at circumcision influences prostate cancer risk, and to determine whether race modifies the circumcision–prostate cancer relationship.

Subjects and Methods

PROtEuS (Prostate Cancer and Environment Study), a population-based case-control study set amongst the mainly French-speaking population in Montréal, Canada, was used to address study objectives. The study included 1590 pathologically confirmed prostate cancer cases diagnosed in a Montréal French hospital between 2005 and 2009, and 1618 population controls ascertained from the French electoral list, frequency-matched to cases by age. In-person interviews elicited information on sociodemographic, lifestyle and environmental factors. Unconditional logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) between circumcision, age at circumcision and prostate cancer risk, adjusting for age, ancestry, family history of prostate cancer, prostate cancer screening history, education, and history of sexually transmitted infections.

Results

Circumcised men had a slightly lower risk, albeit not statistically significant, of developing prostate cancer than uncircumcised men (OR 0.89, 95% CI 0.76–1.04). Circumcision was found to be protective in men circumcised aged ≥36 years (OR 0.55, 95% CI 0.30–0.98). A weaker protective effect was seen among men circumcised within 1 year of birth (OR 0.86, 95% CI 0.72–1.04). The strongest protective effect of circumcision was recorded in Black men (OR 0.40, 95% CI 0.19–0.86, P-value for interaction 0.02) but no association was found with other ancestral groups.

Conclusion

Our findings provide novel evidence for a protective effect of circumcision against prostate cancer development, especially in those circumcised aged ≥36 years; although circumcision before the age of 1 year may also confer protection. Circumcision appeared to be protective only among Black men, a group that has the highest rate of disease. Further research into the differences in effect of circumcision on prostate cancer risk by ancestry is warranted, as is the influence of age at circumcision.

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