Rectal squamous cell carcinoma in immunosuppressed populations: is this a distinct entity from anal cancer?

Squamous cell carcinoma (SCC) of the rectum is rare, but as with anal cancer, risk may be increased among immunosuppressed individuals. We assessed risk of rectal SCC in HIV-infected people.Population-based registry.We utilized the HIV/AIDS Cancer Match, a linkage of US HIV and cancer registries (1991-2010), to ascertain cases of anal SCC, rectal SCC, rectal non-SCC, and colon non-SCC. We compared risk in HIV-infected persons with the general population using standardized incidence ratios (SIRs) and evaluated risk factors using Poisson regression. We reviewed cancer registry case notes to confirm site and histology for a subset of cases.HIV-infected persons had an excess risk of rectal SCC compared with the general population (SIR = 28.9; 95% CI 23.2-35.6), similar to the increase for anal SCC (SIR = 37.3). Excess rectal SCC risk was most pronounced among HIV-infected MSM (SIR = 61.2). Risk was not elevated for rectal non-SCC (SIR = 0.88) or colon non-SCC (SIR = 0.63). Individuals diagnosed with AIDS had higher rectal SCC rates than those with HIV-only (incidence rate ratio = 1.92; 95% CI 1.08-3.42). Based on available information, one-third of rectal SCCs were determined to be misclassified anal cancer.HIV-infected individuals, especially with advanced immunosuppression, appear to have substantially elevated risk for rectal SCC. As for anal SCC, rectal SCC risk was highest in MSM, pointing to involvement of a sexually transmitted infection such as human papillomavirus. Site misclassification was present, and detailed information on tumour location is needed to prove that rectal SCC is a distinct entity.