In recent decades, rates of anal squamous cell carcinoma (ASCC) have increased in many countries worldwide, and women are at higher risk than men, according to a study published online October 27 in the International Journal of Epidemiology.

The results point to the need for improved preventive measures, such as vaccination for human papillomavirus (HPV) and safe sex education, say the authors.

"We found that incidence of anal cancer has been increasing — either in both sexes or in women — in many populations, especially in the Americas, Northern and Western Europe, and Australia," write first author Farhad Islami, MD, PhD, and colleagues at the American Cancer Society, Atlanta, Georgia.

Though anal cancer is still relatively rare, past studies have reported rapidly increasing rates in high-income countries, such as Australia, Canada, Denmark, France, the Netherlands, the United Kingdom, and the United States. Whether rates are increasing in other parts of the world has been unknown until now.

The new study looked at anal cancer incidence in 18 countries across 4 continents, excluding Africa. The analysis covered 19 populations in total and considered US blacks and whites separately. Researchers used data provided by the International Agency for Research on Cancer to evaluate age-standardized incidence rates over a minimum of 13 years for ASCC and anal adenocarcinoma (AAC), two cancer subtypes that arise from different cells. Rates were calculated over a minimum of 13 years (ending in 2007), with an extended analysis in the United States through 2012. The earliest starting period was 1968 (Estonia), and the latest was 1994 (Russian Federation).

Results showed that ASCC accounted for most anal cancers in the countries included in the analysis.

In high-income countries, including Australia, Canada, Denmark, France, Italy, Netherlands, the United Kingdom, and the United States, increasing rates of ASCC over the time periods studied were seen in both men and women. The increase was mostly uniform, except in women in Australia and US whites, for whom rates increased more steeply starting in the 1990s.

Women had higher rates of ASCC than men in all years, except for 1996, when rates of ASCC in male US blacks surpassed female rates.

Rates of ASCC increased only among women in Colombia, Estonia, the Russian Federation, Slovakia, and Switzerland. India, Israel, Japan, Singapore, and Spain showed little change in ASCC incidence among both women and men.

In most countries, AAC incidence remained about the same or decreased. AAC was more common in men than in women.

The extended US analysis showed a continued trend toward increasing rates of ASCC in both women and men, though the increase in black men levelled off in 2010.

While reasons for increasing rates of ASCC remain unknown, it may be related to infection with HPV, especially HPV 16, the authors speculate. Because most HPV infections are cleared in a short period, factors that contribute to continued HPV infection, such as concurrent cervical HPV infection, may be involved. Increased duration of HIV infection related to improved medical management may also contribute. Also, behaviors such as anal intercourse and smoking can increase the risk for anal cancer, the authors point out.

HPV vaccination, which was introduced in 2006, can prevent anal HPV infection, but it may be too early to see the impact of vaccination, according to the authors. In addition, vaccine uptake has been suboptimal, and low- and middle-income countries do not routinely administer the vaccine.

"Certain preventive measures if implemented, including HPV vaccination and campaigns for safer sexual behaviours, would serve to prevent a substantial number of anal cancer cases in the future," they conclude.

They added that condom use can decrease the risk for genital HPV infection but does not provide complete protection.

The authors have disclosed no relevant financial relationships.

Int J Epidemiol. Published online October 27, 2016. Abstract

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