Anal human papillomavirus (HPV) infection and lesions which typically precede anal cancer are very common in men who have sex with men (MSM), according to a meta-analysis published in The Lancet Oncology.

But the findings also suggest that rates of progression to cancer seem to be substantially lower than they are for cervical pre-cancerous lesions in women.

“A comparison of the prevalence of pre-cancerous lesions known as high-grade anal intraepithelial neoplasia (AIN) with anal cancer incidence suggests that most of these lesions will never progress to anal cancer, and progression might occur less often than it does for cervical precancers in women”, explains Andrew Grulich from the University of New South Wales, Australia, lead author of the study.

“Our findings mean that cervical cancer screening strategies cannot be simply extrapolated to anal cancer screening as current proposals suggest.”

During the past 20–30 years, the incidence of anal cancer has been increasing, and is particularly common in MSM and in HIV-positive individuals.

A large proportion of anal cancers are caused by the HPV, with types HPV 16 and HPV 18 responsible for 80% of anal cancers and 70% of cervical cancers. Just as cervical cancer screening led to striking reductions in the disease and death, so it has been suggested that screening for precancerous lesions among MSM could save many lives.

In this meta-analysis, the authors analysed data from 53 studies to determine how common anal high-risk HPV is, the prevalence and significance of AIN, and the rate of progression of AIN to anal cancer in MSM.

They found that most men had anal HPV infection, and high-grade AIN was present in 20–30% of all men. The prevalence of infection with high-risk HPV types was substantially higher in HIV-positive men than in HIV negative men (73.5% vs 37.2%), and precancerous abnormalities were also more common in this group.

An estimated 10% of HIV-positive men develop high-grade AIN every year compared with about 3% of HIV-negative men. But the authors point out: “These incidence rates seem to be higher than would be consistent with our estimate of the prevalence of high-grade AIN (29% in HIV-positive MSM and 21% in HIV-negative MSM), unless many of these high-grade lesions regress.”

Indeed, Grulich and colleagues calculated the progression rate from high-grade AIN to anal cancer to be around one in 600 per year in HIV-positive men, and about one in 4000 per year in HIV-negative men.

By comparison, high-grade cervical cancer precursors (CIN3) have a much higher rate of progression to cervical cancer, which is estimated to be about one in 80 per year.

They say: “The identification of biomarkers to establish which men with high-grade AIN are at highest risk for progression to anal cancer, and which are likely to regress, should be a research priority.”

They conclude: “The substantial differences in the natural history of anal HPV infection to those of cervical HPV infection that this review has identified suggests that we cannot simply transfer cervical cancer screening strategies to anal cancer screening…Large, good-quality prospective studies are needed to inform the development of anal cancer screening guidelines for MSM.”

In an accompanying Comment, Nicolas Wentzensen from the National Institutes of Health, Bethesda, USA remarks: “Implementation of screening for anal cancer in high-risk populations needs more data to estimate the trade-off between benefits (prevention of cancer) and harms (complications related to screening, treatment, and cost).”

Source: The Lancet Oncology