Anal Cancer Incidence in the United States, 1977-2011: Distinct Patterns by Histology and Behavior

Although anal squamous cell carcinoma (SCC) and adenocarcinoma (ADC) are generally combined in cancer surveillance, their etiologies likely differ. Here, we describe demographic characteristics and trends in incidence rates (IR) of anal cancer by histology (SCC, ADC) and behavior (invasive, in situ) in the United States.

With data from the Surveillance, Epidemiology, and End Results (SEER) Program, we estimated age-adjusted anal cancer IRs across behavior/histology by demographic and tumor characteristics for 2000-2011. Trends in IRs and annual percent changes during 1977-2011 were also estimated and compared with rectal cancer.

Women had higher rates of SCC [rate ratio (RR), 1.45; 95% confidence interval (CI), 1.40-1.50] and lower rates of ADC (RR, 0.68; 95% CI, 0.62-0.74) and squamous carcinoma in situ (CIS; RR, 0.36; 95% CI, 0.34-0.38) than men. Blacks had lower rates of SCC (RR, 0.82; 95% CI, 0.77-0.87) and CIS (RR, 0.90; 95% CI, 0.83-0.98) than non-Hispanic whites, but higher rates of ADC (RR, 1.48; 95% CI, 1.29-1.70). Anal cancer IRs were higher in men and blacks aged <40 years. During 1992-2011, SCC IRs increased 2.9%/year, ADC IRs declined nonsignificantly, and CIS IRs increased 14.2%/year. SCC and ADC IR patterns and trends were similar across anal and rectal cancers.

Rates of anal SCC and CIS have increased strongly over time, in contrast to rates of anal ADC, similar to trends observed for rectal SCC and ADC.

Anal SCC and ADC likely have different etiologies, but may have similar etiologies to rectal SCC and ADC, respectively. Strong increases in CIS IRs over time may reflect anal cancer screening patterns.